A4 / A6 Auto and Shift Points
Thread Starter
Racer
Joined: May 2005
Posts: 344
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From: Rancho Santa Margarita CA
A4 / A6 Auto and Shift Points
Auto techs I need some help. I am looking for some understanding on shift points for the auto trans. I have an A4 and I have interest in changing the 3:15 rear gears for 3:73 rear gears.
Now I am thinking if I just swap the gears/diff should be no problem. I.E. if the car shifts under hard acceleration at 5500 RPM then the car should continue to shift at 5500 RPM after.
I continue to read about the need to adjust shift points and make adjustments to the speedo.
Why? Will the car now change it shift point higher to 6,000 RPM or lower it 5,000 RPM ?
What is the truth? does anyone know? I am having a tough time understanding why a auto trans would not shift based on the engines RPM.
I appreciate any knowledgeable feedback on this. Thank You!
Now I am thinking if I just swap the gears/diff should be no problem. I.E. if the car shifts under hard acceleration at 5500 RPM then the car should continue to shift at 5500 RPM after.
I continue to read about the need to adjust shift points and make adjustments to the speedo.
Why? Will the car now change it shift point higher to 6,000 RPM or lower it 5,000 RPM ?
What is the truth? does anyone know? I am having a tough time understanding why a auto trans would not shift based on the engines RPM.
I appreciate any knowledgeable feedback on this. Thank You!
Thread Starter
Racer
Joined: May 2005
Posts: 344
Likes: 0
From: Rancho Santa Margarita CA
Thank you! I thought I was correct with the shift points being RPM based only. I know tuning is needed to maxmize most mods but not always a must, like in this case. I think the confusion may come in with tuners trying to ensure you get all you can out of a modification. So it seems that swaping gears will work just fine without any tuning. I am sure the car will be much quicker and off idle traction more of a problem. 
Thread Starter
Racer
Joined: May 2005
Posts: 344
Likes: 0
From: Rancho Santa Margarita CA
Originally Posted by JohngC62005
If you think you are going to change the rear diff ratio and not tune, you are wrong. Tune is needed. Tune is a must. On the C6 it is a reprogram of the TCM not PCM.
Why am I wrong?
Why is a tune needed?
Why is a tune a must?
Ok, so you may need to reprogram the TCM, but I would like to understand why? What happens if I do not?
Thanks again!!!!
Team Owner
Joined: Sep 2000
Posts: 25,001
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From: Far NW 'burbs of Chicago
St. Jude Donor '13
I don't know about the C6, but the C5 automatic shifted by the speedometer, not the tachometer. So a rear gear change required reprogramming to make the speedo correct. There were lots of headaches when people first started doing this, don't know if they ever got it sorted out.
