just shy of 1,000,000 horsepower. seriously that's a tough question. it depends on your application, tuning, other mods. etc. etc.. TYPICALLY a set of headers will net you 5-7 hp per litre. so on a 3 litre BMW engine, 15-21 horsepower is possible. on a 2.5 12-17 etc. etc.. this is why the big displacement V8 guys can see 40+hp. also how badly are the factory manifolds designed.. BMW seems okay, but I've seen stuff from ford that you can't imagine how the car ever ran.