ORIGINAL: NikolayTT
I believe 4S is a great solution but it is too expensive while I have all the things
I listed (OS.91+Pipe+Header+APC12.5x12.5) and also an airplane where OS is
mounted already. I just wander what in fact .91 could deliver if pushed to high
speed ?
Nick - what does the FX .91 turn that prop at with just a stock muffler or an open header (no tuned pipe)? I have not had any experience with the 12.5 x 12.5 - closest I've used are 12 x 12 (estimate - about 10K rpm with open header) and the 13 x 13 Narrowblade (estimate, about 9K rpm).
Use the tach reading you get without the pipe as a baseline then aim for 1000 - 2000 rpm increase with a tuned pipe.
Knowing what type of airframe you're running the engine in will help too. At rpm levels in excess of 10K, a 12.5 inch pitch prop needs a pretty slick airframe to take advantage of it.