Quote:
Originally Posted by JohnnyGrey
An increase in exhaust diameter results in a reduction of exhaust velocity and therefore scavenging. Yes, a larger exhaust will open up your top end because there is a limit to exhaust gas velocity, but when you lose that velocity on the low end, you lose power. The same principle applies to the intake runners.
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Consideration needs to be taken into account on the configuration, size and type of exhaust system under the vehicle stock. Compression bending, large multi pass restrictive muffler, unnecessary bends in piping all conspire to slow the velocity. In many cases, a higher flow, straight through design muffler, in stock location could provide benefits, if the stock piping is designed well.