370z Auto - High-RPM acceleration issue w/ 3.7 gears
 

Was curious if anyone here has any idea what might be going on.

For my auto 370z I did a differential swap with a manual 370z sport diff which put me from the stock auto 3.3 final drive to the stock manual 3.69 final drive. Ever since then, my car now has some form of hang-up or power loss after 7200 RPM that wasn't there before the differential swap.

It's most noticeable when flooring it from a standstill in auto mode. In auto mode with the ~7500 shift point, there's a slight but noticeable loss of acceleration for the fraction of a second between 7200 and 7500 RPM and it takes longer than normal to rev up through that range.

With using GPS-based "drag apps" on my phone, with my current set up in Auto mode I consistently get 4.8-5.0 second 0-60 times, while if I use manual mode and manually shift at 7200 I drop all the way down to 4.2-4.4 seconds. (I'm aware that Phone GPS-based drag apps aren't exactly accurate, but this is dozens of 0-60 pulls in auto and manual mode that all consistently hit the same, and they line up with the feeling of power loss at 7200+.)

This occurs both with a stock tune and my custom tunes.

Is this something I will need to use a dyno for, or is there some very odd or badly labeled setting that is the key to fixing it? I'm just not sure what it could be at this point or why swapping a diff would cause it.

I use HPTuners if anyone's got experience with it and might know how to fix it with tuning.

Thanks guys.

Going on a wild guess here, but is it possible that the auto ECU must be recalibrated, since it thinks the car is going slower/faster than it should? I'm guessing it's configured to have the basic calculation of car speed from the gear*diff*wheel size, whereas now the diff ratio is changed and the redline isn't where the car thinks it is, and the shift aren't happening when they should happen.

The speedometer is in fact off by about 5% after the diff swap so that is a possibility. 5% is also about the amount past 7200 that is slow to rev.

However I haven't been able to get the speed calibrated properly through HPTuners. There is a speedometer calibration setting but it has zero effect on anything.

I figured it could be something to do with that, but I wouldn't think that the speed reading being off would make the ECU start acting up at high rev's when the majority of the tables are based on engine RPM and not your speed.

If this is the case, is there any way to properly calibrate it through the ECU or do I need to replace something physical in my swapped diff?

Edit: I should also mention that my car is a 2017 but I got either a 2009 or 2010 MT diff. Was there a manufacturing difference between those years that throws off newer ECU's? I know that the older diffs had completely different axle stubs, which I have appropriately obtained and installed already.

Edit 2: I'm dumb, forgot my speedo didn't change until I changed my wheel and tire setup a couple months after the diff change. Wouldn't even make sense for it to do so, so it isn't anything to do with the speed readouts.

I put a 3.69 in my auto and had none of what you refer to.

Even on the stock tune, then on Ecutek.

So I am thinking it may be related to HP Tuners.

Wheel speed (mph) is determined by the wheel sensors.

Tire dia will alter the speedo but rear gears don’t.

It behaves the same with all the same problems even with my stock tune, though I've heard HPTuners isn't very well received. I also had HPTuners tunes on my car before the diff swap without the issues described.

Could be placebo or remembering wrong but I also remember that for some reason my car changed when it shifts in auto mode from around 7250 to 7450~7500 when I turned my shitty hot air intake into a proper long tube CAI. No idea if that has anything to do with it (or how/why that would change the shift point when shift points are determined by the trans controller, not ECU.)

Also, edited my previous post. Speed readout issues were from my tire/wheel setup changing a month or two after the diff change, not the diff. I'm an idiot lol.

Most likely tune related. I would swap over to an Ecutek tune and tell your tuner to input in the upgraded diff data.

Source: I tune cars with different gears all the time, my 09 auto has a 3.9 diff ratio.

you will need to fix it in a tune.. when i did my gear change my tuner had to change the gearing in the tune for it to be correct
HP tuners may not support it so you might want to reach out to them..
ECUTEK and UPREV both support this

All signs point to needing to change my diff gear ratio in my tune, unfortunately HPTuners truly does suck as much as people say it does because there is absolutely no place anywhere in it that lets you change diff ratio for a 370z. Even on the "speedometer" page, which shows diff ratio settings on screenshots for other (mostly USA) cars, there is no setting but wheel diameter which does absolutely nothing when changed. Looks like HPTuners definitely isn't made for our cars, yikes!

Will be moving to ECUTek soon then, for now manual mode works completely fine albeit with slightly lower redline than desired. My brother (who actually has a ton of tuning experience but just wasn't familiar at all with the quirks and problems of HPTuners) and I do all our tuning on our own because it's a lot of fun to us. Granted, we only even used HPTuners because long story short it was either money wasted on a non-returnable tuner my brother got a year ago or say screw it and see how well it works on my car.

Thanks for the input guys, much appreciated! Will come back to here if when I swap over I still can't fix the issue, but I'm absolutely certain now that it's that one setting.