About 15 minutes after the very first engine was test run, it was determined that something was needed to take the waste heat away and keep the engine from turning into a puddle of melted steel. Thus the cooling system was born. Actually, I made this up. I really don't know who came up with the first cooling system design or how they came to decide it was needed. Suffice it to say from the early beginnings it was clear that cooling the engine was as important as keeping the engine running.

The only purpose of the engine's cooling system is to remove excess heat from the engine, to keep the engine operating temperature at its most efficient level, and to get the engine up to the correct temperature as soon as possible after starting. A cooling system is also required to prevent the internal engine parts from melting from the heat of the burning fuel. The pistons would expand so much they could not move in the cylinders resulting in a seized engine.

When fuel is burned inside the engine, only about one-third of the total energy created is converted into power. One third of the energy is lost through the exhaust pipe, and another third is turned into heat, leaving only one third in the form of power that can be used. Heat is the real culprit, because burning fuel produces up to a staggering 4,500° F of heat. Fortunately the cooling system removes about a third of the heat that is produced in the combustion chamber. The exhaust system removes the lion's share of the heat, however the internal parts of the engine, such as the cylinder walls, pistons, and cylinder head, absorb large amounts of heat. If these parts of the engine get too hot, no oil in the world will protect it.

On the other side of the coin, if an engine runs at too low a temperature, it is wasteful, the oil gets contaminated, sludge forms, and fuel mileage decreases. It will also raise the emission levels above specified limits.

There are two types of cooling systems; air cooled and liquid cooled. Most automotive engines today are liquid cooled. Air-cooled engines are more typically used in motorcycles, airplanes and lawnmowers.

The main components of the modern cooling system are:

  1. Radiator

  2. Coolant

  3. Water Pump

  4. Freeze Plugs

  5. Heater Core

  6. Thermostat

  7. Hoses

  8. Cooling Fans

  9. Expansion (Overflow) Tank

  10. Radiator Cap (Pressure Cap)

Cars have to operate in a variety of conditions and temperatures, from over 100° F to minus 40° F. The coolant has to be able to withstand these temperatures and 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 efficient coolants for holding heat but it will freeze at 32° F, to high a freeze point for cars operating in cold climates. To lower the freezing point, we use a mixture of ethylene glycol and water, which we refer to as Anti-Freeze. By adding ethylene glycol to water, the boiling and freezing points are improved significantly.

Even with the use of ethylene glycol the temperature of the coolant can sometimes reach 250° to 275° F and we must do something else to raise the boiling point of the coolant. We do this by adding pressure to the cooling system.

Just as a pressure cooker raises the boiling point of water, pressure raises the boiling point of the coolant increases as you add more pressure. Many engines use a pressure cap of 14 to 15 psi, which raises the boiling point of the coolant about an additional 50°. This enables the coolant to withstand the high temperatures it has to deal with. Below is a simple chart to determine the proper mix of ethylene glycol and water to achieve the proper freeze/boiling protection for the conditions your vehicle operates in.

 

Pure Water

50/50 Anti-Freeze/Water

70/30 Anti-Freeze/Water

Freezing Point

0° C
32° F

-37° C
-35° F

-55° C
-67° F

Boiling Point

100° C
212° F

106° C
223° F

113° C
235° F

The cooling system in your car needs a lot of plumbing to get the coolant to where it has to go. The hoses, lines and water jackets bring the coolant to where it will pick up engine heat and to where it gets rid of it. Okay, where shall we start explaining how the coolant flows? Why not start at the water pump?

Water Pump:

 

Water pumps come in many designs, but all work the same. They use a rotating impeller to force the coolant through the engine block. Impeller type water pumps must turn rapidly to be efficient, and worn or loose drive belts can permit slippage that is not easily detected. The water pump uses centrifugal force to send coolant to the outside while it spins, causing coolant to be drawn from the center continuously. The inlet to the pump is located near the center so that coolant returning from the radiator hits the pump vanes. The pump vanes fling the coolant to the outside of the pump, where it can enter the engine. The water pump forces the coolant into the engine block, where it makes its way into the water jacket in the engine and around the cylinders. The coolant then goes up into and through the cylinder head where it exits the engine.

The thermostat is located where the coolant leaves the engine. There is a bypass that goes around the thermostat and sends the coolant back to the water pump if the thermostat is closed. If the thermostat is open, the coolant goes past the thermostat and through the radiator and then back to the pump. Also before the thermostat is a separate circuit for the heating system. These circuits take coolant from the cylinder head and passes it through a heater core and then back to the pump.

Engine:

Inside the cylinder head and engine block there are passages that are either cast or machined in to allow the coolant to be directed to the areas that need to be cooled. The engine block is actually manufactured in one piece with the water jackets cast into the block. Since combustion temperatures can reach up to 4,500 ° F, cooling the cylinders is very important, as is cooling the areas around the exhaust valves. The area around the exhaust valves is especially critical. Just about all the space inside the cylinder head that is not used for structural integrity is machined out for cooling system use. On the sides of the engine are "Freeze" or "expansion" plugs, which are sheet metal plugs pressed into a series of holes in the block. These are designed to hold the pressure of the cooling system, but to pop out if the coolant in the block ever freezes.

The engine cannot go very long without being cooled. If the cooling system fails for any reason, the engine will soon reach a temperature where the pistons will actually weld themselves to the cylinder wall. Once this happens, you throw the engine out and replace it with a new one.

The Thermostat:

Just as an athlete needs to warm up when they begin to exercise, your car's engine needs to warm up when it is first started. The thermostat is almost always placed between the engine and the radiator as the coolant flows. 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. The thermostat controls your engine's warm-up period by recirculating the coolant until it reaches the proper temperature. When the correct operating temperature is reached, the thermostat opens and allows the coolant to go to the radiator.

The thermostat is a temperature sensitive spring valve that remains closed while the engine warms up. The normal operating temperature for most engines is between 180° F and 200° F. When the right temperature is reached, the spring valve opens, allowing coolant to circulate through the radiator to be cooled.

The center of the thermostat has a 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 (in a 180° F thermostat which is the most common). A rod connected to the valve presses into this wax and as the wax melts; it expands, pushing the rod out of the cylinder and opening the valve.

The best way to test a thermostat is to place it in a pot of boiling water on the stove. As it heats up to its rated temperature, its valve should open about an inch. If it doesn't, then throw it out and get a new one.

The temperature at which the thermostat is designed to open is called its rating and is usually stamped on the body. The 180° F thermostat begins to open at 180° F and is fully open at 200° F. Different engines use different temperature calibrated thermostats. In addition to maintaining the proper operating temperature, the thermostat also controls the speed of the coolant through the engine. If the coolant moves too fast, it will not pick up as much heat as if it were moving more slowly.

Radiator:

Basically a radiator is a heat exchanger. It transfers heat from the hot coolant flowing through its core to the air blown through it by the fan. Most cars today use radiators made of aluminum. Radiators are constructed by brazing thin aluminum fins to flattened aluminum tubes. The coolant flows from the inlet, usually located at the top, through these tubes to the outlet at the bottom. These tubes are usually mounted parallel to each other in a cross flow arrangement, which means the coolant flows from one side to the other. Some radiators have the flow going from top to bottom but are now not as common anymore. The aluminum fins conduct the heat from the tubes and transfer it to the air flowing through the radiator.

Some radiators have what is called a turbulator inserted into the tubes that increases the turbulence of the coolant moving through them. If the coolant flows too easily through the tubes, only the coolant actually touching the tubes would be cooled directly. Since the cooling ability of the radiator depends on the temperature difference between the coolant and the tubes, the turbulence inside the tube mixes all of the coolant so more of its heat is transferred to the tubes. In this way all of the coolant is used in the most effective manner possible.

Radiators have two tanks, either on each side or top and bottom. In vehicles with an automatic transmission one tank will contain another tank to cool the transmission fluid. In essence, this transmission cooler is like a radiator within a radiator. The only difference is instead of exchanging heat with the air, the transmission fluid exchanges heat with the coolant in the radiator.

 

"In order to increase your cooling system's performance you must maximize both the WATER flow and AIR flow."

Water Pumps

There are significant differences between a stock or OEM water pump, and Stewart Components water pumps. Most stock or OEM pumps are built to meet standard performance requirements at relatively low RPM. Stewart pumps are designed and manufactured specifically for high performance applications.

Every pump is designed to exacting tolerances for reliable, long-term performance that meets the requirements for your application.

 

 

In addition, all Stewart high-flow water pumps are designed to deliver maximum flow with minimum power consumption. Stewart high-flow water pumps deliver up to 180 GPM (gallons per minute) of coolant flow (at 8,000 RPM), yet consume just 2.26 horsepower (at 4,000 rpm)!

Pulleys

Using the proper pulleys and drive system is critical to matching the water pump's performance to your specific application. RACE applications require a maximum water pump speed of 6,000-7,000 RPM. For STREET applications, the water pump speed must at least match crankshaft RPM, to a maximum recommended 35% faster than crankshaft speed

Stewart Components does NOT recommend the use of underdrive pulleys on any application. Stewart high-flow water pumps only consume 2.26 horsepower at 4,000 RPM, so the actual savings in parasitic horsepower loss through the use of underdrive pulleys is minimal. In addition, a properly designed cooling system's flow and efficiency are designed to operate at a given speed. In years of testing, Stewart has consistently proven that the engine will lose more horsepower due to higher operating temperatures than any possible gain from underdrive pulleys.

 

Radiator Caps

In a cooling system, a higher pressure equates to a higher boiling point for the coolant. Higher coolant pressures also transfer heat from the cylinder heads more efficiently. We recommend using a radiator cap with the highest pressure rating that the radiator is designed to accept. In general, performance radiators will accept 22-24 PSI, and professional racing radiators will accept a 29-31 PSI.

The coolant will typically only build to 16-18 PSI, due to expansion up to 200°F. However, if the engine does overheat due to external factors, the pressure inside the cooling system could reach as high as 28 PSI. Once the radiator cap has opened and vented coolant, the engine will not cool down until it has been turned off. The radiator cap is basically a "safety valve", so always use the highest pressure radiator cap that the radiator will tolerate. If you are unsure of the pressure rating for your radiator, check with the manufacturer for the maximum recommended operating pressure.

Radiator Cap Location

The radiator cap should always be located at the highest point of the cooling system, and on the low pressure side (after the radiator core).Cross flow radiators mounted higher than the engine are ideal because the cap is on the tank that is connected to the water pump inlet. This configuration offers 3 advantages:

  1. The cap is at the highest point of the system, allowing any air to migrate to the area just below the cap. In the event the cap vents due to excessive pressure, the air will escape first

  2. This area has the lowest velocity within the system, allowing air to separate from coolant even at high engine RPM

  3. The cap is located on the low pressure (suction) side of the system, so it is unaffected by the pressure generated by the water pump.

For cooling systems NOT using a cross flow radiator, mounted higher than the engine, you must use a surge tank. A surge tank is typically a 1 quart tank mounted at the highest point of the system, with the radiator cap on top. The bottom of the tank is connected to the inlet side of the water pump with a 1/2" or 3/4" line. A 1/4" to 3/8" "bleed" line from the side of the surge tank is connected to the highest point of the low pressure side of the radiator. The bleed line allows some circulation through the tank while the engine is running. The surge tank is also large enough to allow the air to separate as the coolant flows through it. Air in the system will then migrate to the area just below the radiator cap, again so that it will forced out first if system pressure exceeds the radiator cap's rating.

In street car applications, an upright radiator (top and bottom tanks, with the cap on the top tank) represents a compromise that will work, as long as the car is not operated at sustained high RPM, like those seen in racing.

Any aftermarket thermostat housing that mounts the radiator cap directly above the thermostat location, or that mount the radiator cap in the top coolant hose, are NOT recommended. Both of those housing styles are poorly designed, and will push coolant out of the cap at high RPM.

Thermostats and Restrictors

We strongly recommend NEVER using a restrictor: they decrease coolant flow and ultimately inhibit cooling.

For applications requiring a thermostat to keep the engine at operating temperature, we recommend using a Stewart/Robertshaw high flow thermostat. This thermostat does not restrict flow when open. The Stewart/ Robertshaw thermostat enhances the performance of the cooling system, using any style of water pump. However, the Stewart Stage 1 high-flow water pump may require this thermostat to operate properly, and Stewart Stage 2, 3, and 4 water pumps simply will NOT operate with a regular thermostat because these pumps have no internal bypasses.

 

Stewart further modifies its thermostat by machining three 3/16" bypass holes directly in the poppet valve, which allows some coolant to bypass the thermostat even when closed. This modification does result in the engine taking slightly longer to reach operating temperature in cold weather, but it allows the thermostat to function properly when using a high flow water pump at high engine RPM.

A common misconception is that if coolant flows too quickly through the system, that it will not have time to cool properly. However the cooling system is a closed loop, so if you are keeping the coolant in the radiator longer to allow it to cool, you are also allowing it to stay in the engine longer, which increases coolant temperatures. Coolant in the engine will actually boil away from critical heat areas within the cooling system if not forced through the cooling system at a sufficiently high velocity. This situation is a common cause of so-called "hot spots", which can lead to failures.

Years ago, cars used low pressure radiator caps with upright-style radiators. At high RPM, the water pump pressure would overcome the radiator cap's rating and force coolant out, resulting in an overheated engine. Many enthusiasts mistakenly believed that these situations were caused because the coolant was flowing through the radiator so quickly, that it did not have time to cool. Using restrictors or slowing water pump speed prevented the coolant from being forced out, and allowed the engine to run cooler. However, cars built in the past thirty years have used cross flow radiators that position the radiator cap on the low pressure (suction) side of the system. This type of system does not subject the radiator cap to pressure from the water pump, so it benefits from maximizing coolant flow, not restricting it.

Coolant, Hoses, Radiators and Fan Tech

UNEQUIVOCALLY WATER IS THE BEST COOLANT! We recommend using a corrosion inhibitor comparable to Prestone Super Anti-Rust when using pure water. If freezing is a concern, use the minimum amount of antifreeze required for your climate. Stewart Components has extensively tested all of the popular "magic" cooling system additives, and found that none work better than water. In fact, some additives have been found to swell the water pumps seals and contribute to pump failures.

In static cooling situations, such as quenching metal during heat treating, softening agents (sometimes referred to as water wetting agents) will allow the water to cool the quenched part more evenly and quickly. The part will cool quicker, and the water will heat up faster. However, an automotive cooling system is not static. In fact, the velocities inside a cooling system are comparable to a fire hose forcing coolant against the walls of the engine's water jackets. If the softening agents actually aided in cooling the engine, the temperature of the coolant as it exited the engine would have to be higher because it would have absorbed more heat.

 

Fans

Electric fans have improved tremendously in recent years, in both quality and reliability. Electric fans now outperform mechanical fans in nearly every application, except towing and dirt oval track racing.

When using a mechanical fan, a properly designed shroud must be used. Most mechanical fans are not designed for high RPM use: they can have serious vibrations problems, due to air turbulence, when run over 6,500 RPM. This is a turbulence problem, not a balance problem, and will destroy the water pump and components in front of it. The large fans preferred by dirt oval track racers can consume up to 18 horsepower at 6,500 RPM. Do NOT run a mechanical fan that is any larger than required for the application.

Flex fans are a poor design for performance applications. They move less air at higher RPM, and only consume a fraction less power than standard fixed pitch fans.

Clutch-style fans are inconsistent and we do not recommend their use for any application, if possible.

Hoses

Standard full-size hoses should be used to ensure maximum flow. Smaller "AN style" hoses decrease flow and hence inhibit proper cooling.

Radiators

Aluminum radiators are strongly recommended. They dissipate heat more efficiently than traditional copper-brass radiators for two primary reasons:

Copper-brass radiators must be soldered together. Solder is a very poor thermal conductor and inhibits the ability of the fins to pull heat out of the tubes. Although aluminum does not dissipate heat as well as copper-brass.

Modern radiator designs incorporate wider tubes with smaller cross sections. This design allows for more contact area per cubic inch of coolant, and allows the radiator to cool substantially better than older designs using narrow tubes with larger cross sections.

We recommend using the largest radiator (as measured in square inch area) that will fit in your vehicle. Thicker radiators (cores) are better than thin radiators, but thickness is no substitute for surface area. The following comparison is commonly accepted: if the surface area is doubled, the potential thermal dissipation is doubled. If the thickness is doubled, heat dissipation increases approximately 25%. This difference occurs because with a thick radiator, the air does not get "heat saturated" before it reaches the backside of the radiator.

Radiators and External Plumbing

Thicker radiators do have slightly more airflow resistance than thinner radiators but the difference is minimal. a 4" radiator has only approximately 10% more airflow resistance than a 2" radiator.

In past years, hot rodders and racers would sometimes install a thicker radiator and actually notice decreased cooling. They erroneously came to the conclusion that the air could not flow adequately through the thick radiator, and therefore became fully heat-saturated before exiting the rear of the radiator core. The actual explanation for the decreased cooling was not the air flow, but the coolant flow. The older radiators used the narrow tube design with larger cross section. Coolant must flow through a radiator tube at a velocity adequate to create turbulence.

The turbulence allows the water in the center of the tube to be forced against the outside of the tube, which allows for better thermal transfer between the coolant and the tube surface. The coolant velocity actually decreases, and subsequently its ability to create the required turbulence, in direct relation to the increase in thickness. If the thickness of the core is doubled, the coolant velocity is halved. Modern radiators, using wide tubes and less cross section area, require less velocity to achieve optimum thermal transfer. The older radiators benefited from baffling inside the tanks and forcing the coolant through a serpentine configuration. This increased velocity and thus the required turbulence was restored.

Radiators with a higher number of fins will cool better than a comparable radiator with less fins, assuming it is clean. However, a higher fin count is very difficult to keep clean. Determining the best compromise depends on the actual conditions of operation.

Double pass radiators require 16x more pressure to flow the same volume of coolant through them, as compared to a single pass radiator. Triple pass radiators require 64x more pressure to maintain the same volume. Automotive water pumps are a centrifugal design, not positive displacement, so with a double pass radiator, the pressure is doubled and flow is reduced by approximately 33%. Modern radiator designs, using wide/thin cross sections tubes, seldom benefit from multiple pass configurations. The decrease in flow caused by multiple passes offsets any benefits of a high-flow water pump.

Gross flow radiators are superior to upright radiators because the radiator cap is positioned on the low pressure (suction) side of the system. This prevents the pressure created by a high-flow water pump from forcing coolant past the radiator cap at high RPM. As mentioned in the radiator cap section, an upright radiator should be equipped with radiator cap with the highest pressure rating recommended by the manufacturer. The system will still force coolant past the cap at sustained high RPM.

External Plumbing

Street-driven vehicles seldom need auxiliary plumbing or coolant lines. SBC race engines with aluminum cylinder heads usually require extensive external plumbing to address two design problems:

Aluminum heads have much smaller water jackets than cast-iron heads because the external dimensions are similar, but the ports are usually larger, the deck is thicker, and the material near the rocker stands is thicker, all leaving less area in the water jackets. This decreased internal area leaves less area in the water jackets.

The siamese center exhaust ports are a design compromise that presents additional problems when aluminum heads are used. The area near the center exhaust valves is thicker, thus allowing providing less surface area for cooling.

We recommend installing a pair of –10 AN lines that connect the rear of the aluminum cylinder heads to the thermostat housing crossover in the front. This step will help offset the smaller water jackets. A pair of -10AN lines connecting the pressure side of the water pump with the area in the center of the cylinder head (just below the exhaust ports) will offset the lack of surface area due to the extra material.

Dirt Oval

Gasoline-powered dirt oval track cars require more water pump and fan speed than a comparable pavement car. This is due to the lower engine speeds in the corners, and the increased possibility of clogged grille screens. A mechanical fan with a proper shroud is a necessity for these applications.

Street Rod

Nearly all street rods are equipped with an upright radiator that positions the radiator cap on the pressure side of the system, thus subjecting the cap to the pressure generated by the water pump. The highest pressure rating cap recommended by the manufacturer should be used. Such applications will still force coolant past the radiator cap when operated at sustained high RPM.

Pavement/Oval Racing

Blocking the grille openings reduces aerodynamic drag and also increases downforce. This has become "standard procedure" for qualifying. However, the engine may be damaged by overheating and subsequent heat-soaking after the engine is turned off.

Advanced Cooling System Basics

Cooling System Basics for Spark Ignition Engines
SuperFlow Advanced Engine Technology Conference December 7, 1992

"If people suffering from heat exhaustion were revived in the same manner most people cool engines, they would be placed in a cold shower set at a trickle. Then the temperature of the water going down the drain would be monitored."

Common Misconceptions

Energy Loss

Following is a typical engine:

Inlet temperature = 180 F
Outlet temperature = 190 F
Coolant flow = 100 GPM
Specific heat of coolant = 1.0
1 HP = 5.2769885 GPM 1 F
{ (Outlet-Inlet)CS} / 5.2769885 = HP loss
{(190-180) 100*1.0} / 5.2769885 = 189.5 H

Basic Functions of the Cooling System

Engines

 



Radiators

Hoses

Large diameter hoses with large radius bends should be used. Never use braided hoses, they will always result in higher metal temperatures.


 

Pressure

The vapor point of water increases under pressure as follows:

Always use the highest pressure cap available. It merely serves as safety valve
that has no function when the system is operating properly.

Coolant

Grill Opening

Pumps

 

In addition to having the proper flow restriction as expressed in GPM @ PSI the cooling system pressure and temperature must be known. All these conditions are duplicated for the bench test.

Energy losses due to driving the coolant pump can only be calculated when all conditions are duplicated and torque and RPM measured. Amp draw of the drive motor is not an accurate measure of the energy required to drive the pump. Torque must be measured with a load cell and horsepower calculated from there.

Most pumps are biased to the inlet side. Most even spaced cylinder heads (IE, IE, IE, IE) are biased to the exhaust end.

There are three basic impeller designs: universal, clockwise, and counter clockwise. The directional specific impellers are more efficient that the universal impellers. The performance of all designs are very similar when installed in the same housing.

Metal temperatures always increase when you slow the water pump down.