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Discussion Starter · #1 ·
Hey,
From my many hours spent on clubwrx I have learned that a lean mixture is bad for your engine. What I don't fully understand is why this is. What is it about the lean mixture that hurts an engine?

Why does an A/F ratio that is too lean...
Burn too hot?
Hole a piston?
Blow an engine? ...etc...

Any actual engineers out there who can explain this to me? Anybody play one on TV?

I have not done any engine mods yet, and when I do I will definitely include some form of engine management. I was just hoping someone could enlighten me about the lean issue.

Thanks, Paul
 

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Paul,

I am a mechE, but I don't play one on TV.

When you run an engine lean, you overheat the combustion chambers and this leads to pre-ignition or knocking. Knocking is hard on the top and bottom end of an engine.

In the worst case scenario, running lean will burn a hole right thru the piston. I know because I've done it (bad distributor).

-Jim
 

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Detonation is bad for this engine. The crank bearings fail under just a bit of it. Lean = detonation. Rich = slow. You need a good map to go fast without buying a new motor.

What kind of engine management do you run?

X
 

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Lean burns hot, real hot in fact.
Things it can do:

Crack cat in up-pipe, throw chunks of cat into turbo...btw, has this been documented?
Cause predetonation(which leads to REALLY bad things).
I guess it could in theory melt pistons in places...although by that time other nasty things prolly will have sidelined ya.
 

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Mostly a question of temperature

The problem with lean mixtures is mostly one of temperature.

Mixture burn temperature is highest near 14.7:1 or stoich mixture that is the target for best emissions. This happens to be the point that all the available oxygen is consumed burning all the available fuel.

At richer mixtures the flame temp is lower because there is excess fuel that never gets burned, and this acts to cool the combustion process.

Max power rich mixture is about 12.0 -12.5 :1 and max power lean mixture is about 13.0 - 13.5:1 (this varies slightly with each engine so you cannot give an absolute number).


Now this assumes you can run that mixture without detonation. In NA engines that is usually the case. In turbocharged engines, the maxpower mixtures are usually lower around 11.0 -11.5:1 or so, because they need the extra cooling to prevent detonation.

The mixture most prone to detonate is near the maxpower rich 12.5:1 so if you go leaner or richer from that mixture you reduce detonation tendency IF cylinder pressures and temperatures are not too high. If the engine is "temperture challanged" ie prone to detonation due to high cylinder tempertures then going lean to prevent detonation does not work because you loose ground by raising cylinder temperatures. That is why most all supercharged/turbocharged engine run best on the rich side of max power rich mixtures.

As you change mixtures you also change the burn speed of the mixture so as the mixture goes rich, you typically need to add timing advance as well to get best power.

Keep in mind that detonation is influenced by many things, mixture temperature, ignition advance, A/F ratio of the mixture, spark plug design and location in the chamber, combustion chamber shape, piston top shape, engine load, engine coolent temp, cam and valve timing, etc etc.

Hope that helps?

Larry
 

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brembo said:
Cause predetonation(which leads to REALLY bad things).
I guess it could in theory melt pistons in places...although by that time other nasty things prolly will have sidelined ya.
Actually, if you are running lean enough, a holed piston can happen VERY quickly.

-Jim
 

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PlatinumWRX said:
Actually, if you are running lean enough, a holed piston can happen VERY quickly.

-Jim
From hot spots or what? Or is it from localized detonation? I know if you run timing advanced too much holes tend to appear in pistons, my GUESS was that it was predet on the upstroke.
 

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brembo said:
From hot spots or what? Or is it from localized detonation? I know if you run timing advanced too much holes tend to appear in pistons, my GUESS was that it was predet on the upstroke.
I has to do with combustion mechanics. Once a small indentation forms in the piston, the flame will always start there and expand, so the indent becomes a crater of constantly increasing diameter and depth. Once a piston begins to hole, it's a lost cause, eventually it will fail, even with a proper air-fuel ratio.

-Jim
 

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Discussion Starter · #9 ·
Wow! Thanks for all the quick replies. I now know that Lean=HOT=bad for engine. I didn't realize that:
1. there could be unburned fuel left in a combustion chamber and
2. that fuel could also have cooling properties in a comb.chamber.

Thanks again, especially Hotrod for that extremely detailed post.
I appreciate it!

Engine management will definitely be in my future. I just need to decide between UTEC, Unichip, and a reflashed ECU. (But that's a topic for another post.)

--Paul
 

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I have looked around and seen that a BOV can cause the engine to run lean. but nothing to back it so far.Can anyone shed some light?or point me in the right direction
 

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BOV can cause an engine to run RICH, becuase it vents to atmosphere.
 

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Shard said:
BOV can cause an engine to run RICH, becuase it vents to atmosphere.
i never understood this could you explain how? i know how a bov works but, i never understood how it made the engine run more rich, also larry was it? thats the best explination i've heard yet of lean and rich
 

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from what I understand, the BOV releases air that the sensors accounted for...meaning you get less air in the engine than the computer thinks, so it pumps more fuel than needed....I might be wrong
 

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yay, a few more weeks on this post and I will know what I am talking about
 

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brembo said:
From hot spots or what? Or is it from localized detonation? I know if you run timing advanced too much holes tend to appear in pistons, my GUESS was that it was predet on the upstroke.
The temperatures present during the combustion process would normally be hot enough to melt your pistons. However during normal combustion I believe that the piston crown is isolated from the heat by a thermal boundary that forms due to the mechanics of the combustion process. In the case of detonation; pressure increases of up to one order of magnitude, and temperatures as high as 18,000F (*) can result. This causes the thermal boundary to break down, and once that happens the piston crown is directly subjected to the elevated temperatures. The thermal transfer properties of the piston are not sufficient to cope (dissipate) with such a thermal differential, and the crown will spontaneously melt and blow through. This would obviously be most likely to occur at the point of localized detonation.

* Temperature quoted from Corky Bell's "Maximum Boost".

-Pace
 

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Dont forget about the emission NOx if your in california- in october we are swithching to the enhanced smog system (you get a free dyno run with your nice smog)----------------------- Hold on------- AWD = No two wheel dynos!!!!!!!!YAY!!!!!! unlesss the make us fing a 4 wheel dyno station that does smogs.. Nah:D
 

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Fortunately I'm pretty sure the "enhanced smog" enforcement including dyno runs will be done on city / county determined basis. I know the bay area will be doing this, and I believe I heard parts of LA or San Diego, but not the entire state. I used to live in a county where I'm pretty sure a dyno didn't even exist. Now I know of maybe two or three dynos in about a 50 mile radius and they all belong to private companies who use them for R&D work. I don't think there's a way in the world they could legislate dyno testing here.
 
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