The A/F Thread

Well from doing a little searching and a lot of reading on air fuel ratios I was thinking it would be best to have it all somewhat in summation.

The first thing that is to be noted that it appears that the best stoichometry for N/A Cars is 12.6-12.8/1 and for boosted cars slightly richer at 11.5-11/1....On higher octane or E85 maybe even a tad leaner. 10.5-11:1 on Pump is normally safer. Boosted cars utilize richer A/F ratios to prevent engine "knock" or unwanted detonation.

Therefore, I've noticed that the factory ECU on DSM"s is running about 14% too rich at about 9.5:1.

Units:
"Lambda" is a commonly used term that is used in place of the air/fuel ratio number, as many devices use or report lambda values. A lambda of 1.0 is equal to the stochiometric ratio (14.7:1 for air/gasoline) and is adjusted accordingly - a lambda of 0.82 is equal to 12:1 air/fuel ratio.
Just as a side note to impress your friends 1 lambda is the technical exact amount of fuel for complete combustion; see "In the lab"

Measuring:
Since there is always some oxygen left in the exhaust gas stream of a running engine, we have an easy way of measuring the air/fuel ratio. An "oxygen sensor" can be used to measure the percentage of oxygen left in the gas stream, and a computer or other electronic device can be used to back-calculate the air/fuel ratio that will result in that particular oxygen percentage. "Narrow-band" oxygen sensors respond with a voltage output that is sent to the computer that is between 0 and 1 volt. "Wide-band" oxygen sensors send a 0 to 5 volt signal which allows for a much higher resolution and are therefore much better for tuning.

Dyno Tuning
It is disputed but some tuners contend that while street driving you will be running approximately .2-.4 richer as opposed to when on the dyno. The dynamics here are that when you are moving though the air while driving, the MAF will see more air flow than when static on a dyno. This will translate to more pulse width at injector = richer.

In the lab:
From a strictly scientific point of view, the optimum mixture of air and common gasoline is around 14.7 parts of air to every one part of fuel for an air/fuel ratio of 14.7:1. At this ratio and under the right conditions, all of the gasoline and all of the oxygen can burn leaving nothing except for the combustion products. This is called the "stochiometric" ratio. It is just like the ratio of 2:1 for hydrogen and oxygen, as when they react (or burn) in that ratio and under the right conditions, everything is used up and only water (or H2O) is left. Fortunately for us, the oxygen in the air is never completely used up when gasoline is burned. The main reason for this is the fact that air is only 20% oxygen, and the remaining 80% is comprised of things that will interfere with a perfect reaction. If one were to mix gasoline with pure oxygen, the stochiometric ratio would be approximately 3:1 and the reaction would be entirely more dramatic (more volatile) with a much greater chance of a "complete burn."


SAFC TUNING

When tuning with safc2 there are two things you gotta keep in mind. The first is that safc bridges the airflow wire to your ecu and then "subtracts" airflow from the signal and reduces the amount of airflow the ecu sees. This tricks the computer into spraying less fuel and the a/f ratios are accomodated proportionally. However, the ecu is blind to the fact that you are actually running more airflow. This makes it continue to have advanced timing according to the airflow it is seeing not the actual airflow which is in the motor. That is the recipe for knock. Now that being said the safc can compensate the injectors down. the formula is simple old injectors divided by new injectors subtracted by 1 in a percentage. For ex for 660cc injectors the formula would be 450/660-1. That will give you a percentage in your case -32%. That would be your start for tune. Provided everything else is running properly fuel pressure is good fuel pump can sustain the extra fuel flow etc that number would be the proper correction. There are many more things to consider like knock, timing, airflow in hz etc before you can really have your car properly tuned. Safc is very limited in this regard.

E 85 Tuning

E85 is a mix of 85% Ethanol and 15% High test non leaded gasoline. It typically has an octane rating of 100-105. Not only does it have a considerably high level of Octane but it burns extremely cool which further inhibits knock. However it does require about 20-25% extra fuel flow in order to attain the traget a/f ratios.
This causes you to have a different formula then the Safc tuning formula listed above. While you will still tune for the same desired a/f ratios on your wideband (perhaps leaner because E85 tolerates leaner conditions) but the formulas for attaining them are as follows.

DSMlink Global Settings

1600cc = -63% Global
1150cc = -51% Global
950cc = -41% Global
850cc = -34% Global

Not recommended to go any smaller. BUT if you really wanted to run them on a low airflow level then you would use...

750cc = -25% Global
720cc = -23% Global
650cc = -14% Global
550cc = 0% Global
450cc = Are you serious??

(450/(Injector Flow * .80))-1.0 = E85 Global
So for 1000cc's

450/(1000*.80)-1 = -47%

The equation above assumes that you are using the stock base fuel pressure for your DSM.
1G = 37.5 PSI
2G = 43.5 PSI

Stevie K

Seems like a nice explination of the air fuel ratio... I added it to the tech article index. :thumbsup:

Just a reminder, please be sure to let me know whenever any of you guys add a tech article or writeup to this subforum so I can add it to the index list.

Thank you. :)

Well from doing a little searching and a lot of reading on air fuel ratios I was thinking it would be best to have it all somewhat in summation.

The first thing that is to be noted that it appears that the best stoichometry for N/A Cars is 12.6-12.8/1 and for boosted cars slightly leaner at 12-11.5/1....If you have other numbers please throw in your 2 cents.
Boosted cars utilize leaner A/F ratios to prevent engine "knock" or unwanted detonation.

Therefore, I've noticed that the factory ECU on DSM"s is running about 14% too rich.


You have it backwards. 11.5 is richer than 12.6. stoich is 14.7 and boosted cars run richer to prevent knock but other than that your numbers are right

You have it backwards. 11.5 is richer than 12.6.

so basically. the lower the numbers, the richer it will run. higher the numbers, the leaner it will run.

Yes. 1:1-14.6:1 is considered rich 14.7 is stoich and 14.8:1+ is lean but I like to keep mine in the 11-12:1 ratios

Actually you really want to keep a/f's to about 11:1

in my tuning experience, it seems that 11.5:1 or 12:1 is riding too much on the edge. Lots of issues came up, at least on the few cars I took that lean.

Also factory ECU runs around 9:1, more gas = cooler combustion chamber

Couple points, since I just saw this. The airflow may be higher with the vehicle moving, but the airflow sensor should add in the fuel for this extra air. Especially if you are on the max load level. I've never seen an AFR change between the dyno and the street using stock DSM MAFs, all over 2.1 g/rev (for 2g ECUs) in ym travels. It's possible that you could be below the max load level, and the airflow increase would move you onto a higher load level, and on stock type ECUs that should result in a richer AFR. If you are over 2.1 g/rev (again, 2g example) however (anyone with a 16g should be here), there should be no change in AFR.

The implication here that caught my attention however is that on a speed density car any change in airflow may not result in a change in fueling, though you would expect the manifold pressure to go up. Not sure. I'll have to keep an eye out for this now that I'm running speed density on the AEM.

I too have found 11:1 to be best on the DSMs, but I can say that the EVOs will tolerate much leaner AFRs, up to 12:1 or even low 12s:1 on pump gas. I still don't like to run it that lean however, some kind of psychological thing perhaps. :D

Pertaining to Crunchy's post, the stock 2g ECU's highest load target AFR is 9.5:1 almost exactly, IIRC. Quite rich, most likely set up this way for warranty reasons. There has been some speculation that the EVO runs even richer. In reality it can't on a stock wired pump so I never caught it on the WB, and I don't recall what the ECU code was targeting. I believe the 1g was ~10:1, but I'm a little foggy on that one. At any rate...

kjewer1
bastard :wink: when the hell you been ?

updated with e85