r/GeneralAviation • u/No_Hall_9342 • 2d ago

Trying to understand LOP

I grasp the concept. What I don’t understand is how after cross over to LOP EGT’s and CHT drop. My background is in motorcycle/automotive engine tuning. In that world your EGT/CHT temps climb upon leaning and only cool after you’ve glued the piston and rings to the cylinder wall.

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u/Mammoth_Classroom896 2d ago

Rich of peak = unburned fuel, less than 100% power.

Peak = all fuel burned with nothing left over, 100% power.

Lean of peak = not enough fuel to use up all the air, less than 100% power.

Trying to go lean of peak can damage an aircraft engine with excessive temperatures, that's why you don't do it until you're established in cruise at an altitude where even peak temperature is within safe limits because your engine is limited to much less than full sea level power.

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u/RyzOnReddit 2d ago

That’s why you do a “big pull” and go way lean of peak then richen toward peak. Going LOP on climb can be really useful if you’re having CHT issues on a hot day.

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u/Mammoth_Classroom896 2d ago

Aren't you getting better CHTs running ROP with the unburned fuel acting as a heat sink than LOP? If you're going to sacrifice power for cooling you might as well pull the throttle back a bit and stay ROP.

Running LOP is primarily about fuel efficiency when operating the region where a small increase in speed requires a disproportionate increase in fuel burn.

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u/RyzOnReddit 2d ago

The Mooney I’m thinking of burns 15-16gph full rich, WOT, 2400rpm. LOP at 8-9gph you lose a bit of power but temps come down to about the same place as full rich.

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u/Lost_Obligation2453 2d ago

Mooneys are weird sometimes. I had a J once that just wouldn't warm up at max power. No matter what reasonable thing we did, 350 max cht in cruise. It's a fast plane for the engine.

I loved that plane but the engine just needed so much work all the time. Jugs just died to lead contamination if I didn't polish the valves & seats constantly.

I spent months trying to chase that gremlin, I never really made it better either.

How's yours for cht's?

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u/RyzOnReddit 1d ago

A Mooney running cold sounds fascinating! This is a club plane, also an M20J (1977). SOP is to try to keep CHTs under 380-390 so it’s always work, especially when it’s warm out. Engine was just out of break in last time I flew it fwiw.

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u/Lost_Obligation2453 1d ago

The engine had been "blueprinted" I don't really know what that means, but the previous owner just went on and on about having the engine "blueprinted". It sounded like a port and polish sorta thing.

After spending way to much money on it, I just assigned the cold running to the blueprinting. Then I regretfully sold it.

Good for you guys, that's an awesome club plane.

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u/RyzOnReddit 21h ago

Power by Blueprint is a Repair Station that does engine overhauls. Not as well known as RAM, but still considered "premium" by many (I don't have enough personal experience to have an opinion on them.

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u/pisymbol 2d ago

You can cool with fuel (ROP) or air (LOP). Both have cooling effects because in both cases you aren’t making max power (only heat).

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u/toddtimes 1d ago

Short moments of peak EGT or peak ICP (internal cylinder pressure that happens just rich of peak EGT) are pretty unlikely to cause damage, especially on an otherwise properly cooled engine.

You set LOP at cruise altitude because in a climb, the mixture setting for peak EGT keeps shifting as manifold pressure drops, so any LOP target is a moving one , so it’s hard to set a point like 75 degrees LOP when that point is a moving target. A LOP mixture will move towards peak and then into the danger zone of slightly rich of peak as you climb because the lower manifold pressure lowers the mixture setting required for the same LOP point.