Basic Cooling System Theory
The internal combustion engine is a very inefficient way to power a car. Most of the energy of the fuel is wasted creating heat. This heat, if not removed, will quickly cause the engine to get so hot that the metal parts will melt. The cooling system is there to remove that excess heat. As engine efficiency has improved over the years, cooling systems have gotten smaller and smaller. In the fifties, the average cooling system contained 24 or more liters of coolant, now, the average is somewhere around 8. My Firefly holds less than 4.The way the whole thing works is really quite simple. The heat is transferred from the hot engine parts to the coolant, which flows to the radiator where the heat gets transferred to the air that is flowing through the fins. Then the coolant flows back into the engine where it picks up more heat and so onThe main problem that occurs is problems with the flow. Flow problems are pretty simple to figure out, usually there is an obstruction such as a stuck thermostat, water pump not turning, collapsed radiator hose, plugged radiator, air bubble in the engine, etc.
Liquid cooling systems are employed by most engines today. A typical automotive cooling system comprises
(1) a series of channels cast into the engine block and cylinder head, surrounding the combustion chambers with circulating water or other coolant to carry away excessive heat,
(2) a radiator, consisting of many small tubes equipped with a honeycomb of fins to radiate heat rapidly, that receives and cools hot liquid from the engine,
(3) a centrifugal-type water pump with which to circulate coolant,
(4) a thermostat, which maintains constant temperature by automatically varying the amount of coolant passing into the radiator, and
(5) a fan, which draws fresh air through the radiator.
For operation at temperatures below 32º F (0º C), it is necessary to prevent the coolant from freezing. This is usually done by adding some compound to depress the freezing point of the coolant. Alcohol formerly was commonly used, but it has a relatively low boiling point and evaporates quite easily, making it less desirable than organic compounds with a high boiling point, such as ethylene glycol. By varying the amount of additive, it is possible to protect against freezing of the coolant down to any minimum temperature normally encountered. Coolants contain corrosion inhibitors designed to make it necessary to drain and refill the cooling system only once a year.
Air-cooled cylinders operate at higher, more efficient temperatures, and air cooling offers the important advantage of eliminating not only freezing and boiling of the coolant at temperature extremes but also corrosion damage to the cooling system. Control of engine temperature is more difficult, however, and high-temperature-resistant ceramic parts are required when design operating temperatures are significantly increased.
Pressurized cooling systems with operating pressures up to 14 pounds per square inch (100 kilopascals) have been used to increase effective operating temperatures. Partially sealed systems using coolant reservoirs for coolant expansion if the engine overheats were introduced in 1970.
Introduction to How Car Cooling Systems Work
Although gasoline engines have improved a lot, they are still not very efficient at turning chemical energy into mechanical power. Most of the energy in the gasoline (perhaps 70%) is converted into heat, and it is the job of the cooling system to take care of that heat. In fact, the cooling system on a car driving down the freeway dissipates enough heat to heat two average-sized houses! The primary job of the cooling system is to keep the engine from overheating by transferring this heat to the air, but the cooling system also has several other important jobs.
The engine in your car runs best at a fairly high temperature. When the engine is cold, components wear out faster, and the engine is less efficient and emits more pollution. So another important job of the cooling system is to allow the engine to heat up as quickly as possible, and then to keep the engine at a constant temperature.
Diagram of a cooling system: how the plumbing is connected
In this article, we’ll learn about the parts of a car cooling system and how they work. First, let’s look at some basics.
Inside your car’s engine, fuel is constantly burning. A lot of the heat from this combustion goes right out the exhaust system, but some of it soaks into the engine, heating it up. The engine runs best when its coolant is about 200 degrees Fahrenheit (93 degrees Celsius). At this temperature:
The combustion chamber is hot enough to completely vaporize the fuel, providing better combustion and reducing emissions.
The oil used to lubricate the engine has a lower viscosity (it is thinner), so the engine parts move more freely and the engine wastes less power moving its own components around.
Metal parts wear less.
There are two types of cooling systems found on cars: liquid-cooled and air-cooled.
The cooling system on liquid-cooled cars circulates a fluid through pipes and passageways in the engine. As this liquid passes through the hot engine it absorbs heat, cooling the engine. After the fluid leaves the engine, it passes through a heat exchanger, or radiator, which transfers the heat from the fluid to the air blowing through the exchanger.
Some older cars, and very few modern cars, are air-cooled. Instead of circulating fluid through the engine, the engine block is covered in aluminum fins that conduct the heat away from the cylinder. A powerful fan forces air over these fins, which cools the engine by transferring the heat to the air.
Since most cars are liquid-cooled, we will focus on that system in this article.
The cooling system in your car has a lot of plumbing. We’ll start at the pump and work our way through the system, and in the next sections we’ll talk about each part of the system in more detail.
The pump sends the fluid into the engine block, where it makes its way through passages in the engine around the cylinders. Then it returns through the cylinder head of the engine. The thermostat is located where the fluid leaves the engine. The plumbing around the thermostat sends the fluid back to the pump directly if the thermostat is closed. If it is open, the fluid goes through the radiator first and then back to the pump.
There is also a separate circuit for the heating system. This circuit takes fluid from the cylinder head and passes it through a heater core and then back to the pump.
On cars with automatic transmissions, there is normally also a separate circuit for cooling the transmission fluid built into the radiator. The oil from the transmission is pumped by the transmission through a second heat exchanger inside the radiator.
Cars operate in a wide variety of temperatures, from well below freezing to well over 100 F (38 C). So whatever fluid is used to cool the engine has to have a very low freezing point, a high boiling point, and it has to have the capacity to hold a lot of heat.
Water is one of the most effective fluids for holding heat, but water freezes at too high a temperature to be used in car engines. The fluid that most cars use is a mixture of water and ethylene glycol (C2H6O2), also known as antifreeze. By adding ethylene glycol to water, the boiling and freezing points are improved significantly.
0 C / 32 F
-37 C / -35 F
-55 C / -67 F
100 C / 212 F
106 C / 223 F
113 C / 235 F
The temperature of the coolant can sometimes reach 250 to 275 F (121 to 135 C). Even with ethylene glycol added, these temperatures would boil the coolant, so something additional must be done to raise its boiling point.
The cooling system uses pressure to further raise the boiling point of the coolant. Just as the boiling temperature of water is higher in a pressure cooker, the boiling temperature of coolant is higher if you pressurize the system. Most cars have a pressure limit of 14 to 15 pounds per square inch (psi), which raises the boiling point another 45 F (25 C) so the coolant can withstand the high temperatures.
Antifreeze also contains additives to resist corrosion.
The water pump is a simple centrifugal pump driven by a belt connected to the crankshaft of the engine. The pump circulates fluid whenever the engine is running.
A centrifugal pump like the one used in your car
The water pump uses centrifugal force to send fluid to the outside while it spins, causing fluid to be drawn from the center continuously. The inlet to the pump is located near the center so that fluid returning from the radiator hits the pump vanes. The pump vanes fling the fluid to the outside of the pump, where it can enter the engine.
The fluid leaving the pump flows first through the engine block and cylinder head, then into the radiator and finally back to the pump.
The engine block and cylinder head have many passageways cast or machined in them to allow for fluid flow. These passageways direct the coolant to the most critical areas of the engine.
Note that the walls of the cylinder are quite thin, and that the engine block is mostly hollow.
Temperatures in the combustion chamber of the engine can reach 4,500 F (2,500 C), so cooling the area around the cylinders is critical. Areas around the exhaust valves are especially crucial, and almost all of the space inside the cylinder head around the valves that is not needed for structure is filled with coolant. If the engine goes without cooling for very long, it can seize. When this happens, the metal has actually gotten hot enough for the piston to weld itself to the cylinder. This usually means the complete destruction of the engine.
The head of the engine also has large coolant passageways.
One interesting way to reduce the demands on the cooling system is to reduce the amount of heat that is transferred from the combustion chamber to the metal parts of the engine. Some engines do this by coating the inside of the top of the cylinder head with a thin layer of ceramic. Ceramic is a poor conductor of heat, so less heat is conducted through to the metal and more passes out of the exhaust.
A radiator is a type of heat exchanger. It is designed to transfer heat from the hot coolant that flows through it to the air blown through it by the fan.
Most modern cars use aluminum radiators. These radiators are made by brazing thin aluminum fins to flattened aluminum tubes. The coolant flows from the inlet to the outlet through many tubes mounted in a parallel arrangement. The fins conduct the heat from the tubes and transfer it to the air flowing through the radiator.
The tubes sometimes have a type of fin inserted into them called a turbulator, which increases the turbulence of the fluid flowing through the tubes. If the fluid flowed very smoothly through the tubes, only the fluid actually touching the tubes would be cooled directly. The amount of heat transferred to the tubes from the fluid running through them depends on the difference in temperature between the tube and the fluid touching it. So if the fluid that is in contact with the tube cools down quickly, less heat will be transferred. By creating turbulence inside the tube, all of the fluid mixes together, keeping the temperature of the fluid touching the tubes up so that more heat can be extracted, and all of the fluid inside the tube is used effectively.
Picture of radiator showing side tank with cooler
Radiators usually have a tank on each side, and inside the tank is a transmission cooler. In the picture above, you can see the inlet and outlet where the oil from the transmission enters the cooler. The transmission cooler is like a radiator within a radiator, except instead of exchanging heat with the air, the oil exchanges heat with the coolant in the radiator.
The radiator cap actually increases the boiling point of your coolant by about 45 F (25 C). How does this simple cap do this? The same way a pressure cooker increases the boiling temperature of water. The cap is actually a pressure release valve, and on cars it is usually set to 15 psi. The boiling point of water increases when the water is placed under pressure.
Cutaway of radiator cap and reservoir
When the fluid in the cooling system heats up, it expands, causing the pressure to build up. The cap is the only place where this pressure can escape, so the setting of the spring on the cap determines the maximum pressure in the cooling system. When the pressure reaches 15 psi, the pressure pushes the valve open, allowing coolant to escape from the cooling system. This coolant flows through the overflow tube into the bottom of the overflow tank. This arrangement keeps air out of the system. When the radiator cools back down, a vacuum is created in the cooling system that pulls open another spring loaded valve, sucking water back in from the bottom of the overflow tank to replace the water that was expelled.
The thermostat’s main job is to allow the engine to heat up quickly, and then to keep the engine at a constant temperature. It does this by regulating the amount of water that goes through the radiator. At low temperatures, the outlet to the radiator is completely blocked — all of the coolant is recirculated back through the engine.
Once the temperature of the coolant rises to between 180 and 195 F (82 – 91 C), the thermostat starts to open, allowing fluid to flow through the radiator. By the time the coolant reaches 200 to 218 F (93 – 103 C), the thermostat is open all the way.
The open and closed positions of a thermostat
If you ever have the chance to test one, a thermostat is an amazing thing to watch because what it does seems impossible. You can put one in a pot of boiling water on the stove. As it heats up, its valve opens about an inch, apparently by magic! If you’d like to try this yourself, go to a car parts store and buy one for a couple of bucks.
The secret of the thermostat lies in the small cylinder located on the engine-side of the device. This cylinder is filled with a wax that begins to melt at around 180 F (different thermostats open at different temperatures, but 180 F is a common one). A rod connected to the valve presses into this wax. When the wax melts, it expands significantly, pushing the rod out of the cylinder and opening the valve. If you have read How Thermometers Work and done the experiment with the bottle and the straw, you have seen this process in action — the wax just expands a good bit more because it is changing from a solid to a liquid in addition to expanding from the heat.
This same technique is used in automatic openers for greenhouse vents and skylights. In these devices, the wax melts at a lower temperature.
Like the thermostat, the cooling fan has to be controlled so that it allows the engine to maintain a constant temperature.
Front-wheel drive cars have electric fans because the engine is usually mounted transversely, meaning the output of the engine points toward the side of the car. The fans are controlled either with a thermostatic switch or by the engine computer, and they turn on when the temperature of the coolant goes above a set point. They turn back off when the temperature drops below that point.
Rear-wheel drive cars with longitudinal engines usually have engine-driven cooling fans. These fans have a thermostatically controlled viscous clutch. This clutch is positioned at the hub of the fan, in the airflow coming through the radiator. This special viscous clutch is much like the viscous coupling sometimes found in all-wheel drive cars.
You may have heard the advice that if you car is overheating, open all the windows and run the heater with the fan going at full blast. This is because the heating system is actually a secondary cooling system that mirrors the main cooling system on your car.
The heater core, which is located in the dashboard of your car, is really a small radiator. The heater fan blows air through the heater core and into the passenger compartment of your car.
A heater core looks like a small radiator.
The heater core draws its hot coolant from the cylinder head and returns it to the pump — so the heater works regardless of whether the thermostat is open or closed.
For more information on car cooling systems and related topics, check out the links on the next page.
Cooling System Components:
Cooling System Components
The radiator is the key to the entire cooling system, it is the part that actually transfers the excess heat produced in the engine to the outside air. It is made up of several rows of small tubes connecting two containers which hold the coolant. Small fins are placed around the tubes to direct air around the outside of the tubes and to help the heat transfer from the tubes to the outside air.
The radiator hoses don’t do much except transport the coolant into and out of the radiator. The normal flow is from the lower hose to the water pump, through the cooling passages in the engine, and back through the thermostat to the upper hose and then to the radiator.
The water pump furnishes the force that moves the coolant around the system. (Duh) Most common problem is a leak that will appear at the small hole that you can see near the shaft if you look closely. This leak isn’t a problem if you make sure that you top up the system on a regular basis.
The thermostat blocks off the main path for the hot coolant to return to the radiator, causing a large portion of it to be directed through the heater core and back to the engine until it reaches normal operating temperature and the thermostat opens. At this point coolant starts to flow to the radiator through the upper radiator hose. This function makes your heater produce heat faster, as well as helps the engine warm up quicker.
Transport coolant to and from the heater core. (duh) Look for leaks at the hose clamps where they connect to the heater core and the engine.
A mini-radiator that is used to heat the air inside the car. The hot coolant flows through the tubes and air is directed through the fins, absorbing the heat and then being directed to the cab or to the windshield defrost vents. When your heater core starts to get plugged up, you will gradually get less and less heat from your heater. A flush might help, but it could also trigger a leak…
In the olden days, cars used plain water for coolant, and it worked fairly well, except when it froze. Modern cars run at a higher temperature and use much smaller cooling systems. The vast majority of cars use a 50-50 mix of ethylene glycol and water for coolant, which also gives anti-freeze and anti-corrosion protection. In extreme cold climates, a mixture of up to 75% antifreeze to 25% water is used. Don’t try to run your car on pure anti-freeze! The engine will overheat. If you use distilled water for the mix, you’ll have significantly less corrosion.
The cap just keeps the coolant from sloshing out of the radiator and it maintains a positive pressure in the cooling system. Since the boiling temperature of the coolant is dependent on the pressure, this feature is necessary for proper functioning of the system.
The thermostat is probably the cheapest part of the cooling system. Mainly for this reason, every customer that comes in the door with an overheating problem is required to say, “It’s probably the thermostat.” This is a law of nature. The thermostat has one job, that is to block the main coolant return passage to the radiator until the engine heats up to operating temperature. Then the thermostat opens and the coolant circulates. If the top radiator hose gets hot, and coolant flows freely into the top of the radiator, then the thermostat is opening. Still, a new one can’t hurt, and most of them are fairly easy to replace, so why not? Be warned though, I’ve seen brand new thermostats that were defective.
The bypass hose allows a small portion of the coolant to circulate when the thermostat is closed, get it? Bypassing the thermostat. Some engines don’t have a bypass hose, but rather a bypass passage that is part of the engine cooling jacket. Not much to go wrong, unless the hose develops a leak, which they do with depressing regularity.
Engine swaps or modifications are amongst the top few in the Mini owners list of desires for their car. Little thought or consideration is given to the cooling system when either of these up-grades is carried out. Largely because very few understand just what the cooling system does and how it does it, and that shortfalls in compatibility between the cooling systems capability and the power output of the engine can spell disaster for the new engine. All this is obviously exaggerated in the case of racing engines. Questions along these lines are popular – in most cases too late to be of use, so a little explanation should go a long way…
Cooling system functions
The internal combustion engine as used in cars is not particularly efficient. Burning a fuel/air mixture produces energy, but because this method of energy produces high levels of heat, much of the energy produced must be dissipated. This is essential to prevent component failure through thermal fatigue. The components most susceptible to failure in this manner are the pistons, piston rings, cylinder walls, cylinder head, valves and associated parts; although excessive heat will eventually cause more wide spread failures. The energy/heat level is regulated by the cooling system, passing into the coolant from the combustion chamber in the head, and partially via the cylinder walls to the radiator then to atmosphere.
The combustion chamber area must be cooled sufficiently to prevent pre-ignition and detonation, problems that are exaggerated by current low-octane, unleaded fuels, and the ever-tightening legislation on lower emissions and lean burn engines. Fortunately the latter does not affect the venerable A-series engine, although those seeking to maximise fuel economy should take note. If an inefficient or inadequate cooling system is used, further losses will be experienced. The higher the combustion chamber temperatures are, the more the ignition has to be retarded to avoid the onset of the aforementioned pre-ignition and detonation. This causes a reduction in engine output; particularly torque that is the mainstay of driving the car. Further torque losses are caused when an engine is running too hot by increased inlet temperatures, creating a less dense fuel/air mixture.
Heat dissipation and temperature control are regulated by the cooling system. A thermostat is fitted to keep the temperature constant and consistent at the required level. Heat dissipation is largely by thermal conductivity. The coolant passes over the hot metal surfaces inside the cylinder head and water jacket around the cylinders where heat is transferred to it as it is at a lower temperature. The coolant then passes into and through the radiator where the heat is passed into the cooler air.
The coolant explained
Water is the most common form of coolant used in car engines. It has excellent heat transfer properties in its liquid state and does an extremely good job when properly controlled. It does have one or two shortcomings though. The worst from a cooling point of view when not controlled is it’s very high surface tension – the thing that allows bugs to walk around on it without sinking.
This surface tension limits its ability to ‘wet’ the metal surfaces of the water jacket, forming a sort of barrier. Because of this, hot-spots can be caused – particularly around the combustion chambers where temperatures are highest. These hot-spots form vapour bubbles by boiling the water despite the fact that the bulk of the passing water is well below boiling point. The bubbles formed on the metal surfaces then act as an insulator around this area, greatly impeding heat transfer. This in turn reduces the cooling systems efficiency, thereby increasing the combustion chamber temperature.
The eventual result is component failure, the piston usually being the first to go, or maybe the spark plug, then the exhaust valve, inlet valve, and so on. The speed at which this can happen can be alarmingly quick, and is governed by the severity of the hot-spot and the dynamic loads on the engine (i.e. foot hard down = max. load = blisteringly quick melt down if there is a hot-spot present).
Anti-freeze is widely used as an additive to water in car cooling systems, and is indeed essential where freezing temperatures are to be experienced. It also raises the boiling point slightly, as well as providing some lubrication for the water pump seals and reduces the formation of rust on the iron surfaces. The reduction of corrosion helps prevent blockages in the radiator. It does not, however, increase the cooling capability of the system. Many people are under the false impression that adding more anti-freeze will solve over-heating problems – nothing could be further from the truth.
No more than is absolutely necessary to provide sufficient protection in the environment in which the car is used should be added. Follow the manufacturers instructions to the letter. Although as standard all road cars have a larger cooling capability than is required to allow for a fairly strong anti-freeze/water mix, bigger or more powerful (tuned) engines will soon render it inadequate.
Water, as previously mentioned, has amazing heat transfer properties, far better than almost any other liquid cooling medium within a vast majority of spheres. It is certainly superior to a mix of anti-freeze (usually glycol based) and water. In fact water has up to two and a half times greater thermal conductivity to, say, a glycol-type coolant given the same operating capacity. As the cooling system works by conductivity – from hot metal to a cooler liquid (as in the engine water jacket) then from hot liquid to cooler metal surfaces (as in the radiator), the coolants thermal conductivity is of ultra importance. Tests carried out by major motor manufacturers have concluded that the improvement of glycol’s thermal conductivity is practically directly proportional to the amount of water added to it. Just to illustrate this, a 50/50 water and glycol mix has about 70% of the thermal conductivity of water on it’s own.
To labour the point so that you are left in no doubt about this, other factors such as the viscosity of the coolant, and the convection coefficient of the coolant in a tube (a complex relationship between the thermal conductivity, viscosity, tube diameter – as in radiator core tube – and turbulent flow of the system) influence the effectiveness of the system. A 50/50 glycol/water mix has roughly four times the viscosity (thickness) of water alone and, as previously mentioned, about 70% of the thermal conductivity. A trial using these factors established that this mix had approximately 50% of the convection coefficient of water only. Or to put it in English, water on it’s own as a coolant is capable of TWICE as much heat transfer as the 50/50 mix. Hopefully this has exploded the ‘more anti-freeze will help’ myth once and for all.
Capability Improvement Options
So what can be done, and when is it needed? The last thing you need to do is install your mega-hyperpower engine with a cooling system that is a wild guess at best, to find that it is woefully inadequate, causing the early demise of your pride and joy. To give an illustration of the standard systems capability, even the Cooper S having its radiator with increased ‘gills per inch’ would over-heat at anything but a steady 70 mph. To all intents and purposes, if you put a 1275 engine in where there used to be a 998, put an up-rated radiator in as well. The standard 998 radiator will cope with the application of a stage one kit, but going to a decent modified head and fast road cam will sorely test it if it is in any other condition than A1 perfect.
Old thinking used to be that more water is the way to go, hence the appearance many moons ago of the special four-core radiator. It certainly helped, giving about 23% more cooling capability over the standard item. But technology moves on a-pace, and the ‘more water’ theory soon bit the dust. The latest breed of radiators only have two cores, but a vastly superior core and fin arrangement, giving around 37% more cooling capability than standard. In fact this type of radiator has been sufficient to cool engines fitted with turbos and having outputs of around 160 bhp. Fitting one of these to any normally aspirated A-series should kill the problem dead. After all, it is better to have too much cooling than too little – you can always blank some off, or fit a different thermostat. A bonus here is that it actually weighs less than the standard set-up, every little saving in weight helps – especially when racing. If it were for any kind of racing in the dirt, I would not fit the two-core radiator. Its gills are easily damaged and clogged by clods of mud. Use the four-core.
Back to thermostats for a moment. It is common practice to remove this and fit a blanking sleeve in a bid to improve cooling. If this is done, you must blank off the by-pass hose, otherwise stagnant areas of water will occur causing the dreaded hot-spots. However, the danger with fitting a blanking sleeve is that the engine may not reach proper operating temperatures, and this can be every bit as bad as running a little too hot. I would strongly advise using a thermostat in ALL road cars, of at least 82 degrees to make sure the correct running temperatures are achieved. A blanking sleeve is not the answer to over-heating problems. I always run a thermostat in my race engines unless bound for foreign shores where high ambient temperatures are experienced. Many folk think that they have to fit a blanking sleeve if they are blanking off the by-pass hose. Not so. Blank off the troublesome by-pass hose then fit a thermostat that has had six or eight eighth-inch holes drilled around the periphery. These holes allow water to circulate before the engine is up to temperature and the thermostat opens.
Fitment of an auxiliary radiator will help if the two-core is not enough – say on a race or rally car. Use the matrix out of the heater box, and plumb this in going from the heater tap take-off, into the back fitting of the matrix, then out of the front fitting and into the bottom hose. Mount the matrix behind the grill for maximum benefit – around fifteen degrees temperature drop can be expected. If you pass the water coming out of the heater tap take-off down the front of the matrix first, you will be blowing hot air across the water going back out of the matrix and into the engine. It is important to know that not taking water out the heater tap take-off will increase the temperature that the number four cylinder runs at substantially due to reduced flow around that chamber. Some folk make the mistake of taking the water out of here and connecting it back to the bottom hose. This is putting un-cooled water straight back into the engine. If you do not want to run an auxiliary radiator or internal heater, plumb the hose from the heater take-off into the top hose. This is the least that should be done.
Ensure you always use the water pump with the deep impellor. These are fitted to everything as standard these days, but 850/998/1098 engines before about 1975-ish had the old shallow impellor type. The shallow impellor protrudes from the gasket face by 7.9mm (5/16”) and the deep impellor by 15.75mm (5/8”). All Metros also have the by-pass hose blanked off in the casting, as do the very late Minis. The exception to the rule here is the 850, there is rarely enough material in the block to be able to run these. If fitting to an old 998/1098 block, it may be necessary to grind some of the cylinder wall away to clear the deep impellor. To help engines that will be run mainly at high rpm, use the Metro 1275 large diameter water pump pulley (4.725” diameter), as this will slow the pump speed down, reducing the onset of cavitation.
There are a couple of alternative fans available. The old two-blade type (that is usually run doubled up to make a four-blade), or the six-blade export type fan. I am ignoring the old metal multi-blade type, as they are not generally available and not that good. The four-blade is very noisy but very good, the six-blade much better than the standard plastic one, but a little noisier.
Apart from this, make sure your hoses are in good condition, and you have the right hose for the right engine, particularly when going from a 998 to a 1275 based engine. The top hose is very much different – the 998 looking like a boomerang, the 1275 one shaped like a question mark. Using the 998 one on a 1275 will put a kink in the hose that will cause a severe restriction. It will also be necessary to change the top radiator bracket. This is caused by the thermostat housing pointing sideways on the 998 and forwards on the 1275. The Cooper S top hose and bracket, or 1275GT versions, are the ones to use.
This is a basic introduction to cooling systems and various well-tried solutions. For further information, see ‘Cooling – Controlling water temperature’ under the cooling section.
Cooling System Modifications for rally car
A. Problem Background
With internal combustion temperatures that approach 1300 oF it is vital that a cooling system be designed that thoroughly and efficiently rejects excess heat. The role of the cooling system is to ensure proper and reliable engine performance within an optimal temperature range in all operating conditions. Over many years of racing it has been determined that traditional cooling systems are a closed system consisting of water pump which pushes coolant through the engine and pulls it through a heat exchanger.
The cooling system design in which we are remodeling is comprised of a water pump which is too robust and a heat exchanger with inadequate surface area for proper cooling.
B. Problem Statement
Redesign the current rally car cooling system so that it can support nominal heat generating loads and surge loads while reducing the overall weight of the system based on the following constraints:
C. System constraints
a. Overall weight is reduced.
b. The system will remain autonomous in reference to the
c. Pump is driven mechanically.
d. Fan is electrical and controlled with a thermal switch.
e. The closed system must be sealed from air intrusion.
f. The closed system must be able to withstand pressures
appropriate for racing conditions.
g. Reduction in size of primary heat exchanger.
II. Statement of Work
A. Method of Solution
When designing a state of the art performance based cooling system an accurate thermal model is necessary for rating the pump power vs. heat exchanger size which is the quagmire of our problem. All solutions must meet these design requirements.
B. Design requirements
a. Coolant is maintained at temperatures between 180-220 oF during operating
b. Cooling system will perform regardless of ambient air temperatures.
c. Cooling system will be able to function under nominal and shock engine rpm conditions without excessive loss of engine power.
d. Cooling system will utilize aerodynamic flow as a means of dissipating heat efficiently while reducing drag
Our cooling system will utilize two different heat exchangers with different performance requirements as a means for efficiently reducing cooling system heat load.
A primary radiator will be used to reduce cooling system heat load at nominal temperatures that range too 200 oF. A secondary radiator will handle shock loads with temperatures exceeding 200 oF. These heat exchangers will be connected in series for maximum heat transfer.
An efficiently designed system of this type will consist of a primary radiator that has ample surface area to support cooling loads during conditions in which the car is moving slowly or not at all. The heat transfer coefficient of our nominal system must be adequate to keep the engine from overheating during nominal driving conditions.
Our shock system must be placed properly so that high pressure turbulent airflow is adequate enough that as temperatures rise coolant will be properly cooled without the need for a second fan. Surface area must also be adequate while not adding excessive weight from excessive surface area. The heat transfer coefficient of the shock system must only be great enough to support shock loads The Solution to these criterion places our secondary heat exchanger in the rear spoiler of the racecar. As a means for conserving weight we will place the secondary heat exchanger in an already existing part as opposed to creating an additional part. The design of the rear spoiler is to direct the horizontal flow of air vertically to effectively create a high pressure area above the car and a low pressure zone below the car creating down force. This down force is a safety as well as performance measure to keep the car from flipping.
By placing the secondary heat exchanger in the rear spoiler high pressure air will flow directly through it dissipating heat from our system. In doing so the necessity of a cooling fan is also decreased allowing us to reduce the cooling systems draw on engine power. Also the aerodynamic effect on the spoiler will be minimal as the integrity andshape of the design will be maintained.
The feasibility of our design is good. It involves thinking differently about theutilization of the heat exchanger. The theory of a multiple radiator design is not a new concept. However the previous drawbacks were the tradeoffs between increasedefficiency and added weight with decreased reliability. With more parts come more opportunities for failure. The most efficient designs are simple and have the least parts.
While testing our design the overall system reliability will be determined. Because of our secondary radiator and additional controls that follow our system is further complicated.
Testing will determine optimization.
The approach for testing and optimizing our system will have us testing for the various temperatures at various rpm as the coolant flows through the engine. We will then use these temperatures and mechanical as well as physical properties of our system materials, components, as well as ambient conditions to design an optimization model for sizing our primary and secondary radiator vs. the size of our water pump and also the power needed for our fan. Once the optimization model has been completed our cooling system will be assembled to the car and tested in a wind tunnel in simulated conditions.
Further testing comes once our cooling system meets the standards for design in a performance scenario.
B. Alternative Design Solutions
Alternative Designs include a system of similar design however the heatexchanger runs in a parallel configuration as opposed to series configuration with coolant flow properly regulated. This would increase the temperature gradient between the two heat exchangers. Also additional parts would be necessary as well as an increase in weight would be a result of this configuration.
Another alternative design contains a radiator with fins made from carbon nanotubes. The advantage to using carbon nanotubes is a density half as great as aluminum and higher thermal conductivity than aluminum. At nearly twelve times greater than aluminum, carbon nanotubes would effectively remove heat from our system.
However this only holds true for a very thin sheet of nanotubes. As they become layered and adhesive polymers are applied carbon nanotubes have a decreased thermal conductivity. Calculations prove that a higher than aluminum thermal conductivity than aluminum is actually negligible in the overall heat transfer coefficient and our concentration should be on increased heat exchanger surface area to raise the overall heat transfer coefficient.
The search for an increase in surface area while not a the expense of losing weight then led to a design using the shell of the racecar as a membrane for which our coolant would pass utilizing the surface area of the entire car as a medium for heat exchange. A thin aluminum shell would be the body of our car with another layer micrometers inside.
Our coolant would flow continuously through this shell. Under this condition the exterior of our car would approach temperatures of 200 oF. Also racecar body would be pressurized at 15psi. As the racecar is in motion it is subjected to flying debris. Should the body be punctured there would be no way to effectively repair it without replacing the shell entirely. Also without a fan, as the car is idling heat transfer to the air is minimized and the possibility of the engine overheating when the car is moving slow or is stopped.
Our final alternative design consideration is a cross flow radiator utilizing a coolant box array. The design consists of coolant flowing in a cross flow-fin configuration in an array of tubes a millimeter in diameter. The array of tubes increases the surface area our coolant would contact with thus allowing for a smaller heat exchanger. When using tubes this small our primary concern is of the tubes getting clogged with debris from our cooling system. Also the tubes would be subjected to aconstant pressure of 15 psi and may not hold up under constant pressure. Also the ove
Due to the conditions of a formula style race car and the cooling needs of the driver, it would be best to use a thermosyphon system. A Thermosyphon is a two-phase passive loop that uses liquid as a working fluid for indirect and direct cooling. Heat from the components enters the evaporator in which two-phase exists (liquid and vapor), the vapor rises in the vapor line to the condenser. The condenser dissipates the heat and returns the fluid back down the liquid line into the evaporator inlet. A Thermosyphon is a continuous cooling system using natural convection. The natural convection works because as the fluid is heated up it will begin to enter the vapor state. As more heat is added the bubbles increase in size and begin to rise (since vapor is less dense than liquid) up the tube into the condenser. Once the vapor goes back to the liquid state it falls back to the evaporator where the cycle repeats.
In the design of a thermosyphon a coolant must be chosen. Water would seem to be a natural choice because of its inexpensiveness and its high latent heats. The only problem with water in a system like this is the high boiling temperature. At atmospheric pressure water boils at 100°C . To bring the boiling temperature of water down to a value that is close to the skin surface temperature, which is approximately 35°C, the system must be vacuumed to approximately 5.6 kPa before use . The amount of heat removed by our thermosyphon system will be what defines our system. The rate of heat removal from any refrigeration system depends on the latent heat of the fluid used and the heat flux of the source combined with the surface area covered by the thermosyphon . Due to the fact that our evaporator is the human body, there is no option of improving on the evaporator. However, what can be done is devise a way to increase the surface area covered by the cooling system to decrease the flow rate of the fluid while maintaining the human body temperature. As stated previously, the amount of heat removal desired is 475W. We will achieve this with water vacuumed to 5.6 kPa. The latent heat of water at this pressure is 2418.6 kJ/kg, and with this value a flow rate can be found . The flow rate needed is as follows in Equation (1).
g hr m kg kg g L m L hr
W Q h fg J s Q kJ kg Q g
.1963 sec 3600 sec 1.006 *1 1000 *1000 1 .7109 /
475 2418.6 .1963 sec
∗ ∗ 3 3 =
= ⋅ ⇒ = ⋅ ⇒ = ⇒
(1) Flow Rate Equation
Before we go into what a cooling system does and how to modify it, you must first understand what the engine does. Plain and simple, an engine burns a fuel to generate heat energy and transforms that heat energy into mechanical energy. Any heat generated that does not get used to make power is wasted energy. How well an engine converts the heat it generates into mechanical energy is known as its thermal efficiency. The cooling system takes heat from the engine, heat that ideally could have made power, so the cooling system actually takes power from the engine. It is a necessary evil, without a cooling system, the engine will overheat and the internal parts will have a very short life. A cooling system also reduces the chances of detonation. With new cooling systems and coolants, it is possible to run today’s engines hotter, which increases thermal efficiency. If you take less heat away from the engine, there will be more energy available to make power. Any heat that is radiated off the engine and out the exhaust system is also wasted heat energy that did not get used to make power, which reduces thermal efficiency. The average engine has only a 25-30% thermal efficiency, so 70-75% of the heat generated never gets used to make power. An average 250hp gasoline engine is actually burning enough fuel to make about 1000 hp, making it a very inefficient machine.
Cooling System Goals
Most people seem to think that all a cooling system needs to do is keep the engine from
overheating. But what is not realized is that if the engine runs too cool, thermal efficiency is lost and power is reduced. Many will argue that an engine has more power when it is cold, but that is only due to the fact that the intake air is colder and denser, actual BSFC is higher. Remember that an engines whole job is turn heat into mechanical energy. Running the engine as hot as possible (limited by the detonation limit) will increase power and provide a lower BSFC.
If steam pockets will form, detonation will limit power. Most of today’s high output street motors using a water/ethylene glycol mixture will be limited to about 200° F before detonation becomes a problem (unless other steps are taken). Another goal of modifying the cooling system is to even out the temperatures of the whole engine, which is not easy to do. All it takes is one hotter cylinder to run into detonation to limit the engines power. It only takes 1 cylinder to limit all of them. Most high performance engines are close to detonation to begin with, so a good cooling system is a must.
Nucleate Cooling Phase
As coolant flows through the system it absorbs heat from the engine parts that it comes in
contact with. As it does this some of the coolant will boil and form tiny steam bubbles (absorbing a lot of heat in the process) on the internal engine surfaces. When these bubbles get larger they become a flow restriction and the flowing fluid pushes them away from the surface and that process starts over again. The process is called the Nucleate Cooling Phase. When the coolant boiling point is too low or the flow rate is too slow, these bubbles can become too large and form steam pockets that insulate that surface from being cooled. This usually happens around the combustion chambers, the hottest parts of the engine. Once the steam pocket forms the surface will rise in temperature (even though the coolant is not overheating) and cause that part to overheat, which can cause detonation and / or other problems.
Types of Coolant
I’m sure that you’ve read or heard somewhere before that water is the best coolant.
This is true as far as being able to absorb heat for a given flow rate, water does do that the best. Water also boils at a lower temperature than other coolants and can develop steam pockets easier, so it is not the best coolant in that respect. A water / ethylene glycol mixture will boil at a higher temp and resist steam pockets better than plain water, the down fall is that it has to have a higher flow rate, but that is easy to accomplish.
The 3rd common form of coolant is propylene glycol, which has the highest boiling point and can run higher than 250° F (average temperature as seen on a gauge) without forming steam pockets, but it must flow at more than twice the speed of a water / ethylene glycol mixture (which means major changes to most cooling systems).
The pressure in the block is higher than the radiator pressure; this is because the pump is building pressure due to the thermostat being a restriction. This pressure raises the boiling point of the coolant and reduces the chance of steam pockets, so never run with out a thermostat (or some form of restriction). The radiator cap will usually hold 15-18 psi, if the radiator holds the system at 15 psi, the boiling point of plain water will be raised to 250° F. The water pump can then make an additional 40-45 psi in the engine and bring that boiling point close to
300° F. So as you can see, pressure is important.
Stock Cooling Systems
Most stock cooling systems pull coolant from the radiator and push it through the each bank of the block; it then goes up through holes in the head gasket(s) to the heads and out the front of the heads to a common exit point. This ok for a stock engine that has no problems with detonation, but the cooling is very uneven. The front cylinders will run coolest and the front combustion chambers will run the hottest. Most stock pumps will also favor one bank. The stock pump used on a small-block Chevy for instance will always favor the passenger side bank. This means that cylinder 2, 4, 6 & 8 get more flow, so the 1, 3, 5, & 7 bank runs hotter. With the center exhaust ports right next to each other, you can see that combustion chambers 3 and 5 will run the hottest; it is in these two cylinders that detonation will usually first start. It seems a little backward to start the coolant at the block instead of the heads; it would make more sense to bring the coolest coolant to the hottest parts first. This type of reverse flow system has been tried with much success, but it is harder to get it working properly and not worth it for car companies to research when the stock system worked good enough on a stock engine.
Mechanical Water Pumps
As I said before, stock pumps rarely flow evenly between banks. On the small-block Chevy you can restrict 1/2 of the block inlet to the even cylinder bank to get more even flow, but the better solution is to use an aftermarket high volume pump that has worked out such problems.
Stock pumps have a stamped steel impeller and tend to cavitate easily when turned more than 6000 rpm, so overdriving the stock pump offers little to no advantages and can actually aggravate any cooling problems. Most aftermarket pumps will use a cast iron or an aluminum impeller that better resists cavitation. Weiand, Howard Stewart and Milidon make very good water pumps for most popular applications, which improve flow, resist cavitation better, and require less power to drive than stock pumps.
Electric Water Pumps
Many aftermarket companies offer electric water pumps. Many of these pumps do not flow well or build sufficient pressure in the block. They are only good for limited drag racing use, and when used they need a high pressure cap to help prevent steam pockets. If you are considering an electric pump, don’t settle for anything that flows less than 35-40 gallons per hour and that may not be enough. Many of these pumps flow less than 20gph and cannot keep up with the demands of street driving. Even a stock mechanical pump has less than 10hp parasitic power loss, so the advantages outweigh the disadvantages of an electric pump. Better aftermarket designs only take 5-7hp at ~6000 rpm, so there is not much to be gained by switching to an electric pump.
You’ll pay over $300 for a decent electric pump that is truly streetable, so you need to decide
if a 10hp gain is worth the cost. Some pumps claim a 15-20hp increase by eliminating parasitic power loss. A 20hp increase is very unlikely unless your stock pump had bad bearings or the impeller was eroded to the point that it was causing detonation problems forcing you to retard time a few degrees. If you are replacing a “good” stock pump, you very rarely see more than a 10 hp increase, probably closer to 5-7hp. Saying that they eliminate parasitic power loss is untrue in a street car that uses an alternator.
The power has to come from somewhere and that somewhere is the alternator. In reality, electric pumps have more parasitic power loss for a given flow rate and I will explain why. Mechanical energy drives the pump; this can be taken right off the crank with a belt as is done with a mechanical pump. In the case of an electric pump, the power comes from the alternator, which is driven from the crank. Rather than the crank directly driving the pump, the mechanical energy is changed into electrical energy by the alternator, then the electric motor on the pump changes it back into mechanical energy to turn the pump. Since electric motors and alternator are not 100% efficient, power is lost in this process. So why do we see a little more power when swit ching to an electric pump when they are less efficient? The answer is simple; they do not pump as much. The average electric pump flows under 35gph, where the average mechanical pump flows twice as much.
If an electric pump flows enough for the application, it can be an advantage. The biggest advantage of an electric pump is that it can be left 4 running with the engine off to cool it off better between rounds. If you have a really good electric pump that can out flow the stock pump at low engine speeds, then you’ll have a big advantage in stop and go traffic. I have yet to see any electric pump out flow a stock pump at higher rpm. If you plan doing any road racing, where the rpm stays up for extended periods, a mechanical pump is the only choice. Drag racing is a different animal altogether. You may only see 9 seconds of wide open throttle, heat may build quickly, but an electric pump and fans can run between rounds to cool everything down. Remember, not everything you see on race cars means it’s better for your street car. Coolant Flow Different coolants require different minimum flow rates, but contrary to popular belief, you cannot make the coolant flow too fast. This rumor was started because people removed the thermostat to gain flow, because they had an over heating problem, and it only aggravated the problem. The real reason they ran into problems is that removing the thermostat also removes the restriction that builds pressure in the engine, so they gained flow, but reduced the boiling point of the coolant in the block. Running a higher flow thermostat and a higher volume pump to maintain pressure, will give no such problems. If you think about it, making the coolant flow twice as fast will also make it flow though the engine twice as often, so there will be more even temperature across the engine. There has been, and still is, the rumor that of the coolant flows too fast, it will not have time to pick up heat. That is nonsense, as long as there is coolant contact a surface, the rate of heat transfer will be the same. Coolant that flows twice as fast also flows through the block twice as often.
Basic Flow Modifications
Most stock systems on a V type engine will have a common outlet for both banks. The outlets of each bank flows directly at each other than must take a 90° turn to return to the radiator. If one side gets hotter (which is sure to happen) the pressure of that side will increase. The increased pressure will increase flow in the hotter bank and decrease flow in the cooler one. The faster moving coolant will cool the hot bank better and the slower moving coolant picks up more heat in the colder side. As you can see, the hot side is getting cooled and the cooler side is heating up. This happens until the banks reverse; the side that was cooler is now hotter and has more pressure. The cyclic flow will continue until the engine is shut off. Smokey Yunick was the first to do studies on the cyclic flow and traced the problem to the outlet. By tapping the front of the heads, and bringing the coolant together in a Y eliminated the cycling.
To truly equalize temperatures throughout the engine is not possible with today’s technology, but we can improve the situation some. To get the best results you must start fresh and build totally custom cooling system. The first step is to tap off the pump and put coolant to the back of the block so the coolant enters at both ends. This helps equalize the cylinder temperatures, but the heads will still be hotter toward the front.
To equalize the head temperatures you must tap outlets at the back of the heads so that all the coolant does not have to pass the front combustion chambers. To further equalize, you can tap inlets and outlets in the center of the block and heads also. At that point the coolant will be flowing basically from bottom to top and is about the best you will get without reversing the flow. Reverse Flow Systems As I said earlier, it makes sense to put the coolest coolant to the hottest parts first to bring the temperatures down as much as possible, the already heated coolant can help bring the temperatures of the coolest parts higher and make everything more even. To do this the coolant must flow in reverse (compared to most systems). The problem with reverse flow systems is that the pump tends to cavitate easier (even with a good aftermarket pump). To limit cavitation, a higher boiling point of the coolant helps and so does a higher system pressure.
Stock Clutch Fans
The stock clutch fan is often tossed in favor of an electric fan. In this can go two ways, often the wrong way. There are a lot of cheap electric fans on the market that are junk. A factory clutch fan and shroud flows more air than most electric fans. I have seen many people go out and buy electric fans because they had over heating problems. Only to find out that the problem is worse. The real issue is probably an inadequate cooling system or a poor choice of electric fan. If the problem is a clogged or too small radiator, buying a new fan is not a solution. I have no problem with using electric fans, but they are not all created equal. If your cooling system was borderline adequate before, an crappy electric fan will make it worse. If you are having an over heating problem, don’t even consider an electric fan until you find the cause of the problem. If the fan is the problem, make sure the electric fan you choose can outflow the original. For every quality electric fan on the market, you’ll find about 10 peaces of just that make the same claims.
In our opinion of flex fans is, they are next to worthless. They can be better than a solid (no clutch) stock fan, and that is the only good thing I have to say about them. They are noisy, and offer little to no benefit over the factory clutch fan. They claim to move a lot of air at low-speeds and flatten out at high speeds to cause little drag. The air hitting the blades is what flattens them out and that takes power to do, so they must have some drag. Maybe not a lot, but certainly more than a clutch fan that is near freewheeling. I personally just do not like them. Flex fans are popular in certain race classes that require an engine driven fan due to the fact that they are light and can take very high rpm. They were popular for a while on the street, probably because they are so cheap and people always insist on buying the “race” parts for a street car. It is very important to do research on these kinds of parts. Race cars do not always use parts because they are the best parts; they are usually the best for what is allowed by the rules in that class. Some of the newer designs of flex fans have the blades curved toward the rotation, this looks like a better design because centrifugal force will assist in flattening out the blades and they should reduce drag over other types of flex fans. I have not had any experience with them, so I can’t say for sure, but they appear to be a better design. Actual testing will tell for sure, but I have not personally tried one. Electric Fans Electric fans can offer many advantages.
They are compact, which can really helps when there are space limitations. They are reliable and simple, so it can make for a clean neat installation. There are some good fans out there that will outflow most belt driven fans, but they are not cheap. Although considering the money you have in your engine, pay 2-3 times as much for a good fan is cheap insurance.
Another benefit of electric fans is the ability to control them however you want. My ECU for the injection system controls my fans and I can over ride that with a switch in the car to keep the fans on or shut them off if I want. Many aftermarket companies also make thermal switches to control fans. When you add and electric fan, there is always the option of pushing or pulling air through the radiator. So which is best? For the most part, the pulling air through the radiator works better. It is not a question of the fan being more efficient as a puller, if the fan was totally sealed to the radiator so there was no leakage, the pusher would be the ticket, but even with a shroud, there is some leakage. A fan does not just flow air through itself straight. A fan spins and causes the air to spin as well. Centrifugal force throws air outward all along the fan as well, but the intake side of the fan is pretty much limited to the area of the fan. When the fan is
in front of the radiator, a lot of air goes thrown out and never makes it through the radiator at all. So when you compare total air moved, with a pusher, less makes it through the radiator than the same fan as a puller. A shroud really helps with a pusher, so I recommend a shroud on all pusher fans. Curving the blades toward the direction of rotation like the new flex fan designs might help electric fans as pushers. The curved blades could cup the air and limit the amount thrown outward by centrifugal force. This is just a theory though, some experimenting would tell for sure. One more downfall for the pusher is that it’s right in the way of the incoming air, blocking the path to the radiator. In general, a pusher is only about 80% as efficient as the same fan as a puller.
To keep is simple; a shroud should be used with any fan to get the most out of it. Any cooling system will benefit a properly designed shroud to pull air through the radiator more evenly at low speeds. A shroud could restrict airflow at high speeds, a simple and effective solution is to cut holes in the shroud and cover them with rubber flaps. At high speeds, air can push the flaps open for additional airflow. Picture a fan with no shroud, there are large portions of the radiator that do not get covered by the fan. This means that if the fan is the only thing moving the air (low vehicle speeds), the radiator is only partially getting used. The areas of the radiator where no air is getting pulled through dissipate very little heat.
Source: www.grapeaperacing.com Copper Brass vs. Aluminum Everyone always want’s to know which is better. Well, that is not an easy question. If both were made from the same thickness material and has the exact same design, the answer would clearly be copper/brass simply because both copper and brass conduct eat faster than aluminum.
A copper brass construction has its downfalls. The tubes, fins tanks are soldered in place. If the solder used cannot conduct head as well as the copper tubes and fins, it will hurt efficiency. Another thing that hurts efficiency is paint. Too many people use what ever black paint they have laying around and never think about how well that paint conducts heat. Only use radiator paint to paint a radiator. Aluminum radiators are usually left bare aluminum. There are companies out there making quality copper brass radiators that are silver soldered together that will cool just as good as or better than many equally sized aluminum radiators. Aluminum radiators also have some advantages. They are lighter, a typical aluminum radiator can weight 10-15 lbs less than a copper brass one the same size and thickness. This is not as much as you would think because aluminum radiators are made from thicker material due to the fact that thicker metal can dissipate heat faster. Aluminum does not conduct heat as fast as copper, so thicker material is used to improve heat dissipation. You will see many aluminum radiator companies off two row radiators, usually 1” or 1 ¼” wide rows. An average aluminum core with two 1” rows is about as wide as a for row copper brass core. The advantage is that there is more fin to tube contact to help transfer heat. As you can see, there is a lot more to choosing a radiator than the material that it’s made from. My personal preference is an aluminum radiator, providing is it a quality piece. They are better looking, lighter (even a small weight loss is still a weight loss), and they work just fine from mild to wild engines. I have used Griffin and Be Cool radiators with great success in some pretty radical street cars.
Cross Flow vs. Down Flow
The main advantage toward one or the other is packaging. Vertical cores place the tanks on the top and bottom, making the radiator taller. This works fine for many older cars with narrow tall grills. Most modern cars have more room from side to said, making cross flow radiators more practical. One positive aspect of the cross flow design is that the radiator cap is on the lower pressure side of the radiator. There should not be much pressure drop across the radiator unless it’s restrictive, however, so this is not a huge advantage. In most properly set up cooling systems, the thermostat is the highest restriction point, which causes higher pressure in the engine, than the radiator.
Single vs. Dual Pass There is always the question of which is better. My opinion is to go with what fits best. A dual pass will have the inlet and outlet on the same side, where a single pass will have one on the bottom and one on the top on the opposite side.
In theory, a single pass is better is more efficient, but not by much. A single pass radiator allows all cooling tubes to get the hottest water all at the same time. The downfall is that velocity is cut in ½ compared to a dual pass, which hurts turbulence and heat transfer rate. A dual pass keeps velocity up, but only ½ of the radiator get the hottest coolant, as the coolant passes through the radiator the second time, it is not as hot and transfers heat at a slower rate. The bottom line here is that there is an up and down side to each. The small overall advantage goes to the single pass fro most applications, but there are applications where a dual pass can be better. Either way, the advantage either way is only a few percent, so use what fits the best. Most of the time, a single pass will fit the best because of the factory inlet and outlet positions.