OS 46SF & Royal 45
Yes, I know these engines are NOT from the current crop but many modellers would have a few of these laying around. The Royal 45 I have is the earlier version that has the removable front housing (looks like an OS FSR clone - nice [)]).
Home brew fuel being 70% methanol, 10% nitro, 10% red Klotz & 10% castor (Castrol M). Air temp about 75F & dry.
Same glow plug (OS 8), same prop (Bolly 10.5 x 6) for all tests. Engines fitted with standard OEM mufflers
OS 46SF = 13,200 RPM
Royal 45 (removable front housing) = 12,900 RPM