Just thought you folks would be interested in my little "mini-experiment" Finally got around to doing a little work on my Dolmar PS420 (only had the parts for a month or so). Had put ~ 5 tanks through it, and took it back to the shop for a free tune (part of purchase), then decided to swap out the cat muffler for a factory non-cat (didn't even mod it, just out of the box). Here what the results were (measured with tinytach Fast-Tach) using the same gas, same carb settings, same bar & chain & ~ 3 minute warm-up on a 64 degree day: CAT MUFFLER: Idle - 2,600 RPM; WOT - 12,000 RPM NO-CAT: Idle - 2,800 RPM; WOT - 12,500 RPM If I am correct (and I am sure the pros here will tell me if I am not): -the cat creates significant back pressure -by getting rid of it, the engine can move more air (not as restricted) -If you are moving more air, you will need to richen up your air:fuel screw -If you are able to move more air and more fuel, that SHOULD translate to more power What I am wondering is why the RPM change amount would be different between idle and WOT (i.e. not both 200 RPM or 500 RPM). Would that be indicative of something? Or is that just a function of piston velocity/venturi effect/Coriolis effect/lunar pull/cosmic rays?