When a turbo or supercharger compresses air, the air is heated up. While this hot air can be fed straight into the intake of the engine (and often is), there are two disadvantages in taking this approach.
Firstly, warm air has less density than cool air - this means that it weighs less. It's important to know that it's the mass of air breathed by the engine that determines power, not the volume. So if the engine is being fed warm, high pressure air, the maximum power possible is significantly lower than if it is inhaling cold, high pressure air. The second problem with an engine breathing warm air is that the likelihood of detonation is increased. Detonation is a process of unstable combustion, where the flame front does not move progressively through the combustion chamber. Instead, the air/fuel mixture explodes into action. When this occurs, damage to the pistons, rings or head can very quickly happen.
If the temperature of the air can be reduced following the turbo or supercharger, the engine will have the potential to safely develop a higher power output. Intercoolers are used to cause this temperature drop.
There are a number of factors that affect the temperature increase that occurs when the air is compressed. Firstly, the higher the boost pressure, the greater will be the temperature increase. As a rule of thumb, if you are using a boost pressure level of more than about 0.5 Bar (~ 7 psi), an intercooler is generally a worthwhile investment.
Secondly, the lower the efficiency of the compressor, the higher the outlet air temp. However, it is difficult to accurately estimate the efficiency of the compressor and even if such a figure is available, it doesn't necessarily apply to all the different airflows that the compressor is capable of producing. In other words, there will be some combinations of airflow and boost pressure where the compressor is working at peak efficiency - and other areas where it isn't. While a well-matched compressor should be at peak efficiency most of the time, in some situations it will be working at less than optimum efficiency. This will change the outlet air temperature, usually for the worse.
Thirdly, the turbo- or supercharged car engine is not working in steady-state conditions. A typical forced induction road car might be on boost for only 5 per cent of the time, and even when it is on boost, it is perhaps for only 20 seconds at a stretch. Any decent forced induction road car will be travelling at well over 160 km/h if given 20 seconds of full boost from a standstill, meaning that longer periods of high boost occur only when hill-climbing, towing or driving at maximum speed. While all of the engine systems should be designed with the maximum full load capability in mind, in reality very few cars will ever experience this. This factor means that the heat-sink ability of the intake system must be considered.
If the inlet air temperature of the engine in cruise condition is 20°C above ambient, then on a 25° day the inlet air temp will be 45°C. After 30 minutes or so of running, all of the different components of the intake system will also have stabilised at around this temperature. If the engine then comes on boost and there is a sudden rise in the temp of the air being introduced to this system, the temperature of the turbo compressor cover (or blower housing), inlet duct, throttle body, plenum chamber, and inlet runners will all increase. These components increase in temp because they are removing heat from the intake air, limiting the magnitude of the initial rise in the actual intake air temperature. As a result, the infrequent short bursts of boost used in a typical road-driven forced-induction car often produce a lower initial intake air temperature than expected. This doesn't mean that intercooling is not worthwhile - it certainly is - but that the theory of the temperature increase doesn't always match reality.
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.
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 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!
When either increasing the size of a factory intercooler or installing a new one for a custom forced aspirated car, care needs to be given to the location that is chosen. The first point to consider is the amount of ambient heat that is present. An intercooler core absorbs heat just as well as it sheds it. This means that an underbonnet intercooler core can easily become an intake air pre-heater if care isn't taken with its location. Turbo cars run especially high underbonnet temps and so a bonnet vent designed for intercooler cooling while the car is under way can easily become a "chimney" ducting out hot air while the car is stationery - hot air that passes straight through the intercooler core. In fact, the behaviour of the intercooler while the vehicle is stopped is very important if you're in the habit of caning the car in traffic light Grands Prix!
By far the best location for an intercooler is in front of the engine radiator. The car manufacturer will have aerodynamically tested the vehicle to ensure that large volumes of air pass through the engine cooling radiator, and so an intercooler placed in front of that is sure to receive a great amount of cooling air. Note that the intercooler should be in front of any air conditioning condenser as well!
The air/air core should be ducted with the cold air if at all possible. Many people simply place the intercooler at the front of the car, hoping that the air being forced through the front grille will all pass through the intercooler. However, if there is an easier path for the air to take, that's the way it will go. Sheet metal guides can be used to channel the air coming in the grille through the intercooler, and foam rubber strips can be used to seal the escape routes that the air might otherwise take.
The plumbing leading to and from the intercooler should produce only a minimal pressure drop. Factory turbo cars often use intake ducts that smoothly increase in size from the diameter of the turbo compressor outlet (often only 50mm or so) to the inlet diameter of the throttle body (perhaps 80mm) and if this can be done, it's an approach which should be followed. Intercooler plumbing should have gentle curves and be as short as possible. Don't forget when you are planning the plumbing that the engine (and so also the blower or turbo!) moves around, while the body-mounted intercooler core does not. This means that some rubber or silicone hose connections must to be incorporated in the plumbing to absorb the movement.
The return duct from the intercooler should be insulated to avoid it picking up heat from within the engine bay. Lagging the pipe with fibreglass or ceramic fibre matting works effectively without being too bulky. The pipework can be finished off with a wrapping of aluminium adhesive tape of the type sometimes used to seal roofs. Also note when planning the intercooler pipework that the compressor cover of a turbo can be easily rotated to allow the outlet to come out at a different angle. This can reduce the number and tightness of the bends required.
Some people believe that if they fit a very big intercooler with large ducts, the volume of charge air within it will unduly slow throttle response. Their concern is unjustified however - throttle response problems (for example, turbo lag) are largely the result of other factors within the forced induction system, not the volume of air within it.
Sourcing the Core
There are a number of ways of getting together a very good air/air intercooler. Those companies specialising in the production of intercoolers (Spearco in the US is one of the largest) have a huge variety of cores and end-tanks available. eBay-based reselllers also have a wide variety of intercoolers available.
An alternative in Australia are the Japanese importing wreckers. While few factory turbo cars have really large intercoolers (and even less factory supercharged cars have them!), there are at least a couple of fairly large ones available. As mentioned previously, the Nissan Skyline GT-R and Mitsubishi Evolution model Lancers all have very good intercoolers. The Nissan Pulsar GTiR also has a large intercooler (pictured), while the Mazda RX7 single turbo Series 4 has an engine-mounted intercooler that has a good flow, despite its appearance. Welding two of the RX7 intercoolers in series has also been shown to work very well.
Next: water/air intercooling
The Complete Guide to Intercooling - Part 2