We flew the Kelly F1GT 80cc and the Tsunami Unlimited 289cc Saturday at Williams, 6835 feet...No problems....Engine speed according to a computer program that uses the sound, about 9500 rpm with our 31 pitch prop....air speed ? 250 ++..
OS 160 glow engines work very well up here wiith 20-6 props....
Altitude is a non factor, you just allow for a little little less power..NO long scientific dissertations...