Forgive my naitivity, but exactly what does an intercooler do. I know it cools the incoming air after the turbo, in order to get a higher charge density at the intake manifold. But does it just allow you to dump more fuel in and give more power for a given engine size, or does it actually increase efficiency as well by cooling the air? I was just looking at the turbo setup on my 4D55 today, and there's a several inch section of 2" hose between the turbo outlet, and the intake manifold inlet and I was thinking "I wonder if I could add an intercooler in there..., and if I would get better mileage, or just more power, if I do?" I'd also have to turn up the fuel screw to take advantage of the higher charge I imagine.
Did any of the VW turbos have an intercooler (old ones, not the TDI)? I know the 2.5 liter 4D56 (which never made it to the US) came with one, but not mine.