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How Lean is too Lean? AFR
Posted by: teddbrown ()
Date: April 13, 2012 01:08PM

We've bounced around this subject a little, but I dont remember if we really nailed it down. I know what I think too lean is, but do we have any GM test records, anybody elses testing, etc thats lets us know where the near magic number is?

The magic number I'm talking about is lean, but not so lean that it costs a valve. Under cruise, I have the test EFI set to 15.5 AFR (increased from 15.2 AFR). I know gas is cheaper than valves, but where does the heat go sonic and burn/drop valves?

Just for information: While testing in light cruise, I start cutting out at 16.5 AFR. The engine does not like it above that at all. I also verified in open loop with WBO2 and its almost like a skip, but again... I was guessing and didnt play long there.

This is not only a EFI question. Since WBO2 sensors are getting affordable, they are being used widely by the Carb crowd for jetting.

Ted Brown
Anderson, SC
68 Convert 110/PG/EFI/DIS
61 Loadside Automatic
130HP/PG/Honda Engine Project
**UPDATED** Corvair Fuel Injection and DIS Page

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Re: How Lean is too Lean? AFR
Posted by: 66cv8 ()
Date: April 13, 2012 01:37PM

IMHO 16.5 pushing it. What is your EGT at this mix?

Joe

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Re: How Lean is too Lean? AFR
Posted by: cnicol ()
Date: April 13, 2012 01:44PM

When I jet a carb, I set it for 15.5 cruise without the slightest problem. One could probably go a couple points leaner but I've not gone there.

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Re: How Lean is too Lean? AFR
Posted by: Bob Helt ()
Date: April 13, 2012 08:04PM

Ted,
One would think that this lean AFR would be a variable always changing due to the incoming air and cylinder head temperature, even at cruise. That is because the onset of detonation is highly temperature dependent. And you must avoid any detonation. So you would need to consider the highest possible set of temperatures any engine is likely to encounter (at least normally) and use those as an AFR point. Lower temperatures could accept leaner mixtures (to a point), so how low a set of operating temperatures are you planning on?

In addition, you would want to consider some possible variations on the spark timing that might occur due to wear and/or possible operator mis-judgements to allow for detonation to occur with your high AFRs.

And lastly you would want to chose all of your AFRs lean points somewhat richer than where any power loss or mis-firing occurs. This includes any transitions from cruise to acceleration.

Regards,
Bob Helt

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Re: How Lean is too Lean? AFR
Posted by: brentc ()
Date: April 13, 2012 08:39PM

It was pretty common on seventies GM cast iron V8's to have a cruise mixture in the 17:1 range with no apparent valve problems, I would imagine that would be ballpark the upper limit of 'safe' from a strictly mechanical reliability perspective-

From a drivavbility perspective I think you likely have already found the upper limit already- thermal efficiency on air cooled engines is off by 5-10% compared to similarly configured water cooled engines and I would suspect that will clip your wings a bit on going superlean. The intake charge is just a little hotter and less dense than it would be in a water pumper, Ed Cole mentioned this as an issue in an interview I read once.

On stock engines the combustion control is pretty weak (lousy machine work, non-functional quench) and running lean makes them so pingy that the timing needs to be retarded to the point the lean mixtures are obviated by the late timing for fuel economy. A little extra fuel goes a long ways to cooling things enough to permit nice ignition advance which really is where the economy and response comes from. With port FI you will always be able to get away leaner than with carbs and the resulting mixture distribution issues, carbs need to be tuned to compliment the worst cylinder in any scenario.

Norrmal carbureted non-Corvair engines engage 'power enrichment' around 7"Hg usually, do you have enrichment as vacuum drops? Great sounding project,

Brent

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Re: How Lean is too Lean? AFR
Posted by: teddbrown ()
Date: April 13, 2012 10:10PM

Currently, the AFRs go from 12.5 under full load to 15.5 at cruise. I took a pretty good aluminum head engine AFR map and "made it my own". I dump a little more fuel and pull some timing on higher fuel temps to help with the ping. We are pretty close to ping free, but there is always a chance ping. I still relocate the knock sensor now and then to try to catch that ping.

So... I think we are saying around 16 should be the max at normal operating temp (or that is what Im guessing). I think I'm maxing out at 15.5 for any future EFI upgrades.

Thanks for the imput.

Ted Brown
Anderson, SC
68 Convert 110/PG/EFI/DIS
61 Loadside Automatic
130HP/PG/Honda Engine Project
**UPDATED** Corvair Fuel Injection and DIS Page

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Re: How Lean is too Lean? AFR
Posted by: brentc ()
Date: April 13, 2012 10:24PM

Hi Ted,

Just a small thought that occurred to me also-

Cast iron engines get 'pingy' around 235-240F water temp, this correlates to around the 375-400F range on a stock Corvair thermistor. If your mixture curve has a high temperature enrichment you might try to shoot for it kicking in at that 350-375F zone-I realize this is a bit hard to guage using 45 year old thermistors and a modern temp sensor that is likely in a slightly altered location.

Brent

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