just curious, first of all, does an HF ecu work with a stock A6 engine and Si harness and all? im guessing it does. and so if so, do you think it would make the car get better mpg? im talking about daily driving and cruising.

 

it would run it but not very good. it HF has a EGR valve and the SI doesnt. it will bring on the CEL. i know the HF manifold is alot smaller so it might have smaller injectors and not give it the correct amout of fuel the SI requires. plus the HF only revs to 5500 or so. try to find an SI ecu if possible.