INTERCOOLERS
All turbocharged cars should have one, low boost turbos can get away with not having one but its ideal to fit one.
when a turbo starts to produce boost above atmospheric pressure it starts to heat up the air, the higher the boost level the more heat is produced, this causes a couple of problems. 1 is that hot air has less oxygen in it so less power is made from it and 2. hot air can cause detanation!
An intercooler cools charge temps down by allowing heat to radiate through the alloy/alluminium core into the fins, cooled by air from the outside, as air flows over the intercooler fast it causes the chill factor which means an intercooler works more efficiently at higher speeds.
One down side to an intercooler is it causes a pressure drop from inlet to outlet due to the air density change going on in between, a well designed intercooler will have very small pressure drop where as a badly designed intercooler will cause big pressure drops, my car is fitted with a standard fiat punto gt turbo intercooler and when i fitted it i noticed a drop of 2psi on the boost gauge! gains are still made due to the fact that there is alot more oxygen in the intercooled air but the boost level has to be incresed to reach the required boost level,,,again giving another power boost.
When it comes to intercoolers bigger really is better, but this means the turbo has to fill it before getting to the engine meaning more lag,,,,,,a small price to pay for tons more power
