Detonation is really the only important factor in the entire LOP subject, and the one that seems to have the most mystery surrounding it.I suspect that resolution would lift the fog surrounding LOP ops once and for all.
Yep.
Refer back to post #41 and the standard Lycoming power chart.
I don't know if Lycoming meant that line to be a detonation line or not....
Oh ye of little faith
Lycoming offers some discussion of the limit line in
Lycoming Flyer Key Reprints (page 44, lower left), but like some of you I prefer straight data. The following charts are from a Lycoming test cell, detonation runs made with 100LL for baseline comparison to the same IO-360 on an ethanol fuel. It takes more than three charts to illustrate the entire limit line as found on the power charts, but you can get a taste.
Look close at the operating temperature conditions. All the runs are made in accordance with FAR 33.47 and its companion AC 33.47-1 dealing with how detonation tests shall be conducted. At least one CHT must be at max and the others close to it, oil temp is near max, and intake air temperature is ~100F......pro-detonation conditions.
First up, 26.8 MAP and 2700 RPM. From the top down you have CHT, EGT, power, BSFC and detonation. The detonation graph is expressed as a percentage of firing events. In this test they pulled mixture from rich through peak to 40-60 degrees LOP. Detonation percentage is zero:
Let's make things more difficult, full throttle, 28.5 MAP and 2700 RPM. Again mixture is pulled from bog rich to LOP. There's a slight nibble of detonation right at peak EGT (less than 5% of firing events) and well lean of best power:
Ok, let's see that detonation limit. This run is again 28.5 MAP, but the RPM is hauled back to 2400 RPM....ouch. It detonates 10%, maybe 12% at ~25 ROP, a little lean of best power. You could even argue power fell off because it started to detonate. Whatever,
it's the limit:
28.5 and 2400, the upper end of the limit line on the power chart. There's your red box for a low compression NA Lycoming on 100LL:
Keep it cool and pay attention to what you do with the prop control, and it becomes difficult to hurt it with the mixture control. You don't need twenty pages of Deakin or a trip to charm school. You really don't need to understand much more than the basics, the very first graph in the first post of this thread.....information published by
both GAMI and Lycoming.
Of course if you run more ignition advance, less octane, high compression pistons, a turbo, or anything else which steps away from the base version, you get to find your own detonation limits.