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Probably been asked a thousand times before but what are the fundamental differences in the models such as the RS4 and the S5 that have the higher power output than the S4? Anything that is interchangable such as improved manifolds or cams?

S5 doesnt have more HP

well, maybe 6ps

S5 is the same unit but with Direct Fuel Injection, hence only slight increase in hp. RS4 unit has DFI and a load of other changes that I am not capable of listing, but it is irrelevant - withouth DFI you can't make an S4 engine into and RS4 or S5 engine. The thing I have read about the RS4 unit is that although hp is up considerably, torque increases are not quite as impressive.

Fair enough - I had been under the mis-impression that the RS4 was turbo like RS6, but must have been confusing that with the original 2.7 version. Still, a remap and exhaust should make this as fast as you ever need if it is not already!!

Does anybody bother to get into the realms of matching the head and inlet /exhaust manifolds up at a machine shop, flowing the manifolds or any other geekey tricks from the old-school?

I've got a port matched and honed inlet. Adds lots of torque low down

JHM in the US do exchange inlet manifolds that have basically been flowed, as far as I understand. As per xyber, reports of increased low end torque abound.

Sorry, I should have read your signature - the JHM stuff is clearly what you have!