“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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