Can someone explain to me what the different nitro contents will have on a motor? I have an O.S LA 40 and I fly at 5000 feet. I also have a Moki 1.35 which calls for 0% to 5%. Thank you.

I just looked up a pressure chart for altitude and it gave a figure of 12.23 psi @ 5000 feet compared to 14.7 at sea level so there's near enough 83% less air at 5000 feet. So from this you could say your engine will have 17% less HP than it would at sea level. Adding more nitro than what you might use at sea level will help boost the HP closer to the nominal sea level power. You can actually use more nitro than the engine could handle safely at sea level because the lower air pressure effectively reduces the compression. Another way to look at it though is that an engine that's 17% larger will give the power of an LA 40 at sea level. An LA 46 fits this perfectly

Other complications though are that the thinner air means less bite for the prop and less lift from the wings but at 5000 feet I doubt it'd be really noticeable.

the more nitro the more power.