Put 5 different side dump or wrap around mufflers on an engine and get 5 different rpm readings. And no, the side dump muffler is not tuned to the engine. It is a bare minumun device designed to reduce sound levels and does a crappy job in the process. If an engine is marketed with a side dump muffler it's to make it easier for the customers and for the seller to make more money. Take the muffler off and power will increase. An engine would have to have an incredibly short wave length port timing to benefit from having an expansion chamber placed so close to the exhaust port.