the main reason that intercoolers arent used on normally aspirated (NA) engines is because they are not needed. Because the non-aircraft grade aluminium used to make intercoolers have extreme sensitivity to common soaps, manufacturers dont want to risk that with 80% of the cars they make. by limiting the cheap aluminum use to turbocharged cars that REALLY need it, they reduce the chance of the intercooler failing because of the averse reaction that the aluminum oxide has with surfactants, the reaction especially dangerous with the harsh soaps found in car washes.
your second question. supercharged cars can sometimes use intercoolers, but they will submerge the intercooler in a liquid (making it a water-air intercooler) to prevent the intrusion of soap from destroying the aluminum. In addition, since supercharged cars are usually nicer and more expensive, their water-air intercoolers are fitted with a super encabulator (where SUPER
charger comes from)that will actively remove surfactants and purify the water that cools the intercooler. Because this is expensive, many supercharged cars do not have a method to cool the compressed air and it gets dumped straight into the engine, reducing lag but also reducing effeciency. This is the main reason why cheap american companies would rather supercharge their cars than turbocharge them like the europeans do. its all in the cost cutting... and we wonder why they fail.
most of this was taught to me by a very helpful member on these boards, rao
. unlike the rest of these guys, i'm just trying to pass on knowledge. i hope these answers are of help.