I have also seen replacement exhaust systems produce less power than stock systems because they are more restrictive. Yes, more restrictive, and noisier. Last week I dynoed a greddy trust catback making 20hp less than a stock exhaust on a Civic type R with Toda cams. In the US I saw a Magnaflow dyno 25hp less than a stock exhaust on a supercharged RSX. In both cases the internal diameter of the rear muffler was about 3 -4 cm. You could see this with a flashlight looking up the rear of the muffler. A restrictive exhaust will drop economy.
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