EASYTIGER is absolutely right. Even if it's true that some have had luck getting the .91 small head engine to work in a Byrojet fan, it's not supposed to. I'd bet that the small-head engines running in Byrojet setups have a lot of visible signs of overheating, too. Use the large-head for Byrojet applications. There is a good and simple reason for it--the small head doesn't have enough cooling-fin area to dissipate that much heat. The cooling air that the vacuum creates is a fraction of what it is in a tractor-fan setup. How do I know? Burned up an engine back in '93 because I didn't want to buy the large head, even though three different people told me to. Good luck!