Flowmaster Muffler.

In my summit catalog there is a Flowmaster Series 30 or 40 muffler that's supposed to boost horsepower by 30. Is this true or is it bull?

bull shit. every company does it, they take the optimal situation for gain and then use those numbers.

maybe they added too many numbers on, 3 horsepower is probably what you're gonna get. Sometimes aftermarket mufflers have been known to decrease horsepower too.

Odd, usually summit wont boast bullshit that extream- thats usually left to less respectable retailers.

You're looking at 15 HP tops. 7-10 is the most probable. Keep in mind it depends on the car and the engine you're putting it in. If you have a really tiny exhaust like the shit you see on honda civics on a car with a big engine, then it'll give you more boost than replacing say a muffler on a dodge truck. Also, keep in mind that the amount of exhaust an engine produces depends on its cylinder capacity and the amount of fuel it burns. You're not gonna produce alot of exhaust and backflow on a 2.0L engine. Exhaust modifications are for tuned cars (forced induction, NOS, etc). Stick to oxygen/fuel boost and playing with ratios. exhausts are a waste of time (no significant numbers).

I gained 300hp...but I should probably mention that my cat came apart and completely blocked the stock exhaust, and it wouldn't even run before I changed out the muffler :D