Doug,

 

I have a problem with your argument.  It seems to be upside down.  In reference I refer you to my 11/2/2010 email entitled "NA Injected Engine Performance LOP".  In any event, for any given proper fuel/air ratio (mixture), there is a narrow timing range that produces efficient cylinder pressures at the correct crank angle range.  The Lycoming engine manual (for my 320) displays the acceptable range for the EGT for best power as 100F to 180F ROP, and best economy as 0F to 80F LOP.  Running 50F ROP is very bad for cylinder pressures as stated by GAMI - thus your argument of moving timing by messing with the mixture is, uh, not good operational practice.

 

Secondly, you imply that the only function of an electronic ignition is to advance the timing.  Au contraire mon ami!  An electronic ignition provides for a hotter, longer, more consistent spark over a greater plug gap that better insures the reliable and timely ignition of the combustible mixture even though the F/A may be at some extreme.  This results in greater efficiency/HP depending on the way one looks at it.  See the pamphlet on the benefits of the LASAR electronic ignition that shows speed increases for the same fuel burn or reduced fuel burn at the same speed of a magneto driven engine.

 

The fixed timing of a magneto (i.e. 25 DBTDC) is a compromise.  Generally speaking, EIs advance the base spark timing minimally until the power, as measured by RPM and MAP, is reduced below 75%.  Bad events, such as pre-ignition, detonation and incorrectly timed cyl pressure, are less likely in this regime (that is why the GAMI lean test is done at less than 75% power).  Furthermore, EGTs (representing F/A ratios) are still maintained in the engine manufacturer's recommended ranges and the timing is a better match for the combustion event pressures.

 

Note that during my best power cruises at middle altitudes (2500 RPM, MAP > 22", EGT > 120F ROP), the EI timing is about 25 DBTDC plus or minus a degree. 

 

Yes, more HP.  Better speed for the same fuel burn holding other conditions equal.

Yes, in all the modes you mentioned.

 

I haven't reached TBO yet, I go too fast to accumulate that much time.

 

Scott Krueger

IO 320 Dual Lightspeed Plasma III EI

 

At the risk of starting a “theological” controversy, I would like to raise some reservations about electronic ignition systems (other than reliability and loss of electrical power)

 

Peak intracylinder pressure (the point in the combustion cycle at which pressure in the cylinder is highest = effective timing) is related to 3 things; spark timing, rpm and MIXTURE. 

 

Why mixture? Because air and fuel mixtures burn at different rates depending on how rich or lean they are.  So at a given RPM, MAP and spark advance you can vary your effective timing by changing the mixture. 

 

·         Want to advance your timing? Change your mixture to a 50 deg ROP mixture – which is the fastest burning.

·         Want to retard your timing?  Make your mixture richer or leaner from 50 deg ROP.

 

And since optimal power is achieved at an effective timing of 16 deg ATDC, advancing the spark timing may or MAY NOT increase horsepower depending on the mixture.  But advancing the timing will likely increase the magnitude of peak intracylinder pressure leading to higher intracylinder pressures, higher cylinder head temperatures and greater stress = shorter enging life.

 

For those of you who are using electronic ignition (I am not):

·         Are you sure you are getting more horsepower?  How do you know?

·         If you are getting more horsepower, are you getting it during all modes of engine operation?  Rich of peak, lean of peak, high MAP, low MAP, etc

·         What is happening to your TBO?

 

D. Brunner

 

The biggest advantage of any of the electronic systems (in my opinon) is that they provide a spark advance that is a function of at least manifold pressure, giving an advantage any time the manifold pressure is much lower than maybe 25 inches.  But there is very little combustion taking place in the exhaust system regardless.  The reason the exhaust temperature rises is that more of the combustion occurs after TDC and that means less of the energy is being converted to work.  The down side of advancing the spark is that since more combustion occurs at the highest cylinder pressure, more heat is transferred to the cylinder head and piston.