Changing AFR with different gears

[turbo_bu]

Did some tuning recently on an engine and happened to notice something that I thought was interesting. It looks like there are some differences between the AFR in 2nd gear vs. 3rd gear vs. 4th gear.

Setup -
350 Vortec with a 6 speed manual transmission
LM1 WB located in exhaust pipe behind the right side header
Logging while doing WOT 2nd though 4th gear runs (simulate drag strip type loading)

Basically hammered it in 2nd gear from ~2500 RPM to 6000 RPM, shift to 3rd gear, engine speed only drops to ~4000 RPM then shift again at 6000 RPM, shift into 4th gear, engine speed goes from ~4700 RPM again to ~6000 RPM.

I have done several of these runs and each time the general trend has been the same. So far, I have adjusted the PE Modifier Based on RPM table to get as consistant of AFR's across the engine speed range. What I have noticed is that for 2nd gear, the AFR is ~13.0:1 [ from ~2500 to 6000 RPM] while in 3rd gear it is ~12.6:1 [ from ~4000 to ~6000 RPM] and in 4th gear it is ~12.25:1 [from ~4700 to 6000 RPM]. For all of these runs, the desired AFR is set at 12.25:1.

If this is consistant with others, then what do you target for???? I understand that this is why most tuners only do pulls in high gear (4th gear or 1:1), but should I be worried about the leaner condition during the lower gears (lower engine load)?

Any help / guidance is greatly appreciated.

[eficalibrator]

This is most likely one of two cases:

1) You still have catalyst overtemp or piston protection active and are not monitoring the actual final desired AFR PID to catch this. or...

2) You wideband is changing its "measurement" with temperature. Temp of the sensor does have a significant effect upon the readings seen by the controller, and correcting for this properly is what separates the $200 widebands from the $1000 widebands. The only "affordable" wideband that I've seen not be as sensitive to this is the NGK AFX.

[ChipsByAl]

What about your fuel trims? I have seen the trims change between shifts and cause a rich condition for the next rpm range. Even without monitoring without a wide band (not recommended) you can see a higher fuel trim and corresponding injector pulse width. Post a log with that data if you can.
Al

[WeathermanShawn]

What tune are your running? I.E. SD/MAF/Stock, etc...

What you are describing is not that unusual (though your AFR swings are a little out of the norm).

As EFICalibrator stated, there are various parameters involved with the actual wideband measurement that may explain it. You might want to check your MAP and engine vacuum. Injector flow is usually referenced to engine vacuum. Are you hitting max MAP's in each gear?

Other than the obvious COTP, etc., normally engine load varies by gear which is pretty self-explanatory. You can log MAF frequencies in various gears and see how your airflow and AFR lines up. Overall, my AFR variances are usually within 0-.4 of desired AFR, per gear.

[gmh308]

Does the PCM/ECM have the charge temp blending table? This will vary the intake charge temp weighting between ECT and IAT over a mix of time & increased airflow. On later models speed is factored in as well.

Just a shot in the dark. :)

[turbo_bu]

Thanks for all the input ...

The car is running a MAF with the fuel trims locked at WOT. WOT = no correction LTFT or STFT.

The catalytic converter overtemp is turned off.

The coolant temperature and intake air temperatures are pretty constant for these short runs. Coolant starts at 175 F and ends at 181 F. The air temp goes down from ~86 F at the start to 79 F at the end. Looking at the MAF data shows nice overlays for each gear and speed. The MAP data does show a slight change vs. engine speed. It starts at 100 kPa and goes down to ~96 kPa by 6000 RPM.

For the tune, I have anything in these coolant temperature and MAP ranges set at 12.25 AFR.

The wideband I am using is an LM1 with a Bosch sensor (LSU) (not L1H1 :(). I do not know how hot it got during the run.

Any other thoughts or something that I am missing?

[WeathermanShawn]

Short of some anomaly with the wideband calibration (or location), it sounds like the issue would be in MAF Calibration Table.

What MAF frequencies are you hitting in each gear at WOT? If it is the same frequency, but differing AFR's..then it is more likely to be an injector or fuel pressure issue.

If you are simply hitting differing (and higher) MAF frequencies in each gear under identical WOT conditions, then your desired AFR from the PE Table is suspect. As always, posting a log and tune helps. It is better to get the MAF Calibration Table down, or the PE Table desired AFR will never be correct.

[redhardsupra]

do few experiments: SD/OL, pureMAF/OL, try to gather data at the same speed, trying both higher and slower speeds, does it happen within first 15 mins of startup or does it occur while the car is thermally stable (>15mins on the highway makes everything very stable)

basically what you're trying to do in which situation this dependency does not occur.