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AFR vs FA for Jeep tuning

Understanding AFR

Discussion and questions related to the course Understand AFR

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am trying to get my head around Fuel/air ratio that is used in Jeep ecu to calculate fuel.

For stioch there are three tables FA stoich, FA stoich even FA Stoich odd.

I was was able to calculate stoich and convert it to AFR by using the following:

stioch AFR(14.7) X stoich FA (0.710) = EQ (1.043)


stoich AFR(14.7) / EQ(1.043) = 14.09 AFR.

In other words when I read 0.071 FA stoich it is equal to 14.09 AFR. Correct me if am wrong

This is the what I could understand however when reading the FA power enrichment I couldn’t do any calculation to understand it more.

Is there a way to do this faster or by using Lamda since when I log EQ commanded it reads lamda 0.877 at WOT. And the FA enrichment at WOT is 0.0100

Chrysler uses Fuel/Air ratio instead of Air/Fuel ratio. It's just a quirk of their ECU. So take your F/A and just do the reciprocal. So 1/0.071 [the F/A ratio] = 14.08 A/F ratio.

BUT keep in mind that the ECU does not assume 14.7:1 (same as 0.068:1) = stoichiometric . It uses a richer ratio. It looks like the stoich is the 0.071 you pointed out, or 14.08 AFR. So if you want to understand AFR from Lambda, you do 14.08 * 0.877 = about 12.35 A/F

Can you post screenshots of the datalogs and maps you are looking at? If your datalogging software allows a realtime formula calculation, You can have it automatically do the conversion formula for you. What you can do is create a custom calculation of A/F and F/A.

Take measured Lambda * stoichiometric A/F (14.08 or whatever you calculated from what the ECU is using) and you get measured A/F.

Then do 1 / measured A/F (what you just calculated) and you get measured F/A ratio. If you can't do it in your datalogging software (no custom formula channels feature) you can post process in microsoft Excel with a formula. It's just more time consuming to do it that way.

attached is the log scan and the maps I got the stoich FA and the FA enrichment.

on the scan am logging FA Enrichment PID, EQ commanded (SAE )

thanks for clearing out FA concept, I understand FA is an additive to stoich FA however the numbers don't add up. also in the calculation you mentioned did you mead measured lamda of a WB or what am logging ?

Attached Files

I checked my VCM Editor & VCM Scanner and I don't have a sample file for a 3.6 Jeep engine, just an early 5.7 liter from a Chrysler 300. I will respond though in this way:

1. If you can, avoid looking at "SAE" signals. The SAE signal is not the value directly used by the ECU in its calculations. It's not an input. Rather, it's an output value meant for a generic OBD II scan tool. Specifically, they are J1979 Mode 01 PIDs. They may be sampled at a slow rate or they may be calculated in a different way in order to meet the regulations. So if you find anything that is not "SAE" for target lambda, target Phi (equivalence ratio, basically 1/Lambda), or target fuel air ratio, use that.

2. In VCM Scanner, go to to Tools --> Math parameters to make custom calculations. Then you can make a formula to have it automatically convert FA ratio to A/F ratio, or lambda, or whatever, as I mentioned above.

3. Under tools-->quantity and units, change away from traditional (American/Imperial) units. In the US we use them because they are customary (we grew up using them), not because they are easy to work with. Set your pressure to kPa (much easier to work with for absolute pressure than psi) and temperature to C and your mass flow rate to g/sec. I know a lot of numbers are quoted in these old units on the internet but for day to day working, they suck.

4. As you know, the 3.6 Pentastar uses narrowband o2 sensors. So it is best to trust your AEM wideband for actual numbers when going WOT, but in lower loads (closed loop) look at your fuel trims too which are based on the stock narrowband sensor. The fuel trims will tell you if something needs to be rescaled (injector, VE tables, etc) or maybe if something is wrong with the engine.

5. The numbers don't add up but there may be other modifiers applied. There could be cat or exhaust manifold temperature enrichment also operating (unless it's disabled). For example, it may have the catalyst temperature target set to 930C or whatever.

Basically these systems work in a similar way: modern Chrysler, GM, Ford, Bosch, Siemens/Continental are all using the same kind of calculations, although, standalones are different and so are many Japanese systems like Subaru. For target fueling is they have the basic stoichiometric A/F (or FA ratio in your case), then they add up all the modifiers based on what conditions have been reached. Need additional power? Power enrichment. Catalytic converter too hot? Catalyst over temp enrichment. Knocking too much? Knocking enrichment. Engine cold and unstable? Stability enrichment.

In the aftermarket tuning world we may turn off some of these enrichments (especially catalyst enrichment) to make it simpler to tune.