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why does everyone thing that backpressure is a good thing? i always thought that it was bad the more backpressure the more power is lost in the engine. less backpressure less restriction. so why it that bad? (besides the loud ass noise that gets on your nerves)
 

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There is a post a little ways down I think about exhaust that details this point nicely. Back-preasure is a myth that existed before we came to understand the importance of exhaust velocity and scavenging effect. So if you go with a small exhaust pipe, you will indeed have backpreasure (aka restriction), and you'll kill your hp & tq. Step it up to the "right" size, and you'll have optimum torque, although not the highest peak hp, because at the extreme, even this size exhaust will start to become a restriction. Now if you go even larger in pipe diameter, your exhaust velocity drops off and you lose torque. You do gain the capacity at high rpms though to make maximum hp.
 
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