2.77 to 3.73 diff swap is ready to test
 

I had the day off Thursday with no pressing agenda. Kids at school, wife at work, nice overcast day, so I broke out the tools. I purchased a M6 diff from a bone yard in Cleveland and had it shipped to my house ($650 +100 freight) so for half the price of a converter we will see how much is to be gained from the gearing change. The 28.7" tall MT's that I run will offset some of the steep difference in the two. I'm hitting the track tonight if Trop Storm Jose doesn't drench the area. The swap is relatively straight forward so I didn't provide detail pics but I would suggest that you call a buddie to help, the pumpkin is very heavy.Attachment 897450

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My preliminary observations aside from the obvious, the upshifts are much more pronounced during deceleration... I didnt seem to notice it with the 2.77 but with this unit, it is clearly felt going through each gear when slowing to a stop. I also have the Vitesse controller that I used to keep at sp4, the car is way too twitchy now at that setting so I rolled it back to 0.

Odd side note; I went and made the change in HP tuners for the correct ratio in the speedo and then took it for a rip in an old industrial park by my house, the car banged the rev limiter on just about every gear. I can paddle shift it and their is no issue though. Did I miss recalibrating another parameter ? I looked and did not see where else it needs to be modified. I will change the tire size spec in HP before I head to the track.

Awesome, can't wait to see how it turns out.

If I am reading what you wrote correctly. You will have to adjust your shift mph's in the TCM to match up to the desired shift mph's. The paddle is taking the auto shifts and mph dictation out of the equation. Since this rear gear is spinning up much faster, it could be the lead time for the shifts that need to be adjusted by lowering the mph.

I would have to think about the shift point effects by changing your tire diameter to actual or just below actual like the stock tire is set up. i.e. the stock tire as per manufacture is 27.5 but the ECM sees it as 26.83. If you put a straight edge on the top of the stock tire and measure to the ground it is 26.83.

What are you cruising down the highway at in 8th rpm wise now? Sat at 60mph and 70mph.

Nice, Def interested in ur track results.

HPT...all u should have to do is change Gear ratio to 3.73 in those two spots. Try messing with Tire size calibration so the tables can auto scale.

What is your Rev limiter currently set at?

Might have to just go in the Shift Speed tables and reduce by 1mph and see if that helps to not bang against Rev Limiter.

I'll look into the shift mph's in the tcm now, thanks for the help. I would have thought changing the gear size in the "gear ratio and tire size" section under "final drive" would have auto corrected it.
8th gear at 60 mph was just around 2k rpm on the stock wheel. 1st gear 5k rpm is like 22-24 mph lol.

Edit; it appears at first glance that it has been corrected. I see the 1-2 shift at 29 mph on the scale.

Rev limiter is currently 6500.

I wanted to try and make an apples to apples in this setup to see if the change is worth it (with its current tune) and then go from there. Do you think tweaking the Shift Speed tables will skew the results or just go ahead and try it if needed ?

Ahh gotcha, makes sense. Well if you can get it to not hit rev limiter by just the two usual tweaks, you are all set with an apples to apples comparison.

If not, I'd raise your Rev Limiter to 6700 to give you some play room.

The fair race is to have it shift at the same rpm which is dictated by mph for shift points. You will have to drop the mph another 1, 2, 3 mph per gear as you go up for it to shift at 6,500 or what ever it was. I think it shift lead time that is getting you.

Thanks for your help, raising the rev limit is next on the agenda.

I tried changing both tire size and gear ratios in the editor (stock tire size on the MT's and even tried a 4.10 gear input) but was still bouncing off the rev for some reason. I did tweak the Full Throttle Shift Speed on the TUTD to 23-50-84-111-152-195 and it shifts on command without delay....so there goes my experiment with that but I'm really just focused in the 60' tonight and then see where it goes from there.

You and Brett have set the bar pretty high so I'll just see how far I can get on stock mani's and all 4 cats.

Good luck...

:popcorn:

This is AWESOME thanks doing this and sharing all this information and eventually your results

Rain tonight. Darn, I will not be around next Tuesday night for the results. Good Luck!

I would have helped but during the day this week my work projects kept me hopping during the day.

How much do you think it weighs? I understand the case is mostly cast iron.

Thanks for the encouragement guys, I may take the drive to Cecil tomm in Maryland for the first time...weather permitting.

The A8 pumpkin (with its fliuid) sitting on the bathroom scale comes in at 101 lbs. It feels like 250 when you're laying on a creeper trying to coax it back in though. I did use a scissor jack and the hydraulic together.

Is swapping the diff really as easy as it looks? Unbolt the axles and driveshaft without removing them, and then just pull the (3?) diff bolts? Don't even drop the exhaust?

How do you remove the cooler lines? (I assume the A8 has the same diff cooler as the 6MT?)