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AFR and STFT adjustments

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Hello,

So Ive started tuning my 9th gen si now that I am getting a little more comfortable and starting to have a more thorough understanding on how the ecu works.

I started datalogging and found that when Vtec engaged, my commanded AFR vs my actual AFR was about 9 % richer, so I adjusted my MAF scaling (g/s) values with the Short Ram intake I have that uses the same diameter as stock, except air flow is less restrictive and the air travel is much shorter which made sense as to why my AFR was off. So, upon make those changes to the MAF and of course soothing out the MAF curve in the process, by removing 9 percent, until eventually removing 7 percent, to only 5 percent around the lower voltages. I logged again and found that I fixed the issue, and AFR CMD and ACT AFR were on point at WOT. The car felt much more responsive at low and high RPM, and the AFR was fine.

Only issue im having is with the STFT. So, I followed Ktuner and Hondata forums on correcting STFT - setting the LTFT to values to 0, logging the data under normals driving conditions as STFT is 0 at WOT, and then make adjustments. Where I am confused is if my AFR has been corrected for WOT, what should I be looking at in the MAF scaling for STFT be within (+/-) 5? I get a range of numbers. Sometimes showing normal -2 to -5 percent, but many times a whopping -16 - -18 percent at idle and cruise! At one point, 6th gear cruising was showing me up to -22 %. Very concerning. Even before my MAF scaling adjustments, my STFT was off. I only calibrated the WOT AFR as the idle and cruise were already meeting targets.

If I have altered my (g/s) values in the MAF where voltage at high RPM is met to adjust the AFR at WOT, how or what have I missed here in terms of the MAF voltages at idle and cruise approx around 1.2 - 2.6 v? Would further adjustments alter the AFR CMD vs ACT AFR? Of course I have to put in the values for the LTFT, which I didn't do when relogging, would that contribute in the STFT numbers im seeing? I am a bit lost here.

I'm not sure I understand what your problem is. Are you using the fuel trims to do your MAF scaling, or are you using the reading of the wideband sensor?

You can't really use both at the same time. When you want to use wideband sensor, you need to disable the trims as they will affect your AFR reading.

If you want to use the trims, you need to add the STFT and LTFT so you get an "error" percentage, and you use this value to adjust your MAF scaling. Most cars disable the fuel trims when at WOT, but there are exceptions like the Subaru FA20 that uses the fuel trims at WOT. I have no idea about the 9th gen SI thought. Also, when you drive the car on the road, you need to be really steady with your throttle pedal input, or your fuel trim values will act crazy one side or another.

Thank you very much that was helpful and clears much of the confusion. Makes a ton more sense now why the car feels more responsive but also why the numbers don’t make sense to what I’m seeing. Im going to assume that if I do remove the trims to adjust MAF using AFR, my AFR will be off.

In terms of going about tuning the scaling, using trims vs using AFR what would be the preferred strategy on the road? Trying to go about correcting the scaling to fine tune to it

It does not really matter honestly, but generally I use the AFR to do the MAF scaling, if you have the ability to log it.

Fuel trims are reactions from the ECU to a past combustion event. STFT for quick reaction, and LTFT for "trends". For the ECU to trim, it needs to interpret the AFR and then react to it. I'm sure that nowadays the ECU are pretty quick to calculate all this, but I like to keep it simple and get my data to the source!

Man you are awesome

I was stuck so I really appreciate your feedback

thanks again buddy 👍🏼

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