look at it this way:
with ONLY an exhaust (everything else remaining stock) you CANNOT go over your cars bhp.
bhp is your cars horsepower at the FLYWHEEL, which is basically how much power your engine produces.
whp is your cars horsepoer at the WHEELS (what your engine actually produces with a tranny, exhaust, etc and friction is minused from bhp.
thererfore, all an exhaust really does is reduce friction (allow more air to exit more freely). therefore, you cannot go over your bhp by just adding an exhaust--because it isnt actually adding power to your engine--it's just allowing your engine to run more efficiently.
(celicas gts usually dyno at around 160 whp. the bhp rating for a gts is 180 bhp. 160 whp + an extra 50 whp = 110 whp. obviously this is not possible.
it's an interesting idea though about an exhaust sucking out air through a reverselike blower. but the bottom line is still that it's not ADDING bhp--so it can't go over 180 in a gts. and obviously an exhaust isn't the only thing restricting your engine so even with no friction from the exhaust you still can't reach the bhp rating of your engine.
sucking the exhaust gas out would be bad, there is such a thing as too much scavanging effect. This will cause the fresh Intake
charge comming in to get sucked out by the pressure difference in the exhaust, at higher rpms during valve overlap. This is a bad thing as you need the fresh Intake
charge to produce power.
You have to find the fine line where most of the exhaust is sucked out, and not spit into the Intake
charge, and where the Intake
charge isn't being sucked out into the exhaust.