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Reduced Throttle

David Z

Well Known Member
There's a concept I've been bouncing around in my head as a thought experiment. Not for or against the idea, just looking to explore the concept with those smarter than myself.

Old piston engine airliners had different power ratings available based on the different grade of avgas in the tanks. Using 115/145 would permit a higher manifold pressure than using 100/130.

Higher compression ratio burns fuel more efficiently. Auto manufacturers are forever looking at how to increase compression ratio to increase efficiency. Our general trend with Lycoming engines is to have a low compression ratio that will be happy using something other than 100LL avgas.

What's the potential for this with our little engines? We could run 10:1 compression, but then put a redline on the manifold pressure gauge and use a maximum of (for example) 24 inHg. All but high altitude take-offs would be partial throttle. This could give us the higher efficiency of the higher compression ratio. Then limiting manifold pressure could control detonation with lower octane fuel.

Am I onto something, completely wacked out? Is there enough of an efficiency difference on our engines to be worth the hassle?

Watching the manifold pressure gauge while setting reduced take-off power is distracting when we really should be looking outside. I'd hate to save $1/hr on fuel just to be distracted on takeoff with the manifold pressure limit and take out some runway lights. Then spend $2000 fixing the plane.
 
Makes total sense...P2 NATOPS had a different chart for 100/130. Purple gas is not really available so this is what we used.

The thought crossed my mind, but I decided on one take 100LL and the other pump gas at lower power settings.
 
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