I have been having problems with an Irvine 53 engine that overheats on midrange.
(See jbover, Irvine 53 problem 4/10/04 in the glow engine forum)
Using Slimline pitts muffler, 11X5 prop, 10% nitro wildcat fuel, GP RV-4 plane, Perry Pump. beneath cowel.

Bench tested today with Stock Muffler, Ran great, no overheating at midrange, left carb settings as is, replaced stock muffler with pitts muffler, Overheated at midrange.
This Pitts Muffler is rope-a-dopin me, but why?
Jbover in the pitts.