Thread: Desired AFR

The desired AFR question came up recently. I've heard that a good WOT desired AFR for the 3800 s/c is 11.6:1. Of cousre this varies based on mods, I guess. Apparently the V8 guys (Mustang, Camaro, TA, Vette) tuners say that 12.5:1 is a good spot. This can be found at places like HPTuners.com.

If what I've read is correct, is there a reason for the difference? I don't have a WB, but am always curious as to why things are the way they are.

Lee boosted engines are best kept on the rich side and that helps keep things a little cooler in the combustion chamber and also they dont make significantly if any more power leaning them out from that mark, but as you mention every car likes something a little different and rule of thumb is a good starting place. NA cars, especially LS1 cars make more power running in the 12.5-13.0 range. This is where our local performance shops tune cars to and have plenty of experience on working on them so know what works best and makes the most power. Intercooled M90 cars can run a little leaner, around 12.0 and be safe because you dont have the extra heat going into the engine and that leaves you a little more head room. NA engines dont have a furnace pushing air into them so they can get away with running leaner than boosted cars. Its basically a trial and error thing that was established from experienced tuners making dyno pulls to verify max power. A LOT of M90 cars are tuned way too rich but people dont know because they dont have a wideband to check. After tuning you cant tell much power gain leaning out from the 11.0 range and the gains are going to be small and probably hard to tell unless testing at the track or on the dyno. NA cars definately have room to gain as they are generally shipped out of the factory very rich and I have heard gains for 10-25 at the wheels on a stock LS1 car just pulling fuel out!

Thanks for the explanation Dave. Rick and I were talking about this last night, and wondering what the differences were. What you said makes sense, and what we thought may be the reason or part of the reason.

I guess someone is going to have to explain AFR to me because I truly thought 12.0 was rich not lean. 14.7 parts air to one part fuel. If fuel remains constant (1 part) and air quantity decreases (12.0) doesnt that make it richer? Gosh, how did I get all this screwed up.

I guess my car was running lean the last time at the track. Of course it was just a scan from the PCM but it showed a 10.7 AFR crossing the line. And of course I assume thats probably calculated from the OEM O2 sensor.

My base PE AF is 12.0 after PE vs Time and everything my commanded is 10.7 or less. I need a WB02.

not going to have a debate/argument, but after much research and asking M90'ed alkied users, we tend to shoot for 10.5, of how the mixture of alky gets used in the equation. thats just a simple answer lol.

Originally Posted by SyntheticShield

I guess someone is going to have to explain AFR to me because I truly thought 12.0 was rich not lean. 14.7 parts air to one part fuel. If fuel remains constant (1 part) and air quantity decreases (12.0) doesnt that make it richer? Gosh, how did I get all this screwed up.

I guess my car was running lean the last time at the track. Of course it was just a scan from the PCM but it showed a 10.7 AFR crossing the line. And of course I assume thats probably calculated from the OEM O2 sensor.

14.7 (lean) parts of air to one part fuel is leaner than 10.7 (rich) parts of air to 1 part fuel.

Lower number on the air the richer the mixture.

What I wonder now, is for those w/o a WB, running too small of a pulley, drowning the engine with fuel to combat KR, how long will it take for the negative effects of cylinder wash to wreck havoc?

So I was correct then? My PCM indicated 10.7 AFR would be considered rich?

Originally Posted by SyntheticShield

So I was correct then? My PCM indicated 10.7 AFR would be considered rich?

Yep that is pretty rich.

Well thats good, means I can lean it out some. I will have to see what my cruise AFR is, cause I have suspected it could be leaned out some as well and push the mpg's up a bit further.

Ive been told/heard that running rich can be a source of KR as well so perhaps thats the reason I had the few degrees I had, who knows.

Originally Posted by SyntheticShield

Well thats good, means I can lean it out some. I will have to see what my cruise AFR is, cause I have suspected it could be leaned out some as well and push the mpg's up a bit further.

Ive been told/heard that running rich can be a source of KR as well so perhaps thats the reason I had the few degrees I had, who knows.

With the tuning tools we have available, we can change the open loop AFR. But when in closed loop, we are not able to control the AFR. It will always be at 14.7 until we get more tables.

Originally Posted by SyntheticShield

Well thats good, means I can lean it out some. I will have to see what my cruise AFR is, cause I have suspected it could be leaned out some as well and push the mpg's up a bit further.

Ive been told/heard that running rich can be a source of KR as well so perhaps thats the reason I had the few degrees I had, who knows.

You are correct about the rich KR, I was shown a live example of that. I know it's relative, but bumping my O2's to 980 iirc caused knock and leaning it back out cleared things right up. Now granted I was running in the 960 range which was still too rich (10.3:1) but it didn't start knocking until about 980.

Originally Posted by Trannyman95

NA cars, especially LS1 cars make more power running in the 12.5-13.0 range. This is where our local performance shops tune cars to and have plenty of experience on working on them so know what works best and makes the most power. Intercooled M90 cars can run a little leaner, around 12.0 and be safe ...

This is the quote that confused people. Sorry, Dave but your wording made it confusing. Going from 12.5 to 12.0 is going richer not leaner. I'm sure this is not what you were saying but that is how it came out.

Boosted cars need to run a little richer. Maybe you were trying to say that an intercooled, boosted car can run a little leaner then just a boosted car.

The only way to know what changes to make and what works best for your car is to scan and use a WB. Everything else involves some guesswork.

You are correct Rick, I mentioned that Intercooled M90 cars can run leaner, leaner than a NON IC M90 car. Commanded AFR tables in the pcm arent exact numbers as to what really happens on the car and more for referrence than anything unless you tune your car with a wideband to specifically match those numbers.

Whew! I thought my world had come to an end or something and that I had SERIOUSLY misunderstood fueling. I have been thinking to myself since then that I could probably pull some fuel and kill the KR or add some air as in like a smaller pulley.

I understood what you were saying Dave. 12.0 IC versus the 11.6 non IC'd.