I have flown more than one CL scale model that have Tongue mufflers on the engine (with throttle) with no problems. My one and only CL carrier model has flown with a std OS muffler and the Tonque muffler, and without a muffler for carrier competition. They work just fine, the OS std mufflers do a better job with the noise.

Fred C.

Not very effective at reducing noise and the ones I've used tended to make the engine run a bit hot.    Mike

A neat trick is to slice the muffler "tongue" off an engine's R/C muffler and use that as an exhaust tube to direct some of the oil away from your model. Does nothing to silence but makes a neat install.

Very true.  In fact, some engines need the back pressure to idle at all.  Considering the scoring emphasis on low speed, sacrificing a bit of high end in favor of reliable idling is a fair price to pay.

Of course the optimum would be to make an oldest exhaust value.  Full high speed and good low speed.