- A third of the engines power is converted to
- Any rise in engine power causes an equal rise in
dissipation of heat is essential to maintain normal engine operating
conditions. Only a third of the heat generated by the engine is converted to
mechanical energy, another third is expelled via the exhaust and the remaining
third must be absorbed by the cooling system.
that an engine producing 150bhp at the flywheel also expels 150horsepower out
the exhaust, but it also means that the cooling system must deal with a
maximum of 150 horsepower of heat energy. It also
means that any rise in engine horsepower should be accompanied by an increase
in the cooling system.
150bhp engine is upgraded to output 300bhp then the cooling system must also
be upgraded to deal with double the heat energy.
Corrosion and Freeze Protection. Two major
deterrents to heat transfer from the combustion chamber to the cooling system
are deposits and air in the coolant. Metallic oxides that are formed in the
water passage that are 12-thousanths of an inch thick can cut heat transfer by
40%. The coolant system should be chemically cleaned as often as possible. The
coolant should also contain an inhibitor that will keep water jackets clean
and free of deposits.
two types of inhibitors; chromates and non-chromates. Sodium chromate and
potassium dichromate are two of the best chromate inhibitors for water coolant
but are toxic so must be handled carefully. The best non-chromate inhibitors
are borates, nitrates, nitrides and are used in water only or anti-freeze
freeze protection is required, a permanent type anti-freeze must be used. If a
solution of 30% anti-freeze is used then additional inhibitors aren’t needed.
But a solution weaker than 30% will need additional non-chromate inhibitors to
Concentrations above 50% will hurt horsepower and should not be used. If
anti-freeze is not required then don’t use any because it reduces heat
ethylene glycol based anti-freeze in all high performance engines. Methyl
alcohol based anti-freeze should not be used because of its low boiling point
and because of its corrosive effect on water pump seals and radiator hoses.
Anti-freeze containing sealer additives should also be avoided unless it’s
used to temporarily repair damage, after which it should be flushed from the
based products used to lubricate the water pump and used as a rust inhibitor
should never be used. A 2% concentration on soluble oil will raise the deck
temperature by 10%.
Eliminating Air Bubbles.
Air can be
introduced into the coolant through a leaking gasket or hose and will reduce
the heat transfer and the efficiency of the water pump.
air out make sure there are no leaks in the system and keep the coolant topped
up. Bleed the system every time a pipe is disconnected. Make sure no air
pockets are trapped in the head or internal heater rad. Some systems will self
bleed and others must be manually bled by an air nipple high up in the block.
Start the engine and rev for several seconds and re-bleed. Repeat until all
air is expelled. With road cars, run it for 15 minutes with the internal
heater on full blast and re-bleed.
Pressurised Cooling System.
coolant system is pressurised to stop boiling after the engine is switched
off. After shutdown the coolant climbs from 90degC to 110degC. By pressurising
the system using a 14psi radiator cap, the boiling point of the water is
raised to 125degC. As well as this, the pressurised radiator cap prevents the
formation of gas bubbles in a number of situations. A road engine may be
driven at full throttle at low engine speed which will cause rapid heating of
the combustion chamber, exhaust port and valve area. At low rpm the water pump
is turning too slowly so water flow is limited and only the radiator cap stops
localised boiling. A similar situation occurs in a rally car after the flying
finish when the car must stop at finish/passage control.
pumps are normally set to give max efficiency at 3000-5000rpm
(mechanical-road). Water pressure created by the water pump prevents boiling
(not the radiator cap). Regardless of the radiator cap pressure, the water
pump will produce 30-40psi in the engine block and head when the water is
restricted by a thermostat. This water pressure packs coolant around the top
of the cylinders and around the combustion chambers to carry away heat and
stop air bubbles forming.
bubbles from the water pump cavitations must be minimised by correct design
and by limiting water pump rotation speed. All water pumps cause bubbles at
excessive speeds. However, some poor pumps will produce bubbles at low speeds
because of economical design. These pumps have smaller blades and more space
between the blade and casing, contributing to poorer flow and more air
are available that have a good close fitting, closed back, cast impeller.
for consideration is the pump speed. In a well designed system, water speed
will increased in proportion to engine speed up to 5000rpm. After this the
rate of increase should drop off rapidly as the pump approaches max
efficiency. However, past 5000rpm it is costing more power to move
proportionally less water.
the rate of water flow thought the engine and radiator and to get water
pressure in the block to 30psi we need to select the correct restrictor size.
If the restrictor is giving the correct pressure but the water temperature is
too low then the pump speed can be lowered.
engine is too hot, we can go to a bigger restrictor. If this doesn’t affect
temperature then we can increase radiator size and/or increase pump pressure.
people remove the restrictor (thermostat) in an effort to decrease
temperatures. This will cause the temperature guage to read a lower
temperature but water pressure in the block and head will be much lower, steam
pockets will form in the hottest areas. The combustion chamber, exhaust valve
and piston crown will then overheat, causing detonation.
solution to better cooling in the engine is to move less water in a more rapid
fashion, rather than moving more water.
delivery of coolant at high velocity to critical areas such as combustion
chambers, exhaust valves, spark plugs and the top of the cylinder head is what
we are aiming for.
standard systems pump water from the water pump into the front of the engine
block to the back, then upwards into the head through the large water passages
interconnecting the cylinder block and the head. It then moves to the front of
the cylinder head. It then exits the engine via the thermostat and into the
steam pockets at the top of the cylinders, a series of vent holes are drilled
in the top of the block. They also allow the free passage of coolant from the
block into the head. As the spark plug bosses are very close to the surface of
the head, there are usually holes in the block that direct water flow from the
block upto the plugs. As a result of these holes, coolant flow velocity around
the outer cylinders and combustion chambers is reduced and heat on these outer
cylinders and chambers is too high while the middle cylinders and chambers run
too cool. To keep detonation at bay, we have to run lower than ideal
compression ratio and less spark advance. Obviously, these flow problems need
to be corrected in high powered engines.
coolant flow path must be altered. The block requires some attention, but it
is the head that really must be addressed. In a modified engine we want to
increase water flow through the head.
increasing the flow of water is not viable because the middle cylinders are
already too cool.
solution is to change the coolant flow path. The bulk of the flow must be
directed straight to the head. This reduces parasitic power losses because
less water is pushed through the block. Frictional losses are also reduced
because the cylinders are running at optimum temperature. So in a road/race
engine 65-80% of the coolant is delivered directly to the head. In drag cars
the block gets no coolant at all. All flow goes into the back of the head.
use the traditional system or the high power split-system of cooling, we still
need to consider the flow of coolant between the block and head. It is at the
back of the head that we want volume flow of coolant; all other holes must
only be as large as is needed to vent air and steam from under the deck of the
block into the head, or to allow metered flow from the block to specific areas
in the head. A single 0.09in hole can drop cylinder deck temps by 50degC.
it is more normal to have holes at about 1/2in diameter because of
manufacturing methods. These overly large holes cause too much flow and the
rear cylinders overheat.
manufacturers then make smaller holes in the head gasket to rectify this
problem by limiting flow and they increase hole size where they want more
Advanced High-Output Cooling.
push high output engines to ever higher boost levels we have to consider
scrapping the standard coolant system and implementing an external coolant
system. Coolant flow is directed to each individual cylinder and then directed
to the radiator.
is achieved varies from engine to engine. The simplest arrangement is to
introduce all coolant flow from the water pump into one side of the block.
However, if the cylinders are not joined, and we are unable to seal the water
passages we will require two or more inlets. The main aim is to get max water
movement up close to the top of the block and we want to equalise cylinder
temperatures as much as possible. Usually we knock out existing Welch plugs
and tap the holes to accept water fittings. We must also cover the hole left
by the old water pump and fill the large coolant flow holes in the deck at the
other end of the block.
decide on the flow path across the head. We want the water to enter at several
points at the front of the head and exit at several points on the back of the
head, usually into a head coolant manifold. The size of the inlets and outlets
can be varied to equalise the temperature across the head.
Radiator Maintenance. Bugs and
debris should be cleaned off the red regularly. If its blocked up with rubber
then the rad must be removed and soaked in solvent for a day and then
powerhosed from back to front. Any fins that are bent should be straightened.
The rad should be regularly sprayed with matt black paint to improve radiating
aluminium should be used. Copper conducts heat better but aluminium is more
resistant to accident damage and damage from stones and chips. Also, because
aluminium is stronger its tubes can be twice as wide.
flow in a four row copper rad and a two row aluminium rad is the same but the
heat transfer of the aluminium rad is 20% better. Bigger tubing means that the
fins have more of a contact area with the tubes and therefore they transfer
cars use less tubes and more fins, whereas race rads use more expensive tubes
and less fins. While both rads will transfer the same heat at lower road
speeds, at high speed the race rad will be more efficient because the fins
stifle less air. The race rads have an added advantage because they allow a
lot more air into the engine bay after passing through the rad.
A road rad
will have 29 tubes per foot with a fin count of 17 per in.
A race rad
will have bigger 22 tubes per foot with a fin count of 10 per in.
If we need
better cooling we can increase the number of tubes only. The fin density
shouldn’t rise. If the rad brings the temp too low then fit a thermostat with
a smaller orifice to get block pressure back to 30psi. Less parasitic losses
and reduced air flow resistance will see a speed increase on fast straights.
also increase cooling without changing the radiator at all. We can modify the
existing radiator by placing a splitter plate half way down the rad end plate.
This will cause the coolant to flow twice as long in the rad which will
provide 15-20% more cooling so long as the water pump is upto it. Obviously
this increases flow resistance, so shouldn’t be used on a competition engine.
Regulating Coolant Temps with a Thermostat.
high-temp thermostat (95degC) is usually fitted to production cars. Only
running and testing on the dyno will reveal the ideal thermostat to use for
best power. But in general, a thermostat that will run the engine at 85degC is
best for power.
to use the correct type of thermostat. Modern engines use a dual-function
bypass type thermostat. To allow water flow out of the head into the radiator
the thermostat closes off the engine bypass port. If a standard thermostat is
used to replace it then this port will remain open and a large volume of very
hot water will circulate in a closed loop within the engine. This may lead to
overheating because of the volume of hot water from the bypass mixing with the
water from the rad and raising its temperature as it flows into the block.
Radiator Cooling Fan.
If a fan
is required then it should always be an electric unit. Side or rear mounted
rads will need bigger than normal fans.
on hill climbs will also need bigger than normal fan size to allow for greasy
uphill stages that are run at max engine load at low road speed.
mount the fan at the back of the rad so that it doesn’t stop airflow into the
engine and so that it is protected from stones etc. The draw of a big fan can
be close to 10amps so make sure the alternator is upto the task, especially if
you are using CD ignition. If you have to run the fan all the time then there
is a problem with your cooling circuit.
switch shouldn’t be necessary on a competition engine; the driver should
operate the fan by a switch. The thermo-switch can be used to illuminate a big
warning lamp on the dash.
Airflow through the Radiator.
pressure build-up in the engine bay may also require the fan to be used
outside of normal operating conditions. When this is the case we must work to
equalise the pressure by providing an escape path for the air in the engine