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This is the reason that backpressure is developed. With turbo exhaust you want as low of velocity (smallest pressure) in the exhaust after the turbine, that's why turbo exhaust is larger in diameter. 300hp semi truck have two 6" diameter pipes, they don't need to be that big, but why not.
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Not true...you still want high flow velocity in a turbo setup.
And as for the 300hp semi truck...I dunno about that. If it is true, then the reason is simple. The 300hp semi truck doesn't have comparable power to a 300hp 4 cylinder. The 300hp semi truck revs really low...so to get 300hp, they have a ton more torque than we ever have. And you need to tune for torque as well.
You heard wrong, Jeff. Backpressure is needed is a myth, or an urban legend. You need high flow velocity, not backpressure.
In fact, it is so important, it is one of Corky's Rules (from Maximum boost): Backpressure in an exhaust system is evil. Go back to the people who taught you that backpressure is good and demand a refund!
If you don't believe me, go to
www.google.com and do a search for backpressure myth. Articles like these are good ones, and come up at the top of the list:
http://www.hondalife.com/articles/backpressure.htm
http://www.uucmotorwerks.com/html_pr...torquemyth.htm
Dennis