ORIGINAL: earlwb

People are sometimes amazed and surprised to see how much RPMs or power is lost with some mufflers.
I have seen as much as a 3,000 rpm drop in performance with some mufflers. Then we have other mufflers that have little to no drop in RPMs when used, and a few mufflers that actually increase the RPMs too.

When I did some experiments to satisfy some of my curiosity, I found that a Fox Eagle IV .60 would turn about the same RPMs using the same prop as a Rossi engine and the same muffler. Both engines really came alive using a Performance Specialties tuned muffler (a Nelson Muffler). Both engines were within a 100 RPMs of each other, which pretty much makes them equal then. The stock OEM Fox muffler, although it works OK, it is more noisy than I like and it restricts the engine power a lot.

Using a 9x5 or 9x6 prop on a .30 is a pretty reasonable assumption to make. The older design .29's through .35 engines typically all ran the 9x5 through 9x6 props. They usually didn't like lugging down so much with the 10x6 props. Where a .40 would run a 10x6 prop OK. But then the modern big bore .25 engines that were bored out for .32, .35 and .36 displacements or even larger too, would turn a 10x6 prop really well.