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Cool Don.
325hp with 6 lbs boost sounds about right.
TurboKraft makes some interesting points of reference.
-658hp through two 2.5" mufflers is 328hp per which is about where you are.
-More importantly, a change from 2.5 to 3" on a 400hp motor made a 7% increase in HP.
Some thoughts.
2.5" is about 4.9 sq in,
3" is about 7.0 sq in (apx 29% more flow)
3.5" is about 9.6 sq in. (apx 33% more flow)
325 hp w 2.5 is 66hp per sq in
400hp w 2.5" is 100hp per sq in
424hp w 3" is 60hp per sq in.
525 hp w 3" is 75hp per sq in
525 hp w 3.5 would be about 54hp per sq in.
325hp w a 3.5" is 34hp per sq in (power-stroke)
At 66hp per nominal sq in, it dose not look like the 2.5" is a big issue.
However, from the example we do knot know if and even further increase in exhaust tubeing size would further continue to make improvement in efficiencies.
The general rule is a turbo dose not benefit from back pressure and the less one can get the better. Thus, bigger is better.
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