If there is a difference between the POH & engine manual, I'd follow the POH.
I think the engine manual information is more for workshop / overhaulers to follow as they work on the engines by itself.
POH is the related to both engine & airframe combination.
And when bot the engine and airframe manufacturer contradict them selves and each other what then?
And no I am not joking.
plus much depends on options do you have CHT/EGT etc...always read the handbooks...they tell you everything!
Well that is half true. It depends, a good EDM or similar, and some good education and you will be 100% better off than just following the POH for doing mag checks. In fact the methods in POH's really just prove the mags have not been stolen.
For those who have well equipped machines learn to use the gear, and even the JPI manuals are not quite perfect, although better than most.
For those who think doing the mag check full rich is the way to go. Think again. I have no doubt the knockers will arrive soon, so do your homework properly and seek the truth. Even in your C152 with no gauges. Tacho will do!
