Fly lean of peak engine operations like a pro with a Continental or Lycoming engine, I will teach you how to extend engine life with lower CHT's and save gas with NO ETA penalty. Remember, your car runs LOP automatically, no reason why you can't run LOP in your airplane manually with RPM and Mixture settings you understand.
ROP stands for Rich of Peak, that is, a richer mixture than the one you need to fly at peak EGT and you are keeping the cylinders cool with excess fuel.
LOP stands for Lean of Peak, that is, a leaner mixture than the one you need to fly at peak EGT and you are keeping cylinders cool with excess air. Fuel is expensive and Air is free!
Now that we know what ROP and LOP mean, we can define a simple rule of thumb to avoid any issue during flight.
Let’s divide our “engine operations” in three parts based on the percent of engine’s power output. To know percent of Maximum Continuous Power (MCP), base your engine settings on the reference tables in your aircraft POH. You must have balanced fuel distribution to each cylinder, correct timing on both Mags and correct spark plug gaps to run smoothly LOP.
High power (more than 75% of MCP)
LOP not recommended.
Medium power (more than 70% of MCP and 2400RPM)
The rule is do not lean between 150 ROP and 40 LOP. Keep hottest CHT below 375, TIT below 1650.
If you go richer than 150ROP the engine will be cooled enough by the fuel. If you go more than 40LOP, the engine will be cooled down by air, in the middle you’re stressing the engine with excessive cylinder pressure resulting in higher CHT's resulting in shorter life span.
Going LOP, will slow climb performance but will save a lot of fuel. The trick here is as TIT or hottest EGT go up in the climb (reduced airflow) you need to slightly lean back to 1600 (TIT or hottest EGT), as cylinders get hotter in the climb you can lean more aggressively to cool TIT/EGT and CHT's. I do this several different times in a LOP climb as TIT goes above 1625, I tap (reduce) the mixture slightly and watch both TIT/EGT and CHT go down, a few more thousand feet of climb, there they are going up again and I lean ever so slightly and they retreat once again. If indicated air speed diminishes below 110kts or engine runs rough (too lean) I would transition to ROP climb.
Medium power (more than 65% of MCP and 2300RPM)
Lean less than 25 LOP or go richer than 125ROP. The reasons are the same as above. The interval between the limits has tightened because the engine internal pressure is lower than before.
Low power (less than 65% of MCP and 2300RPM)
You’re a free soul here, lean as much as much you like. You’re not going to hurt cylinders here, just don't shut down the engine!