How much horse power dose a exhaust system add?

Answer 1

It depends on what engine your talking about and what type of exhaust, further more on how restrictive the base system is.

Assuming your talking about a normally aspirated (non-turbo, non-super charged):
You can have a few diffent types, but a cat-back system (from the catalyst converter back to the tail and including muffler) and a full exhaust system (from engine block all the way back, which includes headers and a cat-ack) is the most common.

Exhaust systems can decrease your power output if they are too big. But with a "correctly sized" full exhaust system, on a 2L 4 cylinder look at about 5% increase, on a 3L 6 cylinder about 7% increase, and on a 5L V8 the increase can be pushed up to 10% with a simple supporting modification like intake. That said, all the figures can be pushed up a bit with a intake mod more then a intake mode does by it's self.

A cat-back will do very little, possibly half of the above figures, if not less. It will produce more sounds then power. Similarly it will sound a lot worse then a full system.

Please note, these figures are an average. It depends on what car, hence how restrictive a exhaust system is.

If you where asking about a turbo or super charged system, for a full turbo-back system (which is similar to a full system, with just one less pipe called a "up-pipe"):
Assuming about a 35% increase in boost pressure to the turbo, then the figures could be double of the above figures (10%, 14%, 20%). Super charged systems don't get too much of a increase in boost so the figures will be less then a turbo figure. Finally, the turbo figures given are fragile because a turbo car greatly depends on other components of the car (eg intake) and environment (too hot and power could drop). Similarity for a supper charged.