RE: Telemetry from 90 powered FAI Formula I
You replace the O-ring every 6 to 10 flights. The heat kills them and the engine does not needle well when it is leaking. For a while, air shrink tubing was popular and was put over the exhaust adapter and pipe to prevent leaks. But at contests you often had to change engines between rounds, so the air shrink fell out favor with people. Also you need to hold the pipe with a spring pulled tight holding the pipe solid to a plywood pad to keep it from rotating or falling off.
I'm somewhat surprised that you could get a .90 in there and balanced, since the Nelsons and ST engines only weighed about 12 oz, and I would guess the .90 weighs nearly double that. Even with the light engines, most airplanes had the servo toward the rear of the wing saddle, and on one design I had the battery pack mounted behind the wing.
BTW, there was a thread some time back where they claimed straight line speeds of just over 200 with a racing .40 on nitro with a F1 design. I know when you put the nose down, these open exhaust engines unload far more than the 5000 rpm that they would unload on a race course. Unlike tuned pipe engines, the rpm is only limited by the aircrafts drag.