Although it is not a performance part per se, an intercooler is nevertheless a fundamental part of a turbocharger system. While there are two types of intercoolers on the market — air-to-air and air-to-water intercoolers, only air-to-air intercoolers are practical on street and endurance type racing cars. Air-to-water intercoolers work wonders on extremely short runs and are ideal for drag cars. You can think of an as a radiator that cools the compressed air that compressor-wheel pumps into the intake manifold. This compressed air is referred to as the intake charge.
Heat is a byproduct of the compression of air and whenever air is compressed, such as when turbochargers and other forced induction systems are used, the air (intake charge) is heated. This is called the thermodynamic effect of compressing air. On an internal combustion, air temperature is important because it affects air density and because too much heat will result in pre-ignition, knocking and detonation. The role of the intercooler is to reduce the temperature of the intake charge. Cooling the intake charge provides two major benefits: it makes the intake charge denser — denser air produces more power; and it inhibits detonation. However, the intercooler has one major disadvantage: it causes a drop in boost pressure! This is inescapable; the best you can do is to minimize the pressure loss caused by the intercooler.
There are a number of things you can do when designing your intercooling system to keep pressure loss at a minimum. You can incorporate a plate-and-shell core rather than an extruder core as the plate-and-shell core produces less flow resistance. The internal flow area of the core also affects pressure loss. Selecting an intercooler with the correct internal flow area is important to keep pressure loss at a minimum. Here I have found that a turbo system that produces a flow rate of 400 cfm requires an internal flow area of approximately 20 sq in. You can use the following graph to estimate the required internal flow area based on your turbo's flow rate.
Once you've estimated an idea of what internal flow area you require, you can determine the actual core size that you require. This is a bit tricky. Typically, only about 45% of the intake charge will come into contact with the heat exchange elements of the intercooler core. So we must first divide the required internal flow area by 45% then divide the result by the core thickness. This gives us the following formula:
While we're talking about core thickness, remember that the intake charge flows through the length of the core and the cooling, ambient air flows through the thickness of the core but that every subsequent inch of core thickness is 40% less effective as the air flowing through it heats up. Thus, a thinner core intercooler with a larger frontal area is more efficient than a thicker intercooler with a smaller frontal area.
You can also improve the efficiency of your intercooler by fitting a duct to it. Without a duct, approximately 25% of the air molecules will pass through the core; the rest will follow the path of least resistance around the core. Any improvement here will greatly increase the efficiency of your intercooler.
If you're running high boost pressures on 91-93 octane gas and a fairly high compression ratio, you may also want to supplement your intercooler with a water injection system.