What does it mean to run rich or lean?

Hey what's up everybody! I was wondering if somebody could explain to me what it is to run rich and lean. I've been reading threads where someone would mention it, but don't go into full detail. So I wanted to know what they meant to further my knowledge of dsm turbos!

for a motor to run correctly it has to have the correct air/fuel ratio so combustion can be present in the cylinders and the motor runs. running lean means that there is more air than fuel. not a good thing, leanness can lead to detonation. running rich is exactly the opposite, you have more fuel than air. this isn't as bad for your motor than running lean but it's still not good. the motor will run to a certain extent with a lean/rich condition, but it won't run very well or won't last as long.

just a quick explanation. you probably could research more on the internet and get a more in-depth version.

for a motor to run correctly it has to have the correct air/fuel ratio so combustion can be present in the cylinders and the motor runs. running lean means that there is more air than fuel. not a good thing, leanness can lead to detonation. running rich is exactly the opposite, you have more fuel than air. this isn't as bad for your motor than running lean but it's still not good. the motor will run to a certain extent with a lean/rich condition, but it won't run very well or won't last as long.

just a quick explanation. you probably could research more on the internet and get a more in-depth version.

Running rich doesn't necessarily mean you have more fuel than air. It just means that the ratio between air to fuel is closer. For example if the desired air to fuel ratio is 12:1 and you are running 11:1 you are running richer than is desired because there is less air to the same amount of fuel.

well yes...that's what i meant, i was trying to put it as simple as i could though, my mistake.

Just to site a quick example for when people refer to air/fuel ratio as x:x, or 11:1 as a sample. That means there is 11 molecules of air (im not sure it's measured in molecules, per say, but you get the idea) to every 1 molecule of fuel. So if the desired air/fuel ratio is 11:1 and you're running 10:1, then you're running richer because there is one less molecule of air per molecule of fuel. Vice versa -running lean, running a 12:1 air/fuel ratio would mean that there is one more molecule of air to each molecule of fuel.

As a reference, on 4g63T's, around 11:1 is considered a good air/fuel ratio on pump gas. On higher octane race gas (117) you're allowed to run a little leaner, like in the ballpark of 12.5:1.

Just to site a quick example for when people refer to air/fuel ratio as x:x, or 11:1 as a sample. That means there is 11 molecules of air (im not sure it's measured in molecules, per say, but you get the idea) to every 1 molecule of fuel.


The 11:1 is just a ratio. Parts of air : Parts of fuel. It is one of those unitless numbers. The units could be soda can (volume), lbs, grams, liters, etc.

The 11:1 is just a ratio. Parts of air : Parts of fuel. It is one of those unitless numbers. The units could be soda can (volume), lbs, grams, liters, etc.


It seems to me that it would have to be some kind of unit. It doesn't make sense that the entire automotive community would refer to something so incredibly important like that and there not be a unit of measurement for it. Maybe I'm missing something, but it just seems like there should be some kind of unit for it. :screwy:

No, a ratio should be unitless because it's unchanged no matter what unit you're going to use.

If you have 11:1 air/fuel in liters, and 11:1 air/fuel in molecules.. no difference. Still 11 parts air, 1 part fuel.

Thanks guys.

No, a ratio should be unitless because it's unchanged no matter what unit you're going to use.

If you have 11:1 air/fuel in liters, and 11:1 air/fuel in molecules.. no difference. Still 11 parts air, 1 part fuel.


Ah, I get it now. I'm just so used to everything having certain units with cars that I forgot basic math rules :grinno:

It is a mass ratio. Its important to mention this, because if it was a volume ratio (for example), it would be very different. We typically look at pounds per minute for airflow, so we tend to covert fuel flow to the same. For example, at 50 lbs/min, and a 10:1 ratio, you would need 5 lbs/min of fuel. Etc. This is the basis of all the calculations in the spreadsheet I set up, and the program wagon put together.