Cam Timing
FTE Leadership Emeritus
Joined: Sep 2000
Posts: 23,174
Likes: 1,678
From: Stephensville WI
Originally Posted by Johnny Langton
You're looking at this wrong.
It's physically impossible for combustion chamber temps to be higher with leaner than stoich(14.64:1)air fuel ratio. The stoich air/fuel ratio for gasoline(14.64:1) is it's highest BTU thermal point-in other words-it makes the highest amount of heat and does the most work at that ratio.
Internal combustion engines are not 100% efficient,so they unfortunately do not make the max power at gasoline's most efficient air fuel ratio-it's somewhere between 12.8-13.1:1 air fuel for most engines-Modulars typically make the most power at 13:1. When someone(like your reference above) refers to a "lean" mixture-they're simply referring to a mixture that's leaner than the air/fuel that makes max power for a given engine. They are NOT referring to mixtures leaner than stoich for gasoline.
JL
It's physically impossible for combustion chamber temps to be higher with leaner than stoich(14.64:1)air fuel ratio. The stoich air/fuel ratio for gasoline(14.64:1) is it's highest BTU thermal point-in other words-it makes the highest amount of heat and does the most work at that ratio.
Internal combustion engines are not 100% efficient,so they unfortunately do not make the max power at gasoline's most efficient air fuel ratio-it's somewhere between 12.8-13.1:1 air fuel for most engines-Modulars typically make the most power at 13:1. When someone(like your reference above) refers to a "lean" mixture-they're simply referring to a mixture that's leaner than the air/fuel that makes max power for a given engine. They are NOT referring to mixtures leaner than stoich for gasoline.
JL
Stoich mixtures produce (or extracts) the most BTUs from a given amount of fuel for use in makling power; That's a given.......but there is a lot more that goes on in the combustion chamber.
The incoming mixture absorbs a lot of heat from the combustion chamber, which cools the chamber and prevents preignition. This also vaoprizes the mixture, which aids in burning it. A lean mixture does not cool the combustion chamber properly, which causes temp in the combustion chamber to climb, causing preignition.
As mentioned in the link I provided, one way to get away with this is to use a colder spark plug. A colder plug transfers more of the heat from the chamber into the cylinder head, where the cooling system can (hopefully) absorb the excess heat.
Postmaster
Joined: Oct 2006
Posts: 4,171
Likes: 12
From: SE Texas
Originally Posted by redford
No, not at all, but I think you are confusing highest combustion temps with max power.
The highest combustion chamber temps you will see is at stoich-14.64:1.
The max power you'll see is approx 13:1.
Leaner than 14.64:1 will yeild cooler combustion chamber temps,and the same is true for richer than 14.64:1.
JL
FTE Leadership Emeritus
Joined: Sep 2000
Posts: 23,174
Likes: 1,678
From: Stephensville WI
OK, a few more snips from various web sites to prove my point:
http://www.asashop.org/autoinc/oct2004/techtips.htm
. High combustion temperatures can be caused by a lean running engine or a faulty or rich biased oxygen (O2) sensor.
http://www.sccaenterprises.com/updat...detonation.doc
The following are some items that cause higher combustion chamber temps.
: If oxygen level in fuel increases (lean) and A/F ratio is not correct
http://www.nichols.nu/tip766.htm
Things like high compression and lean mixtures contribute to high chamber temps.
http://webpages.charter.net/loweperf.../Emissions.htm
While these readings are within acceptable limits, over time the elevated heat from the lean condition will degrade the catalyst to a point of not working.
http://www.asashop.org/autoinc/oct2004/techtips.htm
. High combustion temperatures can be caused by a lean running engine or a faulty or rich biased oxygen (O2) sensor.
http://www.sccaenterprises.com/updat...detonation.doc
The following are some items that cause higher combustion chamber temps.
: If oxygen level in fuel increases (lean) and A/F ratio is not correct
http://www.nichols.nu/tip766.htm
Things like high compression and lean mixtures contribute to high chamber temps.
http://webpages.charter.net/loweperf.../Emissions.htm
While these readings are within acceptable limits, over time the elevated heat from the lean condition will degrade the catalyst to a point of not working.
Postmaster
Joined: Oct 2006
Posts: 4,171
Likes: 12
From: SE Texas
Originally Posted by redford
OK, a few more snips from various web sites to prove my point:
http://www.asashop.org/autoinc/oct2004/techtips.htm
. High combustion temperatures can be caused by a lean running engine or a faulty or rich biased oxygen (O2) sensor.
http://www.sccaenterprises.com/updates/FSCCATechnicalbulletindetonation.doc
The following are some items that cause higher combustion chamber temps.
: If oxygen level in fuel increases (lean) and A/F ratio is not correct
http://www.nichols.nu/tip766.htm
Things like high compression and lean mixtures contribute to high chamber temps.
http://webpages.charter.net/loweperf/loweperf/Emissions.htm
While these readings are within acceptable limits, over time the elevated heat from the lean condition will degrade the catalyst to a point of not working.
http://www.asashop.org/autoinc/oct2004/techtips.htm
. High combustion temperatures can be caused by a lean running engine or a faulty or rich biased oxygen (O2) sensor.
http://www.sccaenterprises.com/updates/FSCCATechnicalbulletindetonation.doc
The following are some items that cause higher combustion chamber temps.
: If oxygen level in fuel increases (lean) and A/F ratio is not correct
http://www.nichols.nu/tip766.htm
Things like high compression and lean mixtures contribute to high chamber temps.
http://webpages.charter.net/loweperf/loweperf/Emissions.htm
While these readings are within acceptable limits, over time the elevated heat from the lean condition will degrade the catalyst to a point of not working.
ALL of those are referring to a mixture that's leaner than 12.5-13:1-not leaner than stoich. I don't know why you cant see this. They are all referencing a mixture that's leaner than power enrichement-NOT LEANER THAN STOICH!
Stoich is the highest temperature for gasoline combustion-PERIOD.
It's simple-highest BTU production is highest combustion temperature-PERIOD.
JL
Elder User
Joined: Aug 2007
Posts: 914
Likes: 1
From: Saskatoon, SK, CA
Wow, and all I said was "correct me if I'm wrong". 
Anyway, I know my experience with this comes from jetting my quad and setting the right air fuel mixture. And by the way, I don't know all the "stoich" business, but if you end up setting it too lean on an air cooled engine (like in my quad) you will heat the crap out of it. This I know from experience. This is what you get for trying to help - now I'm also confused.
Anyway, I know my experience with this comes from jetting my quad and setting the right air fuel mixture. And by the way, I don't know all the "stoich" business, but if you end up setting it too lean on an air cooled engine (like in my quad) you will heat the crap out of it. This I know from experience. This is what you get for trying to help - now I'm also confused.
Postmaster
Joined: Oct 2006
Posts: 4,171
Likes: 12
From: SE Texas
Originally Posted by WildWildWest
Wow, and all I said was "correct me if I'm wrong".
Anyway, I know my experience with this comes from jetting my quad and setting the right air fuel mixture. And by the way, I don't know all the "stoich" business, but if you end up setting it too lean on an air cooled engine (like in my quad) you will heat the crap out of it. This I know from experience. This is what you get for trying to help - now I'm also confused.
Anyway, I know my experience with this comes from jetting my quad and setting the right air fuel mixture. And by the way, I don't know all the "stoich" business, but if you end up setting it too lean on an air cooled engine (like in my quad) you will heat the crap out of it. This I know from experience. This is what you get for trying to help - now I'm also confused.
To help with the confusion:
Stoich is what the O2 sensors in your truck switch at,and is how they provide feedback to the PCM for fuel adjustment. While crusing,and at idle-the PCM keeps the engine at stoich for best economy. When the calculated load on the engine changes to a level that the PCM determines is too much for stoich operation(60% load usually)-it changes the fuel mixture to a richer mixture(power enrichment). This richer mixture will make more power than stoich,and will prevent detonation from an overly hot combustion chamber. Most trucks are around 11:1 from the factory when in power enrichment-this is a very reliable and detonation resistant mixture,but tends to use excessive amounts of fuel. When aftermarket tuners make files for the PCM recalibration-they typically try to get as close to 13:1 as possible to make more power. They also advance the timing as well to make more torque and power.
JL
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