Design Point of the Cessna 172


The following question has been asked recently by a customer who flys a Cessna 172: “…with the D2J, the best fuel efficiency seems to occur near 55% power, say 2350 RPM at 8000 ft. ‘Just wondering what your design point was?”


The answer to this is, the PFS exhaust system for the Cessna 172 was technically optimized at 2450 RPM at sea level. In my flight testing experience, that RPM is the true “peak” point and it is still correct at altitudes as high as 8500 feet. I haven’t specifically tested the Electroair, but I understand how it works and it is the same ignition mapping as the much older LASAR ignition system which I did do some testing on. As the throttle is reduced (actually as the manifold pressure is reduced, but it is the same thing at lower altitudes) the ignition timing will advance as long as it is below 23 inches of manifold pressure. This ignition timing effect may alter the performance and therefore may affect where the peak RPM of the Power Flow is.


Green line represents Power Flow. Notice during over lap, where the exhaust gas valve and intake valve are both open, Power Flow creates a negative pressure to suck out all the exhaust gases.

To figure out what the actual performance curve in your aircraft is for a given altitude (fixed pitch props):

In level flight, set the throttle to 2300 RPM in level flight with it leaned using the same leaning methodology each time. Ie: 50 rich of peak or 75 Rich of peak. Wait 3 minutes for everything to stabilize and record fuel flow, airspeed and engine EGTS and CHTS. Then repeat the test at 2350 RPM, releaning the engine using the same method each time, and after waiting 3 minutes for everything to stabilize, again record fuel flow, airspeed and engine EGTS and CHTS. Add another 50 RPM increment and incrementally go up to 2500 RPM. Then you can look at the data and see at what RPM the best “bang for the buck” occurred. My money is on 2400-2450, but we would love to see your data!

Darren Tilman

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