Originally Posted by
SNS3Guppy
Spark and timing occurs as a function of a fixed magneto position relative to crank position, not mixture setting. The spark fires at the same point BTDC each time. Preignition isn't an issue, though detonation can be. The spark doesn't advance, and there isn't use of a variable timing or advance mechanism on most aircraft engines.
While you are correct that the spark fires relative to crank position and nothing (in a typical GA engine) changes this; in a timing sense (i.e. how many msec before TDC) the spark fires is clearly effected by RPM. One of the 'problems' with LOP operation is that the flame front spreads much slower than, say 50 degrees ROP, so the effective maximum pressure (what the timing is trying to position at 14 ATDC) happens later (in time) LOP than ROP and hence at lower mechanical efficiency. Slowing the RPM makes this happen earlier. 2500 vs 2200 bring the peak power pulse about 25 microseconds earlier (3 - 4 degrees at 2200)
At a constant power setting (ie. 200 hp) and 30 in MP (and everything else constant except rpm and fuel flow), I get a significantly better IAS/Gal running at lower RPM than higher, on the order of 10%. How much of this is due to prop efficiency, reduced pumping loss, better timing of peak power pulse - I have no idea - but it works!
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Back to the more simple question asked. One more point,
If you are not at full throttle and you open the throttle you should always get an increase in MP as you are reducing the intake restriction.
If your question was,
'My POH says I get 75% power at 25/2500 and I am high enough the outside air pressure will only 24 as a maximum MP, should I throttle back to not waste fuel?'
the answer is emphatically no - for all of the technical reasons already posted.