70 L46 pinging and run on

[Michael W.]

Re: 70 L46 pinging and run on

The extra octane in the fuel makes the fuel burn slower so the extra timing advance does not cause a ping (your situation).

There is no connection between octane rating and flame front velocity (burning speed). This is a very common misconception. Detonation is the result of spontaneous combustion (an explosion actually) which would not be avoided by speeding up or slowing down the burn speed.

Interesting read:

http://www.whitfieldoil.com/www/docs/171.284/vp-racing-fuel-

[Duke W.]

Re: 70 L46 pinging and run on

Bill,



The extra octane in the fuel makes the fuel burn slower so the extra timing advance does not cause a ping (your situation). The ping is caused by the point of maximum cylinder pressure happening BEFORE the piston reaches top dead center and starts down in the bore. What you try to time the engine for is the point of maximum cylinder pressure just after TDC at approx 15* down in the bore so it pushes hard on the piston.

No, high octane fuel does not burn "slower", nor does detonation necessarily initiate at the point of maxium pressure, which could only happen before TDC if the timing was GROSSLY overadvanced, and in this case the engine would want to run BACKWARDS!!!

Even under detonation conditions the point of maximum cylinder pressure is after TDC. The onset of detonation is rapid and as little as two degrees intial timing can mean the difference between NORMAL (detontion-fee) and ABNORMAL (detonation) combustion.

It's all here:

http://www.metroli.org/pdf/2012 Nationals tuningseminar.pdf

I put this presentation together (after doing a poll here to determine what guys wanted to hear about) to dispell the myths and misinformation about combustion, "octane", and explain the spark advance requirement for ALL engine opeating condition, not just WOT at high revs.

It's been on the Web for over a year now, but I doubt if more than a handful or two of guys have ever read it, so we have a long way to go, but members of the TDB have a leg up because I don't the link has ever been placed on any other automotive discussion board.

Duke

[Domenic T.]

Re: 70 L46 pinging and run on

Bill,
I have the same problem and as Tim said SLOW the burn rate down with octane.
I have to use pump gas mixed with av-gas and when I add to much pump it will run on, shake, and diesel to a stop with a slight back fire. All I do is add the octane and life is good. I used to retard the timing but then my engine lost power BUT shut down without a bang.
Champion prooved that gasoline will burn/explode at different rates with long tubes, one with low and one with high octane. The low reached the end first as both were ignited together with the same source. I think they even have plug #s that are concistant with rate of burn. ALL my aviation training states that to be true especially when moving a 9" piston down a bore at 15 hundred RPM's (has to be slow & even and requires HIGH octane).
I had a brief encounter with a combustion engineer years ago that measured cumbustion pressures with different fuels (gasoline) burn/explosive rates, and each fuel created a different pressure on the crank shaft. I think he said 1600 to 2000 lbs explasive power on his test bed was the differences BUT a wide range of octane.
Acetelyn VS propane VS nat gas or gun powders etc, different rates & pressures.
This issue will never be resolved on this board and YES you can get a engine to run good on todays gas by modifying it to modern tech.
Many of us have seen the difference as we did it both ways!! Some have never tried it (more octane) and will defend todays pump gas forever.

I for one am HAPPY with using the old factory specs and the way my engines run.

DOM

[Duke W.]

Re: 70 L46 pinging and run on

Bill,Champion prooved that gasoline will burn/explode at different rates with long tubes, one with low and one with high octane. The low reached the end first as both were ignited together with the same source. I think they even have plug #s that are concistant with rate of burn. ALL my aviation training states that to be true especially when moving a 9" piston down a bore at 15 hundred RPM's (has to be slow & even and requires HIGH octane).DOM

As I pointed out in my San Diego presentation, what is going on in an engine cylinder is very different from igniting a quiescent mixture of fuel and air in a spherical or cylinderical test vessel, especially a cylindrical vessel that is much longer than a typical spark ignition engine cylinder bore, and I have seen cylindrical test vessels that are several feet long.

The difference is cylinder turbulence that greatly increases flame propagation rate and increases the rate in an exponential manner from the point of ignition, which renders minor differences in quiescent test vessel "flame speed" measurements for most commonly used fuels to a noise level issue.

An engine only needs sufficient octane to keep out of detonation. Higher octane does nothing and does not change the optimum spark advance range for any given operating conditon.

Emission controlled engines have more tendency to detonate and run-on because their severely retarded spark advance maps heat up combustion chamber boundaries, which can lead to preignition, and preignition can lead to detonation and run-on. Anyone who has read the presentation understands that preignition and detonation are NOT the same thing, but one can lead to another in a destructive positive feedback loop.

Run-on can be eliminated by converting from ported to vacuum advance, which will also usually allow a reduction in idle speed. Most emission controlled engines have higher specified idle speed than their pre-emission antecedents because higher idle speed lowers emissions.

I've never heard of an aircraft engine with a 9" bore. Six inches is about the limit for a spark ignition engine, even with two spark plugs, but a few large reciprocating aircraft engines from the forties may have had bores up to about 6.25"

Can you provide an example of an aircraft engine with a 9" cylinder bore?

Duke

[Bill L.]

Re: 70 L46 pinging and run on

HI Duke,

I downloaded and am starting to try and implant your presentation in my once fertile brain.

For now I will continue to add a little CAM2 until I can really dig in to it. It really does run flawlessly all the way to the redline with that little extra octane.

Bill