jeff gordon tape on grill

Adding tape to the front of the grill helps race cars turn, accelerate and brake more effectively. In turn, the reduced cooling must be overtaken by the cooling system. The new racing radiators can handle extremely high pressures and temperatures.


New radiators, especially those of the double pass design, have been designed to increase the internal pressure of race car cooling systems. Companies like C&R Racing and Griffin Radiators have been involved in racing throughout the world with some of the best race teams in all forms of racing. There, the companies learned the importance of the pressure that is present in the cooling system to aid in gathering heat (calories) from the engine, particularly the cylinder heads above the combustion chambers.




Radiators come in a multitude of dimensions, quantity of rows and configurations such as double or single pass, thickness, and location of the filler neck if one is used on the radiator.

 cross flow and downflow radiators

A radiator with its core running vertically is called a downflow radiator. This design is the most ubiquitous among OEM and racing radiators. In this type of radiator, the water runs from top to bottom of the cooler. A radiator with tubes running horizontally is known as a crossflow radiator. As the name indicates, the water flows horizontally from one side of the radiator to the other. Neither of those designs proves to hold a distinct advantage over the other. However, a particular design may be more convenient than the other. The crossflow design can allow the radiator cap to be positioned on the low-pressure (suction) side of the system, which prevents the pressure created by a high-flow water pump from forcing coolant past the radiator cap at high rpm.



Coolant recovery tankOpen radiators with a cap need a recovery tank to receive the excess coolant that has passed through the cap. The radiator first expels the air at the top of the radiator and then, when the water cools, the negative pressure in the radiator aspirates some water to fill the void the air had occupied above the radiator.



AFCO aluminum recovery tank

Closed radiators require a pressurized recovery tank that includes a filler cap.  Recovery tanks allow for easy and fast burping of the radiator. These tanks must be installed above the highest point of the cooling system.The outlet of the bleed must connect to a surge tank.


Radiator Styles

Three styles of radiators are available to racers:

  1. Open radiators with a cap and a recovery tank.
  2. Closed radiators with a surge can and recovery tank.
  3. Pressurized systems that do not include a cap but instead they utilize an accumulator with an air spring and a PRV.
AFCO tall racing coolant recovery tank

Recovery tanks should be as tall as possible to allow all the air in the water to rise to the top of the tank. These tanks recover the hot water that was expelled from the radiator when the water heated up. To ensure that no air will re-enter the cooling system when the water recycles back to the radiator, the manufacturers install the return hose fitting at the bottom of the tank.


The major factors to consider when choosing a racing radiator must include: 

  1. The form of racing: Circle Track, Road Racing, Drag Racing, Off-Road etc.
  2. What is the average speed that the car will attain?
  3. What is the average RPM the engine will turn and for how long?
  4. What is the possibility of the radiator becoming damaged or plugged with dirt and rocks, and tire rubber?
  5. Will the vehicle be in go-and-stop traffic like in Drag Racing or with a dual purpose street-and-race vehicle?

In any of the above situations using something purpose-built resonates. OEM is for OEM cars. Race parts are made for racing.

Truck off road radiatorThis photo depicts the importance of including the radiator in the race vehicle design. Here, the builder fitted a wide radiator to lodge above the bumper for protection and to fit under the hood. The width and height of a radiator do not matter, whereas the thickness and surface area are crucial to proper cooling. Off road racing cars and trucks require purpose-built racing radiators to sustain the rigors of vibrations, rocks and mud.



How do double pass radiators work?

What makes a double pass radiator more efficient?

Many racers and gearheads believe that a double-pass radiator improves cooling performance. This impression emerged from the erroneous notion that passing the coolant twice through the radiator will make the coolant remain in the radiator twice as long, thus improving the cooling performance. In a double pass radiator, however, only half of the coolant passes through the radiator at one time. Twice the time at half the volume takes us back to the same place as with a single pass radiator. So what is the point of a double pass radiator?Drawing of double pass radiator                                                             

A double pass radiator has the intake and outlet on the same side. Installing a baffle in the tank that separates the inlet and the outlet prevents the water from flowing straight through from the inlet to the outlet. Passing the coolant twice through the radiator does not cool better than a single pass because only half the coolant volume passes through.

Cooling Benefits from Double Pass Radiators

Passing the coolant through one-half of the radiator twice doubles the coolant speed and its pressure.  However, since a double pass radiator reduces the volume of water that passes through the core in half, its pressure also doubles. The increase in pressure in the cooling system is where the cooling benefits emerge. In the heads principally, the pressure reduces small air pockets and it reduces the onset of nucleate boiling and preignition. What is more, the pressure forces the coolant deeper into the recesses that the casting process left behind where it can absorb additional calories.

Advantage of Installation of a Double Pass Radiator

One of the reasons double pass radiators are used is that on some vehicles the inlet and outlet need to be on the same side of the radiator to accommodate the plumbing. Placing the inlet and outlet on the same side of the radiator on a single pass radiator would cause the water to stop flowing through the tubes as the water would enter and exit the same side of the radiator. To avoid this situation, manufacturers insert a baffle between the top and lower portions of the radiator. In this case, the coolant flows in one direction, and it returns in the other. Half the water passes twice through the radiator at a time. 



Running the highest pressure the system can handle is the best method of preventing overheating and detonation.  High-pressure systems reduce hot spots, steam pockets and they increase the boiling point of the water compared to lower pressure systems, while they allow using smaller grill openings to help aerodynamics and downforce.  Radiator caps are rated as high as 30 psi. These caps can only be used with quality brazed radiators vs. the OEM ones that are sealed with epoxy.


radiator cap cutaway

These drawings show how radiator caps work in releasing the pressure, and how they allow the water to return to the radiator when it cools down. racing radiator cap

Generally, 25-psi is the maximum pressure quality radiator caps are rated for.  A high-pressure radiator cap, like those produced by AFCO Racing, is well advised with race cooling systems that do not use a thermostat or restrictor. There, if the cap is installed on top of the radiator, the pressure created by the water pump can lift the pressure valve of the cap and create a condition that can easily be mistaken for a blown head gasket.

It is a good idea to check the cap periodically to ensure it is sealing correctly and the rubber seal has not lost its elasticity.  The filler neck’s sealing surfaces must be free of defects, or the closed system could lose water (and pressure) slowly and cause an overheating issue.  This can cause a condition that appears to be a blown head gasket when it is due to pressure loss and the fluid boiling. This will eventually cause the head gasket to blow. So what came first the blown head gasket or the overheating?


Water pressure gauge auto meter appliedspeed.comAuto Meter warning light

A blown hose down the straight away will send you into the wall in a hurry. A water pressure gauge with a light will let you know if you should back off for the next turn. If the system has lost the majority of its water the temp gauge will not read hot but the pressure one will show low pressure.  A typical case of "Trust, but verify".


Radiator caps come in several configurations. Aim for a higher pressure cap for race applications and heavy duty towing, off-roading and heavy equipment. If you need more than 25-lbs go with a pressurized cooling system from Applied and C&R Racing. Inspecting the rubber pressure seal on race radiator caps should be a part of the in-between race maintenance schedule.

C&R High Pressure System

High-pressure systems produced by C&R Racing do not use a pressure type of radiator cap. Instead, the systems are equipped with an adjustable pressure relief valve. These systems can hold up to 60 to 70 PSI of pressure.


Evans drawing of nucleate boiling

This drawing from Evans Waterless Engine Coolant identifies the areas most prone to boiling in the heads. These spots reside above the combustion chamber and the exhaust port. Once the water reaches Critical Heat Flux, the steam creates a barrier between the metal and the coolant. This condition can lead to pre-ignition, detonation, pinging and severe engine damage. 



The secret of the thermostat lies in the small cylinder located on the engine-side of the device. This cylinder is full of wax that begins to melt at around 180 degrees F . A rod connected to the valve presses into this wax. When the wax melts, it expands significantly and pushes the rod out of the cylinder, opening the valve.


The purpose of a thermostat is to bring the water up to normal operating temperature more rapidly. The thermostat is essential in warming up the coolant before driving. Fast warm up in a passenger or performance car will reduce engine wear and fuel consumption as engines use more fuel when cold.


In racing, it is believed by some that a thermostat or a restrictor is necessary to create slow the coolant flow in the heads. The reality is that restrictors and thermostats helped the cooling process because they created pressure in the system. Today, with high-pressure cooling systems there is no longer a need to create pressure mechanically. The flow is only produced by the water pump. The pressure should be created in the radiator by turbulence and with the temperature increase from ambient to racing temperatures. Turbulence will cause the water to lap against all the inside surfaces of the cooling tubes in the radiator. A word of caution: Run a high-pressure radiator cap of up to the max pressure recommended by the manufacturer of the radiator.

 Water passage restrictors

Restrictors have been used extensively in racing as a means to increase the pressure in the engine and prevent the radiator cap from blowing off. The vernacular had it that the water needs to spend more time in the engine to pick up more heat. Race radiator manufacturers agree that water speed through the radiator and the engine is far more productive than slowing it down. Modern radiators are designed to slow down the flow of the water to build pressure.

Restrictors have been used extensively in all forms of racing. Two thoughts dominated the concept:

1-    The thought that if the water flowed too fast it would not have enough time to cool and it would leave heat from the engine behind.

2-     The belief that the restrictor or thermostat would prevent the pressure from the pump to blow off the cap pressure valve.


Racing thermostat by C&R Racing

These thermostats are produced by C&R Racing in Indianapolis, Indiana. Those units are used on all types of race vehicles but mostly where cold weather can prevail such as in Road Rally and Ice Racing.

Well, neither scenario is correct. The fast flow of coolant and air are king in engine cooling. Slow coolant flow will cause the coolant to pass through the system less often, thus picking up fewer calories (heat). Radiator manufacturers agree that neither a thermostat nor a restrictor should restrict the flow of the water- the flow created by the water pump should be restricted by the radiator turbulence in the tubes Thermostats typically open to an area about equal to the hoses in the system, permitting the water to flow somewhat unimpeded. 

In the past, radiator caps were of low-pressure design. At higher engine speeds the water pump pressure would overcome the cap pressure rating, and the water would leak out. For the past 20 to 30 years manufacturers have made radiators with crossflow designs permitting the radiator caps to be placed at the outlet side of the radiator preventing the flow and pressure from blowing off the cap. 

Do not attempt to run higher pressures than your radiator is designed for. Purpose-built radiators and their components contain reinforced tubes, and they are welded with the proper techniques to contain high pressure.

Do not pre-pressurize your cooling system. Instead, allow the system pressure to build on its own from the engine heat. If you do have a C&R system, the instruction video will guide you on how to check for air in the system by installing some pressure.


“Water flow through an engine or radiator can be compared to someone standing in cold weather with no wind blowing, compared to standing in 30 mile-per-hour wind in the same low temperature. The greater the wind/water speed, the greater the heat removal will be.




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