40rwhp? No way!!! 15rwhp? Wishful thinking!!! I wouldn't expect much out of a cat-back system on it's own. Pair it up with a mid-pipe and you may notice a difference. Throw in some long-tube headers and maybe with a little luck and prayer, you will reach a 40 rwhp gain.
You MIGHT get 5 hp out of a set of mufflers. Perhaps 10-12 out of a full cat back system. Whether you're just buying new mufflers to weld in, or a whole cat-back, shop by sound not HP. The differences in HP are minimal, but differences in sound are dramatic. Pick the ones you like to listen to -- you'll have to listen to them for a long time.
The Flowmasters might give you 5 RWHP +/-
The way how they sound is like +40 HP, but that's wishful thinking.
I dynoed my Challenger with the stock 4 muffler exhaust and I dynoed it with only 2 strait thru 2 1/2 " Flow masters. A full gain of a whopping 8 HP!!! Plus I lost about 10 HP up to 2500 RPM. Thats on a 383 cui 4bbl hot cam setup.
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