Why is running lean bad?

[equate975]

If you run too lean, how does it damage the engine? Aside from not igniting the fuel, I don't see what damage it does.

[UNHCLL]

"Detonation"

Here is good explination of the results: http://www.streetrodstuff.com/Articl...ion/Page_2.php

HTH,
Chris

[equate975]

But if you were to ignite the lean mixture that wouldn't happen. I guess that was my question, if you were to ignite it, what could go wrong?

[TwoJ's]

The lower amount of fuel leads to higher amount of air in the cylinder. When the extra air is compressed, it creates extra heat.

[TwoJ's]

But if you were to ignite the lean mixture that wouldn't happen. I guess that was my question, if you were to ignite it, what could go wrong?

Sure, it will ignite. But the problem is that when detonation (ignition) occurs, if it is not at TDC (or wherever it is designed to detonate) it will create additional stress. Say it ignites early, so the piston in that cylinder will want to go down, but the other cylinders in that bank are still compressing.

You follow? I'm a horrible explainer.

[equate975]

Sure, it will ignite. But the problem is that when detonation (ignition) occurs, if it is not at TDC (or wherever it is designed to detonate) it will create additional stress. Say it ignites early, so the piston in that cylinder will want to go down, but the other cylinders in that bank are still compressing.

You follow? I'm a horrible explainer.

Yeah, I understand that. I was trying to say that if it were to ignite like it would at 14.7:1 when you are running something like 20:1. If somehow you were to get it to ignite just like it should at a richer mixture.

[10/10ths]

You follow? I'm a horrible explainer.

Nah, thats about how I was gonna explain it...

Along with all the "bad for your engine" effects of running lean, your not gonna make optimum power.

[equate975]

Along with all the "bad for your engine" effects of running lean, your not gonna make optimum power.

Yeah I know, I was just curious. So aside from the ignition troubles the only bad deal would be the excess heat.

[10/10ths]

Yeah I know, I was just curious. So aside from the ignition troubles the only bad deal would be the excess heat.

The biggest problem really is the potential for detonation, which you already know is bad, and you may or may not know its happening.

[Charlie]

Predetonation & heat. Lots of it.

-Charlie

[10/10ths]

Predetonation & heat. Lots of it.

-Charlie

Nice Sig...is that at Mid-Ohio??

[Charlie]

Nice Sig...is that at Mid-Ohio??

Si.

-Charlie

[nando]

you guys have your terms confused. there's no such thing as predetonation. there are two phenomenom that are commonly confused:

1) Detonation - the pinging sound you commonly hear. Minorly damaging to your engine.

2) Pre-ignition - this will destroy your engine pretty much instantly. this happens when the air/fuel charge ingites before the spark plug fires.

http://www.hrd-performance.com/13.html (sorry for the background, I have another source for the same article but not on this PC).

also, a lean mixture won't neccesarily cause pinging. but if it's running significantly too lean, you will know it - surging/bucking is a common sign. Also, what's lean and what isn't depends on the engine, the timing, the load, and all kinds of variables. 16-17:1 afr might be fine (I can cruise down the freeway at 16:1 no problem, although it strangely makes no difference in fuel economy from 15:1). Or it might require 13:1 to run right. Trying to tune based on some imaginary "ideal" AFR isn't going to do anything for you.

also, AFR makes a small difference in power, within reason. if you're between 12:1-13:1, a point or two of AFR isn't going to make much difference. Timing on the other hand, affects power in a much more significant way.

running excessively lean could lead to extremely high EGTs, and that's not good for anything (engine, cat, exhaust valves, etc). But running lean won't neccesarily cause pinging, if the timing is set in accordance for the AFR in question -- more advance lets you get away with a leaner mixture. Setting your timing correctly and tuning your AFR based on that is the way to go.