[vlittle]

I'm seeking the answer from knowledgable engine guys for the following observed operation of my engine: O-320, Superior roller lifter crankcase, ECI cylinders, dual Slick mags:

Right from the beginning, this engine has run well with plenty of power. After 106 hours, I checked the timing (I did not check when the engine was new). I found that one mag was about 35 degrees BTDD and the other about 30 BTDC.

I adjusted both to 25 BTDC.

Before adjusting, I could run the engine well lean of peak with no roughness, only an RPM drop off as it got very lean. Now, there is a very distinct point of roughness as I pass peak EGT.

This is not a problem, but it got me thinking: why was it so smooth when the mags were mis-timed? I have my theories, but I'm not an engine guy.

My guess is:

Since the timing was so advanced, it gave the lean fuel/air mixture more time to burn completely in the combustion chamber, thus reducing lean misfire. I always tried to run 55%-65% lean 'best economy' or LOP (when I could), and watched my CHTs for temperature spiking... never saw anything that could indicate detonation.

Perhaps this is why electronic ignitions seem to be better for LOP operation... they advance the timing. If this is the case, it's not a function of having electronic ignition per se... it's just the spark timing that allows LOP.

Any experts out there with an explanation?

 

[dan]

Are you positive you adjusted the timing properly? Impulse coupler(s)?

 

[videobobk]

Advanced mags

FWIW, I just had my timing checked and set by an A&P. He is one of those "been there, done that" types. Both mags were about 3 degrees too far advanced (28 BTDC) and he said to leave 'em alone as I would burn less fuel. It runs great now, so why fight it? I have an E-Mag arriving today and I think the 3 degrees may keep it and the remaining mag more nearly in time across the rpm range.

Bob Kelly

 

[Walter Atkinson]

Be aware that advancing the timing dramatically increases the internal cylinder pressures and at high power settings ROP it significantly reduces the detonation margin.

Advancing the timing is not the proper way to get an engine to run smoothly LOP. Balancing F:A ratios and having a very healthy ignition system is. Advancing the timing does NOT automatically make an engine more efficient. In some cases it will reduce the efficiency. In many cases it will actually reduce Hp output.

Monkeying with the timing without getting good pressure trace data is risky.

Walter

 

[airguy]

Be aware that advancing the timing dramatically increases the internal cylinder pressures and at high power settings ROP it significantly reduces the detonation margin.

Advancing the timing is not the proper way to get an engine to run smoothly LOP. Balancing F:A ratios and having a very healthy ignition system is. Advancing the timing does NOT automatically make an engine more efficient. In some cases it will reduce the efficiency. In many cases it will actually reduce Hp output.

Monkeying with the timing without getting good pressure trace data is risky.

Walter

Absolutely true, though advancing the timing is ONE of the key factors in operating CORRECTLY at LOP. LOP mixtures combust with a slower flame front than a stoichiometric or an overrich (within reason) mixture. Advanced timing gives that slower flame front time to build pressure before the pistion begins it's downstroke - but it's only one part of the puzzle.

 

[vlittle]

Be aware that advancing the timing dramatically increases the internal cylinder pressures and at high power settings ROP it significantly reduces the detonation margin.

Advancing the timing is not the proper way to get an engine to run smoothly LOP. Balancing F:A ratios and having a very healthy ignition system is. Advancing the timing does NOT automatically make an engine more efficient. In some cases it will reduce the efficiency. In many cases it will actually reduce Hp output.

Monkeying with the timing without getting good pressure trace data is risky.

Walter

I agree. I am passing on my observations, not recommending running magnetos with advanced timing.

My timing has been reset to 25BTDC and the engine is running fine. Climbing 2400 fpm solo weight in an O-320 FP RV-9A is a good measure of performance.

My question is really about the affect of timing advance on LOP operations. I'm aware of the issues with detonation margin.

As for intracylinder pressures, how do the electronic ignition guys know what they are? I think they just estimate from rpm and map, and hope for the best.

V