It needs to be that low because it has a very high compression ratio. Any higher and the nitro detonates and the engine will run real hot, then flame out. I think it was made for overseas where low nitro fuel is more common. However it gets good power on cheaper fuel. Not exactly a slouch on power, but not quite as good as a modern .91, but it does burn less fuel. Great for a tail heavy .60 sport plane. Doesn't burn much more fuel than a .60 and gobs more power. Also this is an ABC engine most modern .91's are ringed.