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Cooling

 - A third of the engines power is converted to heat energy.
 - Any rise in engine power causes an equal rise in heat energy.


The 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. This means 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. If a 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.

There are 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 systems.

When 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 be added.

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 transfer.

Use 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 system.

Petroleum 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. To keep 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.

The 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.

Water Pump.
Water 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.



Gas 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 bubbles. Race pumps are available that have a good close fitting, closed back, cast impeller.

Next area 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. 

Coolant Flow Rates.

To control 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. If the 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.

Some 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. The solution to better cooling in the engine is to move less water in a more rapid fashion, rather than moving more water. 

Coolant Flow Path.

Accurate 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.

Most 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 radiator.

To prevent 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.

The 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. Simply increasing the flow of water is not viable because the middle cylinders are already too cool.

The 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.

Whether we 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.

However, 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. The 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 flow. 

Advanced High-Output Cooling.

When we 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. 

How this 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.

We then 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 efficiency.  

Radiator Design.

Copper or 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. The water 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 more heat.  Most road 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. 

Double Pass Radiator.

We can 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.

A 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.

Be careful 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. Rally cars 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.

Always 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. A thermo 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. 

Helping Airflow through the Radiator.

Excessive 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 bay.

[ see the section on intercoolers ]