Quote:
Originally Posted by ninetres
Sorry, but your “theory” is 100% patently false. By your “theory” If there was 1” of exhaust length that was only 1mm diameter it would not effect performance much if at all because the length of the restriction is so short. That’s just not true.
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DG is actually on the right track.
Flow restriction (and the attendant power/torque loss) is not just about the length of a reduced cross section area any more than it's only about what the cross section area was reduced to. And this only addresses the steady flow portion of the problem.
The amount of "backpressure" an undimpled pipe has is probably only a fraction of one psi over the entire length, maybe something like 0.2 psi (I did look at some low pressure gas flow charts for this). Suppose that length from converter to muffler is 5 feet, so that becomes 0.04 psi per foot. Now let's make the crimp such that its resistance becomes 0.2 psi per foot (a good bit more severe than the case here), making the total pipe backpressure about 0.04*4.5 + 0.2*0.5, or 0.28 psi. So you've added less than 0.1 psi of backpressure . . . and to give that some context, each whole psi of backpressure costs about 7% power but you're gaining less than a tenth of that (around half a percent).
Any
change in cross section shape (even if the inside area remained the same) has the potential for affecting system resonances and their effect on tuning. This might even be a larger effect than the steady state part, at least within some rpm range or ranges.
Norm