3.45 to 4.10 gear ratio change - impact on A/F ratio - retune required?

[acammer]

** Disclaimer - I have HPT, but I've only looked at files, I've not purchased credits or the cables to tune my own vehicle. I want to, I'm just not prepared to make the investment in everything, plus a laptop, and a wide-band. I hope you guys will help me out with your thoughts, I know there is a lot of knowledge here. **

I'm putting a 4.10 gear-set in my car tomorrow (mods in sig) and so I contacted a tuner about updating the speedo calibration for the 4.10 gears. They told me that I should strongly consider getting the car re-tuned, as the reduced load and increased acceleration speed of the engine with the new gear ratio can potentially introduce a lean condition.

This is a reputable tuner that I've had several personal friends deal with directly, with great success. However, I'm have a hard time swallowing this - I expected that I'd just need the ratio updated, not 90 minutes on the dyno, and the associated fees.

I did a lot of research today, mostly over on this forum. I did learn that it's common for a MAF tuned car to run lean in the lower gears where the acceleration rates are faster than in 4th gear where they are typically calibrated. My research tells me it apparently has to do with MAF filtering and a delay that's imposed there. I don't know enough about the tune in my car (bought it tuned already) to know if it's a Dynamic, straight MAF, or straight Speed Density tune - but I'd wager it's Dynamic.

I read in one thread that a user was 2% lean in third as compared to fourth, and even leaner in second. The difference between 3rd and 4th is 1.48 in overall ratio (M10 M6 car). My new final drive ratio is a change of 0.65 in overall ratio - about half what you'd see in the difference from 3rd to 4th - so by that the car would be maybe 1% of difference in fueling. Not undetectable, but maybe on the edge of the envelop in terms of overall influence. I certainly wouldn't think it would put me in any sort of dangerous territory.

To add some mud to the water - I added cutouts last fall. These are behind the rear axle, well after the high-flow cats, so I would think they are mostly just adding noise, and not changing much in the overall exhaust flow - but they may have some influence on the tune as well, which may increase the justification to hit the dyno again. They're always open - I should have just done muffler deletes but I wasn't sure if I'd like that much noise. I do!

I have a lot of respect for this tuner, they do a lot of high end work, I would hate to think they're just padding their pockets. I don't think that's the case, but I've just never heard anything like this about adjusting the a/f calibrations when making a rear-gear change. I'm interested to hear what you guys think.

[5FDP]

First time I've ever heard of someone saying that changing to 4.10 gears would require dialing in the fuel again.

[gottogo]

I think it makes sense. Your load levels will be different and so will your shift points. While I don't think you're in dangerous territory, a retune will optimize for the changes.

[5FDP]

I understand how the load can change but if it was tuned good before hand and the airflow tables and torque model is smooth the car should be able to handle the change. I don't see how it could change more than 1-3%, the tuner makes it seem like fueling is going to thrown off by 5-10%.

[jsllc]

American Racing Headers/X-pipe and cats, cutouts behind the axle, Cold Air Inductions CAI, New Era Ported TB. These items change the tune. On a Gen IV E38 or E67I doubt the gear change will make any difference. Nothing in these ECMs is load based really. If it was tuned correctly before (IF) then it is fine now.

[RySmDPT14]

I swapped to 4.56 and just had to alter the shift points via hp tuners... picked up better MPG after with no tuning changes.

[acammer]

This is an M6 car, so this isn't about shift points. This is, supposedly, to correct for a potential lean condition resulting in decreased load and increased engine acceleration in PE.

[GHuggins]

No tuner, even with back calculating maths can tune in the entire air model in an hour anyway. Sounds like a MAF only tune with improper VE which would explain the necessity for re-tuning the fueling. If everything is close, fueling should stay within a percent or two of where it was originally.

[acammer]

I appreciate the inputs guys. I ultimately probably will go ahead and let them retune it. Maybe they can give me a baseline pull, and some insight into what they actually are going to change. I would think if it's a dynamic model, the SD portion of the tune wouldn't change a bit, but the MAF side of things could be impacted by the increased acceleration rate. And if I understand right with a dynamic it's just MAF over 4,000rpm anyways, so I could see where there is potential for change. I do think, as some of you have also contributed, that it's a minor change.