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By Julian Edgar
Intercooler Efficiency
An intercooler will do two things - it will lower the temperature of the intake air and at the same time, cause a slight drop in boost pressure. The latter comes from the restriction to flow caused by the intercooler. Some restriction is unavoidable because the flow through an efficient intercooler core needs to be turbulent if a lot of the air is to come in contact with the heat exchanger surfaces. However, if the pressure drop is too high, power will suffer. A pressure drop of 1-2 psi can be considered acceptable if it is accompanied by good intercooler efficiency.
Intercooler efficiency is a measurement of how effective the intercooler is at reducing the inlet air temperature. If the intercooler reduces the temperature of the air exiting the compressor to ambient, the intercooler will be 100 per cent efficient. It will also be a bloody marvel, because no conventional intercooler can actually achieve this! Typical figures for a good intercooler are around 70 per cent.
Intercooler Types:
Most intercoolers fall into two categories - air/air and air/water. There are also those special designs that cool the intake air to below ambient temperatures, using ice, the air-conditioning system or direct nitrous oxide sprays, but they will not be covered here.
Air/Air Intercoolers:
Air/air intercoolers are the most common type, both in factory forced induction cars and aftermarket. They are technically simple, rugged and reliable. An air/air intercooler consists of a tube and fin radiator. The induction air passes through thin rectangular cross-section tubes that are stacked on top of the other. Often inside the tubes are fins that are designed to create turbulence and so improve heat exchange. Between the tubes are more fins, usually bent in a zig-zag formation. Invariably, air/air intercoolers are constructed from aluminium. The induction air flows through the many tubes. The air is then exposed to a very large surface area of conductive aluminium that absorbs and transfers the heat through the thickness of metal. Outside air - driven through the core by the forward motion of the car - takes this heat away, transferring it from the intake air to the atmosphere.
Described above is what is normally called the intercooler 'core' - the part of the intercooler that actually effects the heat transfer. However, there also needs to be an efficient way of carrying the intake air to each of the tiny tubes that pass through the core. End-tanks are used for this, being welded at each end of the core. While some cores are 'double-pass' (the inlet and outlet tanks are at one end separated by a divider, while at the other end the air does a U-turn), most cores are single-pass, with the inlet at one end of the core and the outlet at the other.
Good intercooler manufacturers have two specifications available - the pressure drop at a rated airflow (with the airflow often expressed as engine power), and the cooling effect (normally expressed as a temperature drop at that rated flow). However, many intercooler manufacturers have no data available on either of these factors! To some extent this doesn't matter greatly - the design of the intercooler is normally limited by factors other than heat transfer ability and pressure drop. Because an air/air intercooler uses ambient air as the cooling medium, an air/air intercooler cannot be too efficient - simply, the bigger the intercooler, the better. In fact, the maximum size of an air/air intercooler is normally dictated by the amount of space available at the front of the car and the size of your wallet, rather than any other factors!
It's easy to see how cost is a vital factor - those forced induction cars produced by major car companies as homologation specials (either for rallying or circuit racing) have quite huge intercoolers that dwarf the ones fitted by the same companies to their humdrum cars. Nissan used an air/air core no less than 60 x 30 x 6cm on their R32 Nissan Skyline GT-R and the Mitsubishi Lancer Evolution vehicles also use huge intercoolers. The "bigger is better" philosophy can be clearly seen at work in these cars.
Many factory-fitted intercoolers are undersized. Air/air cores no larger than a paperback book can be found in turbo cars with a nominal maximum output of 150kW. Cars equipped with this type of intercooler can be held at peak power for only a very short time before the increasing inlet air temperature causes the ECU to retard timing or decrease boost. A car fitted with this type of tiny factory intercooler is almost impossible to dyno test - the intake air temp rises so fast that rarely can more than one consecutive dyno run be made before the intake air temp is so high that the engine detonates... On the other hand, the aforesaid Skyline GT-R has a measured intake temp of 45C on a 35C day at 1 Bar boost and a sustained full-throttle 250 km/h!