Cooling Systems 101
General Insight
The cooling system has a tendency to be an afterthought that many race teams, privateers, and
owners look at when building a car. Many of these vehicles are extensively modified and
upgraded, from thousands invested in upgrading OE parts or buying a drop in crate engine; the
cooling system protects these investments, keeps them running at peak performance, and most
importantly, and helps you finish the race each time. The next few pages will give you a general
overview of cooling systems, a deeper description of how radiator cores affect cooling, and how
to properly design a radiator for your application.
General Overview
Cooling systems serve one purpose, to keep an engine from exceeding a temperature that will be
detrimental to performance under all operating conditions. With standard operating temperatures
near the boiling point of water, pressurized systems were introduced. Today, nearly all cooling
systems, both OE and race oriented, are pressurized. This pressure raises the boiling point and
creates a factor of safety so hotspots in the head do not begin boiling. If boiling begins,
detonation will likely occur and the problem will only compound until the engine overheats.
Every PSI of pressure in the system increases the boiling point three degrees Fahrenheit at sea
level (the boil point is 212°F at sea level).
Typical Cooling System Configurations
There are three main styles of cooling systems that will be outlined. They all perform similar in
function but there are reasons for utilizing each as well as one being better suited for the
application. The three styles of radiators are an open radiator with a recovery tank, a closed
radiator with a surge can and recovery tank, and finally a pressurized system that utilizes an
accumulator with an air spring and a PRV.
Most OE manufactured cars consist of an open style radiator with a recovery or overflow tank.
We refer to radiators that have a fill neck and radiator cap on them as an open system. For more
information on these systems refer to section “Recovery Systems” on page 7. The other typical
set-up we see on OE and lower level race applications are referred to as closed system. The
radiator cap and fill neck are no longer placed on the radiator (closed system); but instead, are
located on the surge tank. These systems can still utilize a recovery tank to capture excess fluid
when the system is at operating temperature. A closed system with surge tank is more functional
than an open system. For more information on these systems refer to section “Surge Tank” on
page 8.
The final setup, which is becoming more prevalent in race applications, is a pressurized system.
A proper pressurized cooling system utilizes an accumulator which is similar to a surge tank.
This accumulator has a predetermined air space which acts as an air spring. It also incorporates
the PRV (Pressure Relieve Valve) which is adjustable and eliminates the radiator cap from the
system completely. It has a quick disconnect to add pressure over that generated from normal
temperature expansion.
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Cooling Systems 101
The air spring allows the temperature
expansion to compress the air without
lifting the PRV. The water will not
compress. If the system is all water and no
air spring, the PRV will lift during the
temperature expansion cycle. The PRV
setting in relationship to the size of air
spring, water volume, and the amount of
pressure added at ambient will determine
the maximum water temperature possible
without opening the PRV. Some teams
are running as hot as 290°F on a regular
basis without opening the PRV.
Heat Transfer
Heat transfer principles are the inner
workings behind cooling systems.
Components in these systems are always
seeking thermal equilibrium. This is the driving force behind a substance of high temperature
(high energy) transferring heat (energy) to a substance of cooler temperatures (low energy). This
transfer of heat will continue to occur until thermal equilibrium is met. This is the first law of
thermodynamics, which is a function of the conservation of energy principle. The two main
areas of heat transfer are through the cylinder wall and the combustion chamber portion of the
cylinder head with the water, and the radiator with the ambient air.
Contributors to Heat in the System
The main source of heat in the system is introduced by the engine, more specifically the amount
of horsepower the engine is producing. One factor that can be overlooked in heat production is
the engine tune itself. Depending on tune, an engine can place a massive load on the cooling
system while likely making less power. Advanced timing and/or lean air/fuel mixtures will place
more heat into the cooling system than a properly timed engine with stoichiometric (chemically
balanced) air/fuel mixtures. A restrictive exhaust or improper valve timing can cause excessive
heat to build in the combustion chamber and ports, placing a higher demand on the system.
Ignoring possible mechanical and tuning problems, engine output and operating conditions need
to be discussed and decided upon. When considering the radiator capacity necessary, the engine
output that you are using continually must be examined, not the maximum amount of
horsepower the engine can produce. For example, a 1,000hp drag car making a pass at the local
drag strip will put less heat into the cooling system then a 500hp late model making laps around
the track for 15 minutes continually. The drag car’s cooling capacity only needs to be enough to
cool the engine while idling and the short 8 second sprint down the ¼ mile. On the other hand,
the 500 hp late model, utilizing all the power as often as possible will place significantly more
heat into the cooling system. On average about 1/3 of all the heat produced from combustion is
put into the cooling system.
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Cooling Systems 101
Core Construction
Copper and Brass radiators were used for many years in OE and race application. Aluminum is
now the material of choice. Copper does have a higher thermal conductivity than aluminum,
which is a measurement of the ability a material exhibits to transfer heat; however, the copper
fins are paired with a brass tube and lead solder. The lead solder slows heat transfer from the
tube to the copper fin. When an aluminum core is brazed, the tube and fin are the same material
and are brazed into one consistent part. The aluminum core weighs approximately 30% of a
comparable copper and brass core. These characteristics make aluminum radiators attractive to
the performance crowd. Finally, since these cores are able to be TIG welded, customizing these
radiators is much easier and more readily achieved.
Radiator thickness vs. frontal surface area is a topic of large debate and controversy. Most
people “upgrade” their stock radiator with a thicker core, assuming that thicker must be better.
While in certain cases an increase in cooling capacity is achieved, a thicker core does not
necessarily dictate that a radiator will cool more efficiently then a properly sized thinner core.
Air is the necessary ingredient in producing a cooling system that will perform well. As core
thickness grows, air flow volume and velocity will go down, which hurts heat rejection. We
have witnessed numerous O.E.M. radiators that continually are becoming substantially thinner
than in years past. Basically, doubling a radiator’s available frontal surface area nearly doubles
the available heat rejection, while doubling the thickness of a core does not.
Fin pitch, with units of fins per inch (FPI), and tube spacing play a large role in how efficiently a
core will dissipate heat. Adding more FPI will increase surface area for heat transfer to take
place. Decreasing tube spacing (shorter fin heights) does the same thing while decreasing waterside pressure drop through the
core. Most O.E. and slower
racing applications keep the FPI
around 15-18, while NASCAR,
IndyCar, and Formula one will
run fin pitches as tight as 25.
Offroad applications tend to
loosen the fin pitch to around 12
in an effort to allow dirt and mud
to pass through the core instead
of becoming trapped. We have
seen this adversely effects
cooling performance by
removing a multitude of heat
transfer points (fin to tube), and
recommend using a proper
grill to block any debris
instead of less FPI. Fin pitch
and tube spacing both affect the air-side of the core, with tube spacing making less of an impact.
In general, cores that have closer tube spacing (short fins) are more durable, withstand higher
pressures, and offer an increased cooling capacity.
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Cooling Systems 101
Single Pass vs. Double Pass vs. Triple Pass
When people talk
about multiple pass
radiators, they are
referring to the
amount of times the
water passes
through the core
from entrance to
exit. A single pass
has an inlet and
outlet on opposite
tanks and no
internal baffles.
The water will pass
through the radiator
once in this
configuration.
There are two styles
of single pass radiators, down flow and cross flow. Crossflow radiators place end tanks on the
left and right, while downflow places end tanks on the top and bottom. A double pass has both
inlet and outlet located on the same tank, with a
baffle in between, the water must pass through
the radiator twice in this configuration. Finally,
there are radiators that have been built to pass
the water through the core three times. The inlet
and outlet would be on opposite tanks and
opposing corners, with two baffles, one in each
tank. Rarely will the case present itself that a
triple pass radiator is warranted, and extra care
should be taken to consider water side pressure
drop. Double passing a radiator effectively
doubles the core’s tube length (header-toheader) but cuts the height in half. One concern
that needs to be considered when going from a
single pass to a double is the additional waterside backpressure. Typically going from a single pass to a double pass will increase heat transfer
approximately 7%, depending on water flow rate (see Water Flow Section on page 6).
Airflow
Airflow could be considered the most vital aspect in a radiator based cooling system. Air is the
medium that allows the radiator to transfer heat. Consequently, more air flow leads to more heat
rejection. The best way to do this is to create a pressure differential. High pressure before the
radiator, low pressure behind it, and this differential will be the driving force for fresh air to
move through the core. Why doesn’t everyone build a front fascia that incorporates a large
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Cooling Systems 101
opening to feed the radiator you ask? OEM and race teams alike are constantly worried about
drag and lift forces. While a large grill opening is great for heat transfer, it is not conducive to
the vehicle’s aerodynamic profile.
Application
Vehicle speed, grill opening, and radiator ducting all affect a radiator’s ability to reject heat.
Vehicle speed is NOT the velocity that a radiator core will see; it is significantly less at the core.
This can vary greatly from model to model. The grill opening can make a large change in the
amount of air that will go through a
radiator. The goal here is to feed the
radiator fresh ambient air. Under ideal
circumstances, the radiator inlet should
be perpendicular to airflow; this will
capture high pressure air which will
create the pressure difference necessary
for air to move through the core, we refer
to this as air side pressure drop. Scoops
and deflectors can be used when
idealistic circumstances cannot be met.
Duct work between the grill opening and
the radiator is a great way to ensure the
incoming air will go through the radiator, not around it. Duct work, when designed well, also
encourages even distribution. A well designed air box will gradually open from the grill opening
size to the radiator core, this helps maintain air velocity and pressure. The air post-radiator must
have an area of low pressure to go to, which is generally underneath the vehicle. If the air cannot
exit, high pressure will begin to form and the air will become stagnant.
Fans
Fans are necessary for moving air through the
core at speeds less than 35 MPH (vehicle
speed). Fans are used on street and race cars
alike for low speed conditions when
supplemental airflow is necessary to achieve
adequate heat rejection. Electric fans are
effective for most applications. They do not
depend on the engine RPM so they can supply
maximum airflow even when the vehicle is
idling. Electric fans do have a downfall in
which the motors are power limited and
require high levels of electrical current. This
is when mechanical fans become a necessity.
Generally this tactic is used in heavy
machinery and thick cooling packages where
vehicle speed is slow and the core air side pressure drop is high. Dirt late models also have to
rely on mechanical fans for additional cooling when electrical fans do not make the cut. Fans
directly mounted to the core will only pull air through the area in front of the blades; this is
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Cooling Systems 101
acceptable in certain applications. Another option would be to shroud the radiator to allow the
fan to pull through the entire core. When utilizing a shroud at high speeds pressure can build up
before the shroud due to restricting the airflow path. Trap doors and flappers can alleviate this
pressure build up while remaining closed at low speeds.
Water flow
Water flow is the other side of the equation. The water pump determines the flow rate, we
recommend starting with a one to one (crank to water pump) pulley as a starting point. The
water flow rate and tube area will determine the water velocity through the core. Water velocity
controls the turbulence of the water inside the tube. As velocity increases, so does the
turbulation within the radiator tubes. This turbulence will increase the heat rejection. As a rule
of thumb, the lower water flow rates will see better heat-rejection with a double pass core
configuration. This is because it is effectively increasing velocity, and thus turbulence. With a
high enough flow rate, the velocity is high and a single pass configuration will increase heat
rejection. Water flow rate and core size can be calculated to achieve maximum heat rejection
and performance. Too much water flow for a given core size can cause back pressure. A
radiator core is a restriction that the water pump must overcome. That said, sometimes a single
pass can be more beneficial than a double pass when water flow is high enough.
Pressure throughout the system will vary determined due to different restrictions at each location
in the system. The system will be at less
pressure before the water pump than after it.
However, system pressure will be at higher
pressure before the core and lower afterwards.
These pressure differentials happen throughout
the engine due to the changes in water passage
cross section. Bernoulli’s equation explains
this phenomenon well. The cylinder head is
the location that needs the highest pressures to
reduce boiling risks. Maximizing system
pressure will reduce risks of detonation due to
these low pressure spots.
Other parameters/considerations
Recommended Operating Temperature
With each application and engine, preferred operating temperatures change. In general most
O.E. and race cars will run coolant temperature between 180 and 210 degrees Fahrenheit.
Typical oil temperatures will vary from 210 to 240 degrees Fahrenheit for both applications as
well. Running at these temperatures will minimize excessive wear and keep the engine
continually running for many miles. On race applications it is not unheard of to run much hotter
than these temperatures, NASCAR Sprint Cup cars tend to run 240°F coolant and 260°F oil, and
as high as 290°F with a properly engineered pressurized system. Most oils do not have a flash
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Cooling Systems 101
point below 400°F; as long as pressure is sufficient some road race vehicles run oil as hot as
300°F.
Thermostats
A thermostats sole purpose in the motor is to quickly bring the temperature of the water or
coolant up to operating temperature, and then maintain that temperature. We never recommend
a block off plate or any other form of temperature controlling device.
Radiator Caps
Running the highest pressure the system can handle has been stated more than once in this article
and it should be emphasized again. Higher pressure systems will suppress hot spots, steam
pockets, and increase the boiling
point past that of a lower pressure
system. Generally, 30psi is the
maximum pressure a quality
radiator cap is rated for. It is a good
idea to check the cap periodically to
ensure it is sealing correctly and the
rubber 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 really it
is due to pressure being lost and the
fluid boiling.
Recovery Systems
Keeping the system full is the goal with a recovery system. As temperature increases, the
coolant expands and builds pressure. As this pressure exceeds the cap’s pressure rating, water is
pushed out into the recovery tank. When the fill neck and radiator cap is located on the low
pressure side of the radiator (cross flow
configuration) and at the highest point, any
air in the system will be pushed out first (a
positive situation). When the engine is
shut off and cools down, the coolant
contracts and vacuum is created. Radiator
caps incorporate a vacuum valve that opens
when the radiator is under a vacuum. This
will pull fluid from the recovery tank and
keep the system full. Recovery tanks
should be kept close to the cap and at a
similar height. It is not advised to run this
style of a system without a recovery tank
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Cooling Systems 101
as air will continually be introduced into the system.
Surge Tank
Other names for a surge tank are expansion tank, air separator, and header tank. These devices
are utilized when the radiator is below the engine’s supply of water and also completes a better
system. Filling a cooling system that is not the highest point on the vehicle will introduce air
into the system. Surge tanks remedy this by being placed as the highest point of the cooling
system. These tanks will also help separate out air in the system and when the cap’s pressure is
met, expel this unwanted air to the recovery tank. In most cases the bottom of the tank connects
to low pressure (pre-water pump), and the smaller upper fitting connects to a high point on the
engine or radiator to help removed trapped or aerated water. This creates a low pressure
chamber that isolates the radiator cap or PRV from pressure spikes in the cooling system.
Coolants
There are three types of commonly used coolant in the automotive world. Ethylene glycol and
the less toxic propylene glycol in a 50/50 blend is the choice most O.E. and street cars utilize.
However, water is still accepted as the most efficient coolant in the automotive world. Distilled
water is suggested as many of the impurities are absent. Quality anti-corrosives and seal
lubricants are available to keep the cooling system operating long term, and we strongly suggest
the use of one. As of this date, C&R has yet to test any product that transfers heat more readily
than what pure water will do. We continue to test any and all products that make this claim.
It’s a System!
The big thing to realize here is everything works together. When designing a purpose built
system for an application, each of the aspects above must be considered, analyzed, and
scrutinized. You cannot tighten fin pitch without considering how it will affect the air that goes
through the core. A thinner core that increases airflow more than a thicker variant in the same
size can shed more heat and be lighter depending on airflow volume and speed. Double and
triple passing a radiator can increase heat rejection but water-side pressure drop must be
analyzed. A/C condensers, oil coolers, and large front mount intercoolers impede air and
increase the temperature reaching the radiator. As a result, the cooling system must be thought
of as a complete system in order to optimize heat rejection.
Things to Remember
-Create the biggest difference in temperature possible between the water in the radiator and the
ambient air. The radiator exchanges heat with the ambient air, the bigger the difference between
the two, the more heat that can be rejected.
-Provide the core with plenty of ambient air. To reject heat, air needs to be present and it needs
to move through the core continually.
-Run as high of pressure as the system can safely handle. This protects the engine from possible
detonation, overheating, and catastrophic failure.
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