Seems to me that rpm changes effectively adjust ignition timing. For example, if you're running at 2500 rpm and the spark occurs at 25 btc, the flame propagates at a rate that produces max pressure at some point after tdc, lets just say 15 degrees. If you reduced the rpm to 2200 and the spark occurred at 25 btc the flame would propagate at the same rate as before but the piston is moving slower. Seems to me that the max pressure would occur at somewhere less than 15 after tdc, effectively advancing the ignition.

 

Most agree that the majority of the advantage of adjusting spark timing happens at lower power settings. To me, it looks like the advantage is better fuel economy as opposed to more power. Couldn't one simply pull the prop control back to gain this same advantage?

Bill,

 

Let's turn this around a bit.  Reduced power settings can be achieved by lowering the RPM, diminishing the MAP or both.  Suppose I am flying at 12000 feet with an itsy bitsy NA engine with no ram air recovery running ROP and fixed timing.  I will have been forced into a reduced power setting because at WOT I would see a MAP of approximately 18" while operating at 2500 RPM.  Regardless of compression ratio, there was much less density for my air pump to work with and, in order to have an A/F ratio as indicated in my engine manual, I have leaned to say 8 GPH - perhaps 100 ROP.  But alas, the fire doesn't burn as fast and the piston isn't getting the full benefit of combustion - some maybe even finishing up in the exhaust pipe.  An adjustment might be to advance the spark, start the fire sooner and move more of the combustion event back into the cylinder.

 

If the spark is advanced, things change. More of the fire burns in the cylinder. CHTs are higher, EGTs less.  But, so is the peak EGT less.  In re-leaning the engine for 100 ROP, one notes that the fuel flow has dropped to 6.5 GPH.

 

Now for the question. Is it economy or power?  Well, the MAP and RPM have remained the same.  Wait, I have a Constant Speed prop.  Has the pitch changed?  Has the speed changed?  Perhaps a bit.  But certainly the speed has not dropped.  Same speed for less fuel.  That is economy but it is also power since I have achieved that speed more efficiently.

 

Without advancing the timing, when I pull my prop back to match the lower fuel flow (thus reducing power further), my plane economically slows down.  Of course, this means I will have to stop short for a Krispy Kreme donut, more coffee to keep me awake and more fuel for the plane.  All because I wasn't fast and efficient.

 

With a tail wind, an EI and running LOP I can travel to the end of the rainbow where there is no lightning.