Re-program for rear-end change
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From: Albuquerque NM
Re-program for rear-end change
A few days ago someone posted here that you do NOT have to reprogram the PCM on an A4 when changing 2.73, 3.15 or 3.42 gears because they all have the same size ring gear. The speed sensor is on this gear and therefore the speedo accuracy would not be affected. Can anyone verify this?
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From: Murphy TX
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Re: Re-program for rear-end change (Vettepartz)
To be precise, you do not have to reprogram in order for the speedometer to be accurate. However, you still have shift points to worry about. In most cases you'll want to reprogram your PCM on an A4 car. You might have another look at that thread, particularly C5-TECH's response if you want someone with credentials:
PCM re-program required for MN6 switch to 3.73s?
[Modified by ToplessTexan, 5:02 PM 1/16/2002]
PCM re-program required for MN6 switch to 3.73s?
[Modified by ToplessTexan, 5:02 PM 1/16/2002]
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From: Coral Springs Florida
Re: Re-program for rear-end change (ToplessTexan)
Absolutely ..Postively ...100% you HAVE to reprogram the PCM !
Call Steve at www.ttspowersystems.com.he will answer your questions with absolute answers.:D
Call Steve at www.ttspowersystems.com.he will answer your questions with absolute answers.:D
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From: S.W. Ohio. . . . . . NRA Life Member
Re: Re-program for rear-end change (vstella)
Absolutely ..Postively ...100% you HAVE to reprogram the PCM
[Modified by Mike Mercury, 6:10 PM 1/16/2002]
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From: S.W. Ohio. . . . . . NRA Life Member
Re: Re-program for rear-end change (Mike Mercury)
Speaking C5's here... with the 2.73, 3.15, 3.42, and 3.73 the ring gear has 41 teeth. The ring gear is where the VSS takes pulses from. There is no speedo error when going from any of these ratios to any other of these ratios.
Now speaking specifically A4's here... but shift points are a different problem altogether. If your ratio change moves the WOT shift RPM point above the rev limiter, you will have driveability problems.
to quote an expert on this:
"HOWEVER, for the automatics, the relationship between engine rpm and mph does change, and thus has an impact on the shift points. You DO need to program for this..."
and another:
"The "Exciter" for the VSS is on the outer circumference of the differential carrier, independent of the ring and pinion. As long as it has the same number of "teeth" on it, the speed will always be correct no matter what gear ratio is run."
and yet another:
"Ring count, 41, is the same on all of them (which is why no reprogramming is necessary for speedo correction.) "
[Modified by Mike Mercury, 6:38 PM 1/16/2002]
Now speaking specifically A4's here... but shift points are a different problem altogether. If your ratio change moves the WOT shift RPM point above the rev limiter, you will have driveability problems.
to quote an expert on this:
"HOWEVER, for the automatics, the relationship between engine rpm and mph does change, and thus has an impact on the shift points. You DO need to program for this..."
and another:
"The "Exciter" for the VSS is on the outer circumference of the differential carrier, independent of the ring and pinion. As long as it has the same number of "teeth" on it, the speed will always be correct no matter what gear ratio is run."
and yet another:
"Ring count, 41, is the same on all of them (which is why no reprogramming is necessary for speedo correction.) "
[Modified by Mike Mercury, 6:38 PM 1/16/2002]
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From: Murphy TX
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Re: Re-program for rear-end change (wallstAL)
I think that post refered to a 3.73 being installed in a M6 car.
I'll try again... Who maintains the FAQ? It would be great to get this topic in there. (Mail sent to the FAQ Gods. :) )
[Modified by ToplessTexan, 5:53 PM 1/16/2002]
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From: Roswell GA
Re: Re-program for rear-end change (ToplessTexan)
OK - Several references to "Loaded ie TTS" reprogrammed cars. I have seen loads of posts stating that the shift points are changed via the reporgramming. I have NEVER seen anyone with a "Loaded" A4 and 3.42 gears and I know there a lots of them out there indicate what the "New" shift points are.
What are your shift points?
Is this some kind of propriety information or some thing. Are not most all of them set about the same. And just what if the percentage change of the 3.15 to 3.42 happens to move the shift points up to about the same place as the "put there on purpose" reprogrammed ones - seems like good furtune.
There may be other things like firmness, traction, torque, fans and other stuff changed via reprogramming. But as far as drive ability it would seem the 8.6% rise in shift points just might be very close to the right "hot" setup.
[Modified by KingTut, 7:27 PM 1/16/2002]
What are your shift points?
Is this some kind of propriety information or some thing. Are not most all of them set about the same. And just what if the percentage change of the 3.15 to 3.42 happens to move the shift points up to about the same place as the "put there on purpose" reprogrammed ones - seems like good furtune.
There may be other things like firmness, traction, torque, fans and other stuff changed via reprogramming. But as far as drive ability it would seem the 8.6% rise in shift points just might be very close to the right "hot" setup.
[Modified by KingTut, 7:27 PM 1/16/2002]
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From: Murphy TX
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Re: Re-program for rear-end change (KingTut)
KingTut, you have a good question. I think Mike is gambling on just that. I'm not PL'ed but my WOT shift points are 6200 + change, rev limiter at 6400. Stock topend. I can dig up the MPH values if you're interested.
I suspect you are correct that the WOT shift points might be just about right for a more or less stockish car (including tire size) on a 3.15 to 3.42 swap. The thing to watch out for would be ensuring that it's under the rev limiter ;) I don't think either the 2.73 to 3.15 or 3.15 to 3.73s will make it. :nonod: Also, a lot of folks will choose to swap out the converter at the same time. Things like clutch lock up, slippage codes and so forth might also require some fiddling with code. Part throttle shifting is anybody's guess :D
But if your point is 1) reprogramming is not ABSOLUTELY necessary and 2) it's a bad idea to blindly reprogram without knowing what you've got, I'm with ya. :yesnod: :yesnod: :yesnod: I'm interested in hearing how Mike's project turns out.:yesnod:
I suspect you are correct that the WOT shift points might be just about right for a more or less stockish car (including tire size) on a 3.15 to 3.42 swap. The thing to watch out for would be ensuring that it's under the rev limiter ;) I don't think either the 2.73 to 3.15 or 3.15 to 3.73s will make it. :nonod: Also, a lot of folks will choose to swap out the converter at the same time. Things like clutch lock up, slippage codes and so forth might also require some fiddling with code. Part throttle shifting is anybody's guess :D
But if your point is 1) reprogramming is not ABSOLUTELY necessary and 2) it's a bad idea to blindly reprogram without knowing what you've got, I'm with ya. :yesnod: :yesnod: :yesnod: I'm interested in hearing how Mike's project turns out.:yesnod:
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Re: Re-program for rear-end change (ToplessTexan)
I don't think either the 2.73 to 3.15 or 3.15 to 3.73s will make it.
But even with the 3.15 to 3.42 upgrade, one can still improve shifting and torque management with a reprog. I personally didn't want to spend the plus-$400 if it wasn't mandatory.
The one thing I would like seen put-to-rest is the mis-information about speedometer error correction needed after a carrier swap.
I hope to put more miles on my Corvette this weekend. But at 80 miles since the upgrade, all has worked flawlessly (knock on wood) :)
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From: Murphy TX
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Re: Re-program for rear-end change (Mike Mercury)
The one thing I would like seen put-to-rest is the mis-information about speedometer error correction needed after a carrier swap.
I hope to put more miles on my Corvette this weekend. But at 80 miles since the upgrade, all has worked flawlessly (knock on wood) :)
This is with the stock converter, correct? What's part throttle shifting like?
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From: S.W. Ohio. . . . . . NRA Life Member
Re: Re-program for rear-end change (ToplessTexan)
This is with the stock converter, correct?
yes; since my setup is kinda unique, I guess I should mention this more often.
? What's part throttle shifting like?
No change at all as far as driveability goes. I do notice more acceleration in all gears at any throttle amount... not day/night though , but pleasingly noticeable. :)
I thought I detected some slight shift/downshift different "feelings" at first, but then couldn't detect it as I drove more miles. Maybe the PCM relearned and corrected. It is only an 8.6% change when going from the 3.15 to the 3.42. So a partial acceleration shift from first to second that occurs at 3000 RPM would now happen at 3280 RPM (with the stock TC).
yes; since my setup is kinda unique, I guess I should mention this more often.
? What's part throttle shifting like?
No change at all as far as driveability goes. I do notice more acceleration in all gears at any throttle amount... not day/night though , but pleasingly noticeable. :)
I thought I detected some slight shift/downshift different "feelings" at first, but then couldn't detect it as I drove more miles. Maybe the PCM relearned and corrected. It is only an 8.6% change when going from the 3.15 to the 3.42. So a partial acceleration shift from first to second that occurs at 3000 RPM would now happen at 3280 RPM (with the stock TC).
