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Quickie A/F Setup

I was asked to store this post away somewhere for future reference. For now, I'll just dump it in here as is and maybe come back later and edit/expand upon it. It's really just documentation of my personal process for running through a “quickie” setup with a new car using DSMLink. It's intended to help the user get a handle on whether fuel delivery and airflow metering all make sense.


Fuel trims

First, if you don't already know what fuel trims are or how they're used by the ECU, then I would strongly recommend you start by reading through Fuel-trims: What They Are and How They Work.

Knowledge of this fundamental concept is key to understanding everything else below.


I generally monitor CombinedFT and AirflowPerRev at idle. I want to see CombinedFT cycling around +/- 8% or so.

You'd expect to see 0.24-0.28 g/rev for AirflowPerRev at idle 1), but I don't get too caught up on that if the fuel trim is inline and global fuel settings make sense for the injectors that are installed.

If fuel trim at idle is not where I want it, I either use the lower airflow sliders (log MAFRaw to see where to make adjustments) or I use deadtime to tweak things. If I'm running SD, I'll use the SD VE table instead of the airflow sliders.

Whether I make changes to airflow or deadtime usually depends on whether AirflowPerRev is inline or not. If AirflowPerRev is off from the expected value in the opposite direction from CombinedFT, then I adjust the lower airflow sliders more than deadtime.

If AirflowPerRev is around what I'd expect, I might focus on deadtime instead for idle fuel trim adjustments. Just keep an ear out for little misfires at idle that can sometimes be introduced with deadtime adjustments, particularly if the injectors are not well matched at low pulsewidth.


Next I do some cruising around, again watching CombinedFT to see where it likes to hang out. I don't really have a good airflow figure to watch for at cruise because it depends much more on throttle position and road inclination than idle airflow does. So it's hard to pick a value there.

I just watch CombinedFT and try to adjust airflow sliders or fuel settings (primarily deadtime and/or global fuel sliders) accordingly. I would consider airflow slider adjustments if I know I have reason to suspect airflow to be off a bit (maybe I'm dialing in a hacked MAF or something). I would focus on fuel settings more if LTFT were off for both idle and cruise.

Automated adjustments

NOTE: With the release of V3, we've added a more “automated” method of doing the above idle/cruise dialing. Be sure to read about it on our MAF Adjust page. Or, if you're running SD, read the SD Adjust page or the SDRatio Adjust page.


Then I do a few pulls and look at BoostEst around 5500-6000 RPM to make sure it matches with what the gauge was reading. Some deviation is expected, especially as revs increase. But more than a psi or two and I feel like I want to tweak the airflow sliders some more. You can calculate the amount to move the sliders if you understand absolute pressure and how to calculate scale factors. Otherwise, just lower the MAFComp sliders if BoostEst is above measured boost or raise MAFComp is BoostEst is lower than measured boost.

Once that's done, I'm usually pretty happy with the setup and can start tuning.

Other checks

If you have a WBO2 wired into the ECU for datalogging, then you can do even more checks and verification. In that case, I set the estimated A/F Ratio properties to match my global fuel settings and see where the estimated A/F Ratio differs from the measured.

In all cases, I constantly do cross checks. For example, if CombinedFT at idle is positive by a good bit (significantly more than 5%) yet AirflowPerRev is also higher than expected (0.24-0.26) and my deadtime has already been increased a good bit from where I'd expect it, then I have to question what's going on because the numbers don't add up. Maybe fuel pressure at idle is lower than what I assumed it was, for example.

Automatics and 2.3L engines will generally have a slightly higher AirflowPerRev value at idle.
quickieafsetup.txt · Last modified: 2024/03/15 11:16 (external edit)