Minimum... I did some tests and the stock Bisson robbed nearly 800 rpm! The Supersonic stole about 400-500... That IS significant. The Supersonic had straight exhaust pipes and the Bisson's were swaged down a it.

As many know, cutting them off will return a couple hundred rpm putting it back inthe range of the Supersonic... I haven't tested a slimline...

The sotck muffler is hard to hide and many cowls require some cutting on the side... but I'd rather see a little muffler and gain 400-500 rpm anyday...

With that said... if these other engines could use the DA stock muffler they would see an increase as well... most can't but the ZDZ can... same header pattern.