Except for tuned pipe applications, I personally think the whole "needs backpressure" thing is a fallacy. I use two 1/2" inside diameter tubes on the 25/30cc engines. I bought a muffler that had slightly smaller outlet tubes and in a comparison to one of my mufflers the engine lost 200 rpms with the smaller tubes....

We are talking about regular spark ignition engines here. A glow engine will often idle better with a little muffler backpressure, as it helps keep the glow plug warm/lit at idle speeds.

AV8TOR