The sun is just setting here, but 15 minutes ago we had pretty close to standard atmospheric conditions here, and it was an excellent time for an important test I wanted done with a minimal correction factor. I was testing “Corvair Fast Burn” ignition timing settings.
OK, here is the concept: If you…….
A) Have a tight quench area in the head, as SPA machines them,
B) Have the correct spark plug (the recommended Denso’s, nothing else)
C) Have the carb set for the Correct air/fuel ratio at wide open throttle
The engine can be set to make 98% of it’s potential power output, with greatly reduced ignition advance, giving it a very, very wide margin of safety against detonation. Even with reduced octane fuels.
NOTE, this isn’t the air fuel ratio we use. This is just the one captured in the picture. The actual ratio is 12;1 to 12.5:1 . -thanks, ww.
19 Replies to ““Corvair Fast Burn” Ignition timing settings”
That IS radical thinking. But 11.5 is still pretty rich. Is this about the leanest “safe and sane” zone to be in at full power? And what about when you back off to 2500 or so, something in the neighborhood of 75%
Gordon, that is just what was captured in the picture during warm up, it is not the correct figure, we are shooting for 12.3 to 1 at wide open throttle.
First of all, thank you for the hard work you have done on this subject. There are websites that are packed full of Corvair “experts” who talk about concepts with no testing to back up their talk. So again, thank you.
My question is this – how does this data translate to those of us who are building or flying 2700cc or 2775cc engines? I don’t want to assume that the data correlated directly. Not without asking.
Terry, I’m going to do further testing on 2700s, but as a general rule, builders should start looking at 28 degrees as a new number for 2700’s.
We had discussed that number in the past. I just wanted to make sure that the number had not changed. Thank you.
Does this mean a little adjustment in your standard 32 deg. Advance in your distributors?
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Tim, 32 was suggested as the upper limit of advance for engines running on 100LL fuel only.
Also, the distributor doesn’t have 32 degrees of advance. 32 degrees is a total setting, not the advance in the distributor.
Is that the lambda cable unit or the actual black box one? I tried tonset the lambda cable one up in a turbo car and it would always lose calibration.
Would 26* timing be alright on a fixed timed magneto fired 1/3 corvair?
Brandon, you might have a tough time hand probing it without an impulse coupling
The magneto i plan to use is a slick surplus magneto with an impulse coupling.
Data are data!
You’ve established that at STP, with RPMs in the 3300s and fuel-air mixtures near 11.5:1, that ignition advance past 26° provides very little additional power. I don’t *think* this would be true for more typical flying conditions in the SW, say 3000 RPM at 8000′ with temps 20+° above standard.
Such large changes in environmental conditions and volumetric efficiency usually require substantial changes in timing for efficient burning. (Of course, there is low destructive detonation probability at such conditions.)
Andy, all good points, but the 11.5 was just a number I photographed, I took about 50 pictures to get the digital camera to capture the full 3 digits on the digital instrument, so I just used that image, it isn’t the actual air-fuel ratio we use.
OK to sum it up you you are saying that on the 3.0 and the 3.3 corvairs we only need a total of 26 degrees advance? That is 6 to 8 degrees at idle and 20 to 22 degrees additional mechanical advance? Am I understanding it correctly? Just want to be sure since timing is so critical to keep from blowing up and engine. Thanks for all you do William.
What are the three numbers under your distributor? The middle one is your distributors advance. It not likely 20 nor 22 degrees
David, your concept and math is correct in your example. However the middle number, on the few distributors I’ve seen, is around 16. If your total advance (at WOT) is set to 26 degrees and with a mechanical advance of 16 degrees the static,or starting timing would be around 10 degrees. Checkout this posting for more information.
I’ve been doing my homework here and although I am sure this will get more clear at an operations college and when I get my hands on a MOP manual, I am trying to get clear on the total advance (for a 3,000cc). From a piece you wrote Ignition Timing on Corvairs (Jan 2014), you suggested 32° for 100LL & 30° for 93 auto fuel. In Critical Understanding #4 (Dec 2016), you suggest 30° for 100LL and 28° for 93 auto fuel. I assume that these new suggestions (26° for 90(?) auto fuel) are based on better actual testing and also for giving better detonation resistance. Or am I mis-interpreting the differences?
Kevin, as with every other new document it aviation, the previous material is superseded, and you are to follow the new recommendation. There are other factors that have changed in the same time period; we use different plugs, slightly revised compression height clearances, and newer cooling set ups, all of these factors come into play. If you bought an O-200 continental, and just looked at a 1970’s document, it would list the timing at 28, but these is an AD from the FAA stating the timing has to be reduced, and this came out much later. All engines that are well supported have revisions to information.
Got it. Thanks.