Originally Posted by IO540
On the plus side, Deaking suggests that one cannot achieve detonation unless the CHT is above about 450F and 75% power (this was a TIO540 test I think). This suggests that one isn't actually going to damage the engine by incorrect leaning at any "normal" cruise setting
You're going to think I've got it in for you with recent posts, so please let me assure you this really is nothing personal.
But if you are going to quote Deakin's advice, I feel it's incumbent on you to get it correct!
Deakin is extremely outspoken in his belief that our engines, for longevity, should NOT be operated continuously (i.e. in the cruise) at anything like 450 degF CHT, or anything approaching 75% power unless (in that case) you are well, well away from peak CHT (i.e. well removed from circa 50 degF ROP EGT ... indeed, at 75% power the APS recommendation is to operate either richer than 140 degF ROP on the leanest cylinder or leaner than 30 degF LOP on the richest cylinder!). Even climbing at 450 degF CHT is a complete no-no for Deakin, although it is a value less than the 'Limitation' promulgated by both TCM and Lycoming (and therefore the Limit contained in our POHs).
His advice (in his Avweb articles, at the 2-day Advanced Pilot Engine Management Seminar in Ada and also in his on-line newsletters to APS graduates) is that 420 degF CHT is "an absolute-do-something-right-now redline", 400 degF is "a working limit ... engine monitors should be set to show/sound an alarm at this level", and that 380 degF should be "a working target which allows doing just about anything with a well set up engine".
Similarly, regarding power setting, he is dismissive of Lycoming's advice (to their publicly-demonstrated chagrin) that higher HP engines (especially the turbos) can be operated in the cruise (for reasonable TBO) at anything like 75% power if you are anywhere remotely in the region of peak CHT on any cylinder. He even believes that TCM's comparable figure of 65% may be on the high side, and strongly recommends a more conservative approach (for engines from either source) of 60-65% power.
Further, he is at pains to point out that, whilst anything other than light detonation is obviously disasterous for even short periods (and APS students get to watch the effects of a highly-instrumented test-rig engine being taken into light detonation ... so that's obviously not immediately catastrophic), you don't need simply to prevent detonation to achieve long cylinder lives and avoid premature (and potentially disasterous) failures. All it can take, he claims, to produce problems over a period of time is very high CHTs (i.e. >420 degF) resulting from very high internal combustion pressures due to mixture settings that are inappropriate for the other engine parameters.
You can agree or disagree with his advice (although it is based on ten plus years of hard data from the GAMI test rig). What you cannot reasonably do is to suggest this is actually not what he is saying!