The theoretical way a tuned pipe/silencer works, has been explained again and again in RCU.
I wrote a full explanation four, or five months ago.

You could search the old RCU pages for these explanations.


The matter is: a high RPM tuned exhaust system allows the engine to combust 30-50% more fuel-air mixture, than it would with a standard, expansion chamber muffler (15-30 more than with an open exhaust).

The engine thus can produce more horsepower to the same ratio.

The gain in horsepower is translated to the third power, in RPM terms.

If an engine's RPM with a given prop is raised, say, from 12,600 to 14,400, the horsepower gain is not 14%, but 49%.


A high torque, low RPM pipe, like the Bolly pipe in the MAN engine review for the ST .90, allowed the engine to make the same maximum horsepower at 20% lower RPM. This means a torque (and BMEP) gain of 25%, at the respective RPM of maximum HP.
Very respectable, from where I am looking.