I heartily agree with W8ye on the OS91. Not the best effort by OS. I have a ST G90 on a Stick with a 16oz tank in it and a 13X6 MAS on it. Another fellow has the OS91 in a Cap580 with the same tank and prop. I can pull him....not by alot but I can pull him and he has a cleaner plane. He has also sent the engine back and tried it again and has now sold it. He works in a Hobby shop in Orlando and has verified that they have had many returns on these engines. Another guy just bought a Tower 75 and his Stick runs right with mine...a very powerful engine. Also have a radar gun and we checked speeds of the three planes. Stick/G90=103mph, Stick/Tower 75=101mph, and the Cap 580/OS 91=101mph. Tower has a Hell of a punch for the money in my book!