Ignition timing

[Bones]

Does anyone have a good discussion on how ignition timing is determined for a given engine?

[hgerhardt]

I wondered that too, as I have an IO-360-A3B6D (angle-valve 200HP) in my -6 that's spec'd at 25 deg, when most 8.7:1 angle-valves are spec'd at 20 deg. I heard from a friend who allegedly asked a Lycoming engineer why that was, and he said it was so that Mooney (the original application of my engine) could make the performance numbers it promised. Apparently, the engine made just enough more power at 25 deg to make the numbers. That begs the question of if the engine could tolerate more timing, why weren't ALL of the 8.7:1 engines re-rated at the higher advance? As far as I can tell, Mooney 201 J's didn't have any better cooling than the usual spam can of the day and therefore weren't any more or less tolerant of higher advance. Anybody have any more insight?

Heinrich Gerhardt
RV-6, flying

[sprucemoose]

Originally Posted by Bones

Does anyone have a good discussion on how ignition timing is determined for a given engine?

I assume you mean more than just looking at the number on the data plate?

Magnetos are kind of like fixed pitch props. They are a compromise, designed to run OK over the whole range of RPM/ MP. Electronic ignition, with variable timing, is like adding a C/S prop to your engine.

Don't you need to update your picture now?

[Bones]

The real question was, "how did they come up with number on the placard."

[elippse]

Originally Posted by Bones

Does anyone have a good discussion on how ignition timing is determined for a given engine?

Combustion chamber shape has a lot to do with detonation performance. Fixed timing is a compromise of the different temperatures and pressures that can be expected at airports all over the world to give some degree of detonation margin. For instance, if you are taking off from a sea-level airport where the pressure is 31" and the temperature is -30F, your density altitude is going to be about -6700' and you are going to considerably exceed 100% power at rated rpm if you give the plane full throttle.

[Geico266]

Originally Posted by sprucemoose

Electronic ignition, with variable timing, is like adding a C/S prop to your engine.

While I agree EI may be of some limited benifit and aids in starting, it's not like adding a CS prop to an engine, not even close. The HP gains are usually greatly exagerated by the guys making the EI's. I've taken an IO-540 from EI back to Slicks with very little, to NO difference in performance & fuel burn. Certainly, IMHO not worth price or the risk involved. Granted, I'm pretty much turned off to EI's right now due to recent in flight experiences, the materials used in making the EI's, and other problems I have heard about in talking to other pilots. After market EI on Lycomings is just not worth the risk at this point in time. JMHO.

Not trying to start a war, JHMO.

[David-aviator]

Originally Posted by Bones

The real question was, "how did they come up with number on the placard."

For sure they don't pull it out of their butt, Larry. There has to be lots of engineering considerations concerning burn rate, compression, detonation, and cooling at high power settings.

There may be something magic about 24 degrees, however. When I was checking myself out with Subaru and had the OBDII scanner hooked up, it too ran at about 24 degrees at take off power. Pull the throttle back and timing advanced as far 42 degrees BTDC.

So far, I don't think anyone who really knows about this stuff has checked in here....

[AlexPeterson]

Originally Posted by Bones

The real question was, "how did they come up with number on the placard."

"They" probably all croaked long ago....

[hecilopter]

Just a guess, but I would expect that 25 BTDC is the optimum setting for max power for a Lyc when at 75% of rated HP at full throttle (ie. at ~8000 ft density altitude). Based on that, when running in the same conditions, I would bet electronic ignition is at about 25 degrees as well. When you pull the power to less than that, the EI will advance the timing and give a more optimal fuel burn. It would be interesting to see the fuel burns/timing advance of the same engine equipped with Mags and then swapped out for EI when run at 75%. It would make sense that the efficiency is about the same at 75% power, then the EI would be a little more efficient when the power was pulled back. I have Slicks and normally cruise with the throttle to the stop to minimize pumping losses and about 2350 rpm, leaned to 1320 EGT. I routinely see about 201 MPH true (174 knots) at that setting and see about an average of 7.5 - 8 GPH at the pump. I don't think EI would help me much there. JMHO.

[Geico266]

Originally Posted by hecilopter

It would make sense that the efficiency is about the same at 75% power, then the EI would be a little more efficient when the power was pulled back.

I agree in theory, but in practice when I run LOP I burn 10.2 GPH @ 170MPH with dual Slicks. Those were the same numbers I had with the EI. Certainly, nothing like what is claimed by the EI manufacturers. Granted conditions change, temps, baro pressure, all that. Nothing I could not live without, or (for what ever reason) take the chance on another inflight ignition failure.