Originally Posted by Captain Smithy
You are correct, I don't think you can ruin an engine at power check RPM but depending on how much you've leaned the mixture it will start pinking. If it's a long taxi (not usually at my home base, but at my old airport it was a very long taxi) then I'll have the mixture back pretty far.
I highly doubt it. In order for detonation to occur, you need high cylinder temperatures and very high peak internal pressures, none of which you have available at run-up power setting. Have you ever checked where in the EGT curve are you exactly when you do ground leaning for taxi operations? It takes a lot of mixture lever movement to get to best power, let alone peak EGT. By "not leaning too much" on the ground, you're probably on a good path to kill yourself one day. If you lean the engine on the ground, do it brutally - right to the last point when the engine is still running smooth. By doing that, you have ensured that the engine is getting the minimum fuel required for combustion at idle power and - most importantly - if you forget the mixture before takeoff (I have and I'm probably not the only one), the engine will just cough and may even quit if you don't retard the throttle or increasing the mixture. However, if you don't lean the mixture enough on the ground and try to take off, the engine just might develop more than rated power, since there won't be any excessive fuel - and
that is when detonation would develop quite rapidly, right to the point of engine failure if not recognized very soon. If you don't understand the concept of ground leaning, just leave the mixture full rich - it's cheaper and safer in the long term.
Here are couple of quotes from Lycoming Flyer Key Operations:
First, we must know that cruise power for Lycoming normally aspirated engines is generally considered to be 55% to 75% of the maximum power for which the engine is rated. At these power settings, the engine may be leaned at any altitude.
It is also important to understand that leaning to roughness at the engine manufacturer’s recommended cruise power is not an indication of detonation, but indicates normal characteristics of distribution to the individual cylinders. The roughness indicates that the leanest cylinder has become so lean, it is beginning to miss. This is typical of an engine with a float-type carburetor. Damage, to an engine from leaning does not occur at the manufacturer’s recommended cruise power, but takes place at higher than cruise power.
On the other hand, with any direct-drive normally aspirated Lycoming engine, the pilot can and should lean the mixture at any altitude as long as the aircraft is in cruise configuration at 75% power or less.
Again, we repeat that maximum leaning (peak EGT) does not damage an engine at the engine manufacturer’s recommended cruise power. Damage is caused by maximum leaning at higher than recommended cruise power where the manuals do not spell it out or allow it, and when the aircraft does not have a complete set of reliable engine instruments to protect the power plants. Excessive leaning under the latter high power conditions can cause detonation and/or preignition and possible engine failure.