[smoknrv4]

It needs to be a rear exhaust to fit in a fan unit...a side exhaust won't work. Also when we say at least 20000k rpm...thats ground peak rpm, 45 DF engines will unload to 24000k and higher in the air, which is necessary for thrust to weight ratio of the aircraft, the side exhaust engine is probably for sport flying use and will not reach those rpms without destroying it first, the 40 rear exhaust engine may work, but you'll have to mount it in the fan and then tach the fan to see what kind of ground rpm your getting to see if its enough, mind you with the front intake carb it may be difficult to mount in the fan without hacking it up.