“Corvair Fast Burn” Ignition timing settings

Builders:

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.

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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.

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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.

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Above, the digital Air-Fuel meter from this story:Shootout at the Stromberg corral in action. When I say the carb must be spot on for the setting to work, It means you have to have the correct model carb, jetting as we recommend, not something that kind of looks like it.  If you have a carb running lean, it will be very prone to detonation, no matter what other factors are at play.  When you read that 115/145 octane fuel was used on the last of the piston powered airliners and bombers, the dual rating of the fuel is its comparative Octane rating for running lean (the first number) and rich (the second number) The Octane of the fuel you are using means nothing if it is running lean.

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OK, the results. I use the panel of the run stand as a note pad, because it is easier than writing on a clip board in a 125mph wind. The first number is the total timing advance, the second number is the full static rpm of my test prop.

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LooK:

35 degrees (never use this) is 3360 rpm. This worked because it was a cool day, and I have everything set perfectly, on a hot day, sustained power at this setting would be on the ragged limit of detonation.

Now look at 26 Degrees: It turned 3340 rpm. That is 98% of the power output, but with 9 less degrees of ignition advance.  Note this set of tests was conducted with 90 octane gas. I will very shortly have a more formal recommendation to setting the timing on 3.0 and 3.3 Corvairs, but as a starting point, there is no need to use more than 26 degrees. The power output difference is hardly measurable, but the detonation resistance is radically increased.

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Wew.jr.

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About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at www.FlyCorvair.com and in more than 50 magazine articles.

19 Responses to “Corvair Fast Burn” Ignition timing settings

  1. Gordon Turner says:

    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%

  2. Terry Hand says:

    William,

    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.

  3. Shupert, Timothy says:

    Does this mean a little adjustment in your standard 32 deg. Advance in your distributors?

    Timothy Shupert
    CADD Technician

    Office: 715.834.3161 • Direct: 715.831.7642

    ShupertT@AyresAssociates.com
    http://www.AyresAssociates.com

  4. Brandon Scott Edmonds says:

    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?

  5. Andy Elliott says:

    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

    • 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.

  6. David says:

    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.

  7. 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.

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