I assume you mean more than just looking at the number on the data plate?Does anyone have a good discussion on how ignition timing is determined for a given engine?
Does anyone have a good discussion on how ignition timing is determined for a given engine?
Electronic ignition, with variable timing, is like adding a C/S prop to your engine.
The real question was, "how did they come up with number on the placard."
The real question was, "how did they come up with number on the placard."
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.
Oh, and the two spark plugs per cylinder?? Yes, they do provide a measure of redundancy, but they also help in establishing a uniform combustion flame front within the relatively large diameter combustion chamber on our 4 banger a/c engines.
Hope this helps...
I couldn't pass this up. My point being that, it's for the reason stated above that I wouldn't mix mags and lightspeed or for that matter any other types of ignition systems on the same engine. Non uniform flame front IMHO is worse for an engine than poor timing.
Here at the University of British Columbia we do engine testing. We set the timing to best torque. This is likely what Lycoming did when developing their ignition timing specs.
Bob Parry
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.
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25 deg is the setting for sea-level full throttle at rich mixture.The LSE ignition advances timing from the 25 deg static setting based on both MAP and rpm. At 8000' palt you would have well over 5 deg advance just on MAP. Most likely you would have 32 deg to 37 deg advance with a normally-aspirated engine.
That still doesn't explain why all IO-360-A-series engines EXCEPT -A1B6D and -A3B6D are spec'd at 20 degrees and those two are spec'd at 25 degrees (which have identical cylinders, pistons and compression ratios to the rest of the IO-A models).
I think you are mistaken. I believe most, if not all, non-turbo Lycs are 25 btdc.
Sorry Bob, I'm right. If I could figure out how to post a pdf, I'd post the page out of the Lycoming Operator's Manual here. I emailed it to you at your bobvicky address.
The standard timing was 25* btdc and the change was to 20*btdc. The SI that deals with the change said it was done primarily to reduce CHT's in cruise and to increase detonation margin during cold weather operation at high power. It's SI 1325A if you are interested.
I wondered that too, as I have an IO-360-A3B6D 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. T
Heinrich Gerhardt
RV-6, flying