Q: I see 400° to 415° CHT on takeoff and 360° to 400° in cruise, depending on leaning, cowl flaps setting, etc. What’s acceptable on the O-540 Lycoming?
If I don’t lean much it’ll do 330-350, but that thing is thirsty running like this.
A: Scott, if you’re seeing 400° to 415°F during takeoff and 360° to 400° in cruise, you couldn’t ask for anything better, as long as you are confident in the accuracy of your instrumentation.
The maximum cylinder head temperature allowed for continuous operation on most of the Lycoming O-540 series engines is 500°F.
For best service life, from my personal experience, run the CHT between 380° and 400°.
If you have a six point CHT probe system, this would allow you to compare readings from all cylinders under different power settings. Of course the rule of thumb is when leaning, lean to the leanest cylinder.
The thing is, on a normally aspirated engine with a carburetor, the lean cylinder may be in a different cylinder depending on the specific power setting.
The reason for this, as I’ve mentioned in the past, is because the fuel distribution in an engine with a carburetor is poor at best and each cylinder is likely to be receiving a slight difference in the fuel/air ratio. The different lengths of the intake pipes contribute to this condition, unlike a fuel injected engine where a specific amount of fuel is delivered to each individual cylinder.
I think you are right in the ballpark with what you’ve got now and I see no reason to make any changes in your operating procedures.