FWIW:
Run your engine and adjust High speed needle for max rpm (with NO exhaust setup)
Do this in short bursts and do not over heat the engine .
Now add the muffler of your choice.
re set high speed as needed - do not run long periods - keep engine cool.
OK --which in cowl setup actually exceeded open stack rpm?
anybody ?
Do same test with the best quiet can muffler on a very short header .
Check rpm
any gain?
In honest tests - engines kept cool - we have never seen any gain with an in cowl muffler
Never.
some losses are awful.
some minimal
Using a very short header and a good can - again NO gain but very quiet operation
Tuned setups?
they add power - NONE of the others will.
Check with DA if you doubt this .
Most tests on mufflers do not allow for cool down and proper needle re setting
extended WOTrunning, progressively lowers next tests.
Ask DA if you doubt this.
The stock DA can is very good at minimal losses but is noisy