I bought a Hangar-9 P51 today at my local club swap meet, had an OS 91 installed in it. The original owner powered it up, and we tached it at just over 9000 RPM with a 15x6 prop. There were too many people wandering all over the airstrip, so we didn't get it in the air.

I came home and looked up the specs on the 2-stroke 91, and it mentions that it should turn a 13x8 at about 15000 RPM....

What should I realistically expect out of this motor if it were in strong condition? I think the 15 inch prop may be a bit much, but it shouldn't cost me 6000 RPM. This is the first 2-stroke I've owned in quite some time... is it OK to run the 30% heli fuel in it that I use in my 4-strokes?

i run 15 x 4 w on some of mine along with 14 x 6 both of those being apc in the 9k range richen

Usually with glow plug OS-8, the APC 14x6 gives me 11800 on the ground and 13x7 goes to 12000, all in fuel 10% nitro. With 20% and more % nitro the power is not going up
and the engine overheads. Naturally when I doubled the spacer under the head and use cooler glow plug I tried again and 14x6 went to 13000. Hope this direction is OK to go
to up to 30% nitro, or what I would do is to add more castor oil and methanol to the 30% heli fuel, it is cheaper and safer. Anyway, 15x4 is pretty good thing when the speed
is not a goal and for heavy model might be even better to run 14x4W(wide) with OS.91.

I was only considering 30% in the plane because I use that in all of my 4-strokes, and I also use gas in my giant scale... hate to have to stock and carry around a 3rd can of fuel! Sounds like as long as there's plenty of lubricant, it may not help, but it shouldn't hurt.

Looks like today's rain is clearing, maybe I'll get this thing to the field today and see if I bought a bargain or not!

In fact the Castos oil as lubricant is cooling much better than any synthetic one, and also delays the ignition timing.
Of course if you keep the 30% nitro fuel as a must, then the simplest is to place a second washer inder the head,
as they do in the helli-engine versions. Still with 30% nitro the engines get "older" much faster especially if they
are not cooled very well inside; that is what the Castor oil can only do. I have no experience with 4-stroke engines
and if there is something specific with their fuel; I gues only that high nitro % is getting you only more stable low
rpm but not much more power (if any at all) if you do not prevent the detonation due to premature ignition.

With propellers in the normal range, you can run your O.S. Max .91 engine from the high 9,000's to just about 11,000 RPM. Depends upon the propeller used. If you have a propeller that will let the engine run in that RPM range, and it lets you fly the model the way you want it to fly, you have the "correct" propeller.

15,000 RPM is much, much to high an RPM figure for normal running. That's where O.S. either found the power peak, or the maximum RPM you could run the engine before instant damage (the "red line" speed).