is it true that if you swap out the stock exaust on a 91 civic si that it actually decreases the horsepower?

 

It decreases backpressure.
You will lose some bottom end, but the engine will breathe better on the top end.
Top end is where you keep it for racing.
You dyno peek number will also show a gain.
so I guess the answer to your questing is 'no'.

 

of course it also depends on what exhaust setup you change to

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 91_civic_si_guy &raquo;</TD></TR><TR><TD CLASS="quote">is it true that if you swap out the stock exaust on a 91 civic si that it actually decreases the horsepower?</TD></TR></TABLE>
Depends. If you swap a smaller and/or lesser flowing exhaust on, then you'll lose power . . . if you have gigantinormous piping on an engine that doesn't need it, you'll slow down the flow velocity, which will cause backpressure and therefore a loss in power . . .

You'll want the proper sized piping for your setup - ideally you want the exhaust to be as free flowing as possible.

Rostr02 cut my exhaust off at the cat with a sawzall on Sunday. I definitely gained a few ponies up top