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Old 08-08-2005, 12:52 PM
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Why do lean engines run hot?

I know its pretty much a given that lean engines run hot - glowing red headers being one symptom.

I'm curious as to why.

My guess is that, in a lean-running engine, not all the oxygen ingested is consumed and, since combustion of the fuel isn't 100%, there's enough oxygen in the exhaust that the unburned fuel is essentially burning while in the headers.

In a rich-running engine, more of the oxygen is used so, even though there is a lot of excess fuel in the exhaust, there's not enough extra oxygen to support any combustion in the headers.

Am I close?
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Old 08-08-2005, 12:56 PM
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Basically you are close. Fuel is also a coolant.
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Old 08-08-2005, 02:28 PM
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In my experience, a rich condition usually causes headers to glow red.
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Old 08-08-2005, 03:20 PM
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Ckucia,

You are close enough for gov't work. There is an actual curve that shows relative heat vs A/F ratio. Around stioch or a little lean is usually the hottest, but you can still make things red with a little rich. At that point you are splitting hairs.
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Old 08-08-2005, 03:21 PM
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Retarded ignition timing and a rich mixture will cause headers to glow red. I never had a lean mixture make my headers glow and have no idea if that is possible. I guess higher combustion temps could exit the exhaust valve and make a header turn red.

A high oxygen content burn is hotter than a low oxygen content burn. Like oppening the oxygen valve on a cutting torch. So there will be high combustion temperatures with a lean mixture, but not sure if it is hot enough to make a glow.
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Old 08-14-2005, 11:03 PM
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why do lean engines run hot?

Well they do and they don't depending on what you are asking.

Based on my 40 years experience flying small airplanes that all are equipped with manual mixture controls, watching an exhaust gas temperature gauge while leaning the mixture, as you lean the egt will get higher, then peak, as you continue leaning, the egt will decrease. Best SAFE power is about 150-200 degress rich (for fuel cooling), best power is peak egt, and best cruise economy is 100 degrees lean. Rarely do we use peak egt for power unless taking off at high density altitudes when maximum available power is absolutely necessary.

As far as red headers are concerned, that is usually the continued combustion of the fuel burning in the header. The exhaust coming out of the cylinder into the header does NOT find fresh air there to increase burning as some believe, but this is a continuation of the combustion process because the cam has blown the slow-burning mixture out of the exhaust valve. Insufficient ignition timing causes the combustion process to begin too late and not be complete when the exhaust valve opens, causing excessively high egt readings and red pipes. Lean mixtures generally cause a "colder" pipe because the mixture has less fuel to burn. i.e. lean/cold egt. Have you ever tried to determine which cylinder is not running correctly by checking the temperature of the pipes? The weak cylinder will be colder than the others. Motorcycle mechanics should know this. If the timing is correct, the headers will not be as hot as with late timing. Individual cylinders are affected by unequal fuel distribution (or misfiring ignition) sometimes corrected by staggered jetting.
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Old 08-14-2005, 11:43 PM
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Quote:
Originally Posted by xntrik
Well they do and they don't depending on what you are asking.

Based on my 40 years experience flying small airplanes that all are equipped with manual mixture controls, watching an exhaust gas temperature gauge while leaning the mixture, as you lean the egt will get higher, then peak, as you continue leaning, the egt will decrease. Best SAFE power is about 150-200 degress rich (for fuel cooling), best power is peak egt, and best cruise economy is 100 degrees lean. Rarely do we use peak egt for power unless taking off at high density altitudes when maximum available power is absolutely necessary.

As far as red headers are concerned, that is usually the continued combustion of the fuel burning in the header. The exhaust coming out of the cylinder into the header does NOT find fresh air there to increase burning as some believe, but this is a continuation of the combustion process because the cam has blown the slow-burning mixture out of the exhaust valve. Insufficient ignition timing causes the combustion process to begin too late and not be complete when the exhaust valve opens, causing excessively high egt readings and red pipes. Lean mixtures generally cause a "colder" pipe because the mixture has less fuel to burn. i.e. lean/cold egt. Have you ever tried to determine which cylinder is not running correctly by checking the temperature of the pipes? The weak cylinder will be colder than the others. Motorcycle mechanics should know this. If the timing is correct, the headers will not be as hot as with late timing. Individual cylinders are affected by unequal fuel distribution (or misfiring ignition) sometimes corrected by staggered jetting.

That was a very informative post, thank you.
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Old 08-15-2005, 01:21 AM
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This post interests me too. I am trying to tune my Q-jet equipped 283 for absolute maximum mileage (gas at nearly $6/gal. here in Germany now). My question is this: If I tune the cruise mode to the leanest possible state will the combustion temperature be so high that I could burn valves? I intend to leave the A/F ratio in the power range at a safe 12:1 or so. I would think that the low load combustion could not cause a problem with the valves, despite being extremely lean. Apparently WWII planes were run extremely lean while cruising, without causing any problems (I think Charles Lindenburg had developed this fuel-saving strategy for fighter planes).
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Old 08-15-2005, 11:17 AM
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one of the posts i read i thought one of the guys said that running too lean could melt the pistons. is this true. i cant remember which post it was or i would post it.
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Old 08-15-2005, 03:18 PM
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Bel-Air,
I tried pretty much the same thing in my 355 Jimmy. What I found was using an interior vacumum gage is that by leaning the jets my vacumum reading was up to 5 points lower at cruise, in other words I was pushing harder on the gas to hold cruise speed, I got worse gas mileage overall. With proper jetting and watching the gauge my mileage increased, still not great, 15-16 MPG, but better than the 13 MPG I was getting with the lean jets. Using the gauge I found that at cruise you can back off the gas a bit, going for a higher vacumum reading (carb closed more), and maintain speed, it's the little bits that add up. The hard part is not lead footing it the rest of the time and remembering to watch the gauge

In the WWII fighters their quest was distance, so they would lean the mixture and by doing so go slower and go further. I remember reading a book by Sabro Saiki (sp?) who was able to almost double the range of his Zero fighter doing this but that the plane was very sloppy to fly, at just above stall speed, again the object was distance, how long it took to get there didn't matter.
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Old 08-16-2005, 09:58 AM
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lean running hot?

Thanks,
OK, at the risk of opening a bigger can of worms...

First, carburetor mixture controls were invented because as altitude increases, air density decreases and your gas flow does not.... so your mixture gets excessively rich. Same thing happens if you drive your car from sea level to the mountains. You lose power because there is less air density, and your fuel mixture is getting excessively rich. Viva la fuel injection !!!!!!!!! Viva la TURBOS!!!!!

There are 2 distinct airplane conditions I believe you are confusing.

There is an airspeed/power setting for the longest time in the air...... which usually is quite a slow airspeed= the lowest power setting that allows the airplane to maintain altitude, maybe 30% sea level power. This is about the indicated air speed at which the airplane climbs at the steepest angle over the ground. "maximum angle obstacle clearance climb airspeed", but you get that airspeed and instead of full power to clilmb you use minimum power to maintain altitude. Use this if you are lost and want to save fuel while you're figuring out where the heck you are! or if you are just putzing around sight seeing.

There is an airspeed/power setting for maximum distance over the ground (miles per gallon) in airplanes, this is about the indicated airspeed for the maximum rate-of-climb in feet per minute. You get that airspeed and use just enough power to maintain level flight at that airspeed, maybe 45% sea level power (I never checked). It is the airspeed that has the least aerodynamic drag produced, i.e. most efficient airspeed. That is why airplanes climb best at that speed.

Since we want to go somewhere, there is also the economy cruise about 65% sea level power, normal cruise 75% of sea level power, and the full power settings. And yes I am aware that at about 8,000 feet density altitude maximum normally aspirated power available is only about 75% of sea level power which is the maximum rated cruise power of most piston engine aircraft. So yes, you are running wide open throttle at 8,000 feet, and leaning the mixture a lot. (Think of drag racing in Denver at 5000 ft. on a summer day: density altitude might be 8,000 ft.

I drove my turbo car to Leadville, CO. the highest town in the US, and the car could hardly pull itself away from the stop signs until the turbo started spooling... viva la 12 pounds of boost!!!!)

Speed costs money, how fast do you want to go?

Oh, gas mileage guys..... "You can't make a silk purse out of a sow's ear". There is no "magic carburetor" that will make your 4wd truck get 50 mpg nor make your big block muscle car get 30. If you have a decent tune-up already, fine tuning might net you another 5% mileage.
Thanks, glad you like the info. I hope it encourages you to research more.

Last edited by xntrik; 08-16-2005 at 10:23 AM.
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Old 08-16-2005, 10:36 AM
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I'd look into this...
http://autos.groups.yahoo.com/group/pes_acetone/
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Old 04-07-2007, 12:10 PM
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mixture

Rich mixture burns faster and lean mixture burns slower. If the headers glow red the timing is too retarded for the mixture present. You either need to advance the timing farther or richen the mixture so the burning occurs in the cylinder instead of the exhaust/header. If your vac advance isnt hooked up it could cause this.
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