A vs B Curve with a Single PMag

Tried calling Emag Air yesterday but it was late on a Friday. Here's my musings for which I need some opining...

I normally want to travel efficiently as possible so I'm usually 55-60% power as LOP as I can run smoothly on a carb. I'm running on a Slick Impulse on the left and a PMag on the right.

At low power settings (pilot or altitude induced) and leaned, the PMag advances the ignition to the point were the Slick Mag is dead weight. With the PMag spark doing all the work this tells me the combustion process is taking longer because of the single flame front. This would lead me to believe a more advanced (B curve) ignition would provide greater efficiency in this condition. However, if I switch to the B curve I would loose some detonation margin and also high-power/low altitude efficiency with the ignition advanced too much.

What are you thoughts regarding combustion delay with a single PMag?
Is the only way to get the best of both A and B curve worlds manually manipulating the advance in real time with EI Commander?
There's some conflicting information regarding the static/base timing of the A and B curves, I've read 20/25 respectively and 26.6/30.8 - Does anyone know what is correct?

This is on a RV-9A - O-320-E2D with 8.5:1 pistons FP prop

Not knowing which engine, I can't speak with certainty. However with my IO360 and IO390, I found no real improvement, even high up, with the B curve. After playing with it and burning some fuel during testing, I have the EICommander so I can change the timing easily, mine was left on the A curve.

You might be surprised how much the Slick with fixed timing is still contributing, even with so much advance on the Pmag side. Even though that spark is late to the party, it still burns off the side that the other flame front may not get to until much later. So it does still result in an increase in chamber pressure at the critical 5--15 degree ATDC point.

Try turning it off to see the difference.

The slick is not dead weight and you do have two flame fronts. You can experiment yourself. Turn off the mag in flight. Should see EGTs rise and airspeed drop once the mag stops sparking, more so when LOP, with the engine wanting more advance. Effective timing (i.e. what gets you to peak pressure) is a blended combination of the two different plugs spark advances.

You are seeing the limitations of a pre-packaged, non-adjustable system. You need different timing for different combinations of several different factors, most notable being MAP and mixture.

scsmith said:

You might be surprised how much the Slick with fixed timing is still contributing, even with so much advance on the Pmag side. Even though that spark is late to the party, it still burns off the side that the other flame front may not get to until much later. So it does still result in an increase in chamber pressure at the critical 5--15 degree ATDC point.

Try turning it off to see the difference.

Click to expand...

I did this when running LOP under my normal cruise conditions (55-60% power)...there is no drop when going from Both to Pmag only and when I go from Both to the Slick the engine barely runs and I see a 200-300 RPM drop. The lack of an RPM drop on the PMag led me to believe timing is advanced far enough in this situation it's doing all the work. I'll have to look at EGT's again to confirm.

The Slick is definitely bringing something to the party. Remember that the end game is peak cylinder pressure at the optimum crank angle. How that happens does not matter whether the combustion process is initiated with a magneto, electronic ignition, laser beam, or flint.

PCP is a composite of the two flame fronts in an aircraft engine, and compensating for the significantly retarded magneto contribution is the key to achieving optimum efficiency. The question is if the Pmag A or B curve is appropriate. Maybe yes, maybe no. The only answer is found in a instrumented, altitude compensated dyno room, or in situ testing.

Toobuilder said:

The only answer is found in a instrumented, altitude compensated dyno room.

Click to expand...

Dang...just sold mine

Had a lengthy conversation with Brad at EMag Air...He thinks going from A to B will have little effect in cruise but will have a big (negative) effect at takeoff. He said advancing to B curve is too much to do all at once. In fact he said to try the A curve retarded a couple degrees using the clocking method, which falls inline with the recommendations on VAF.

He did say that in LOP economy cruise with Emag advanced out 9-10 degrees the Slick isn't doing much. He cites that the combustion process timing has mostly to do with the initial kernel of flame starting from the ignition source and since the Slick is late to the game (even though the burn rate is 35-40 deg of revs) it's not doing a whole lot. This is confirmed by doing a inflight mag check and running from Both to Emag only, has no drop in RPM.

avrojockey said:

...
What are you thoughts regarding combustion delay with a single PMag?
Is the only way to get the best of both A and B curve worlds manually manipulating the advance in real time with EI Commander?
There's some conflicting information regarding the static/base timing of the A and B curves, I've read 20/25 respectively and 26.6/30.8 - Does anyone know what is correct?

This is on a RV-9A - O-320-E2D with 8.5:1 pistons FP prop

Click to expand...

The B curve (no jumper) is essentially for an A65 low compression engine.

The A curve (jumper in) works just ok for a 8.5:1 but can be configured for better performance.

See this thread for more details (and check your PM's).
http://www.vansairforce.com/community/showthread.php?t=167948

Best power is ~25* BTDC for most of us. Additional advance will make CHTs hotter and reduce climb/top speed performance. That is why many PMAG users are now running A curve or retarding the timing by 3 teeth on the flywheel.

rwtalbot said:

Best power is ~25* BTDC for most of us. Additional advance will make CHTs hotter and reduce climb/top speed performance. That is why many PMAG users are now running A curve as well as retarding the timing by 3 teeth on the flywheel.

Click to expand...

For a 149 tooth ring gear, that?s 360/149 = 2.4 degrees/tooth, so retarding by 3 teeth means 3*2.4 = 7.2 degrees. Might be a bit more than necessary. I do 1.5 teeth or 3.6 degrees.

Erich

erich weaver said:

For a 149 tooth ring gear, that’s 360/149 = 2.4 degrees/tooth, so retarding by 3 teeth means 3*2.4 = 7.2 degrees. Might be a bit more than necessary. I do 1.5 teeth or 3.6 degrees.

Erich

Click to expand...

It works out really well if you didn't install the jumper and still on the factory B curve. Nothing to come loose or break. Update my previous post.

Simple setup is, find TDC, then go 2 degrees past TDC on the downward stroke. Do the blow in tube or whatever sets TDC for your style EI, that takes two degrees off the base timing, then run the most conservative advance profile.

When LOP at higher altitudes you do not need much past 28 degrees maybe 29. At idle they do run nice with 30+ but who idles their plane everywhere

When ROP at sea level and take off, your traditional mag is actually firing the plugs around 23 degrees +/- a bit every time. This is how they were intended.

An EI fires very precisely every time. Think about the effect on effective timing and Theta PP.

For those who have never seen traces of cylinder pressure on a piston aero engine before, believe me it makes a difference.

Hope that helps.