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On 8/15/2010 9:03 AM, Ed Anderson wrote:
The engine doesn't know or care how this increase in pressure comes about - well, actually it does. If you use a turbo or supercharger you are also heating the air through their compression process (which is why you need an intercooler in many cases) thereby moving closer to detonation. Ed, this sentence summarizes the advice I received from the Aussie rotary expert, Leon Promet (anyone know what became of him?) and others. The way he put it, using the non-turbo (9.7:1) rotors in a turbo installation gets you an extra 30 HP "for free" because this part of the increased pressure doesn't involve heating the air via the turbo. You still have to deal with the potential detonation either way if mixture and timing are wrong.
My experience with detonation (or pre-detonation) was much less severe than yours. I heard 'popping' at high rpm during run-up at around 44MAP. Reduce the throttle a hair and the popping ceased. This was with a big fixed pitch prop that I could only turn at around 4900 rpm. I had the feeling that it was rather like flooring a car in top gear on a hill. (remember in the old days before ECUs and knock sensors when that would get you a popping sound?). I was able to fly my first 40 hours with no issues from detonation by keeping the throttle away from the area that caused it during runup. I never experienced (or at least heard) any popping once airborne, even during a high-speed run at 44 MAP and 6300 rpm. I once tried hooking up a knock sensor to the intercom without success. I never had any damage to the plugs.
When I installed the adjustable IVO prop my static rpm when from 4900 to 5600rpm and the popping sound disappeared (analogous to changing gear on the hill?). I haven't experienced any popping since. I think Tracy's ECU does a good job of providing a rich mixture at high rpm to keep you away from detonation.
Regards,
John Slade
Cozy IV, N96PM
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