Effect of new gear on shift point?
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From: Kingston, Ontario, Canada
Effect of new gear on shift point? Not sure if I should post this in gear or tuning section but I'll start here. Just went from a 3.43 gear to a 3.73 to allow running a 28 inch tire. Obviously the mph is off on the speedometer by approx 10km per hour but other than that no drivability issues at all. Since I'm running a 26 inch street tire and 28 inch drag tire I'm assuming I would need two separate tunes to adjust for gear and tire size. My question is if I leave things as is for the street, with a 10km per hour speedo variance, how would that impact shift point at wot? Is shift point commanded by mph, rpm or a combination of the two? Appreciate any assistance,,,,tks
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From: Ann Arbor, MI
At throttle up to about 75%, the shift points are only set by mph. Above that, the trans does not shift until both the "WOT Shift RPM" AND "WOT Shift MPH" are reached. (The 75% can also be specified.) To simplify tuning, I set the WOT Shift speed table values 10% low so that I only need to change the WOT Shift RPM table to try different shift points at the track.
If you don't care about your speedometer accuracy, you can change gears and tires anyway you like without changing the trans shift tables and the car will shift at the exact same RPM as it did before. Of course once you correct the speedometer, you will need to change both the shift speed tables. IIRC, in HP Tuners, when you change the rear axle ratio, it changes the trans shift tables automatically for you.
In detail:
3.42/3.73 = .917
Therefore, all shift speed table must be reduced by multiplying them by .917
Example, if your 1-2 WOT shift speed was 43 mph, it will now be 39.
I hope that makes sense and answers you question.
If you don't care about your speedometer accuracy, you can change gears and tires anyway you like without changing the trans shift tables and the car will shift at the exact same RPM as it did before. Of course once you correct the speedometer, you will need to change both the shift speed tables. IIRC, in HP Tuners, when you change the rear axle ratio, it changes the trans shift tables automatically for you.
In detail:
3.42/3.73 = .917
Therefore, all shift speed table must be reduced by multiplying them by .917
Example, if your 1-2 WOT shift speed was 43 mph, it will now be 39.
I hope that makes sense and answers you question.
Thread Starter
9 Second Club
iTrader: (4)
Joined: Aug 2009
Posts: 317
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From: Kingston, Ontario, Canada
At throttle up to about 75%, the shift points are only set by mph. Above that, the trans does not shift until both the "WOT Shift RPM" AND "WOT Shift MPH" are reached. (The 75% can also be specified.) To simplify tuning, I set the WOT Shift speed table values 10% low so that I only need to change the WOT Shift RPM table to try different shift points at the track.
If you don't care about your speedometer accuracy, you can change gears and tires anyway you like without changing the trans shift tables and the car will shift at the exact same RPM as it did before. Of course once you correct the speedometer, you will need to change both the shift speed tables. IIRC, in HP Tuners, when you change the rear axle ratio, it changes the trans shift tables automatically for you.
In detail:
3.42/3.73 = .917
Therefore, all shift speed table must be reduced by multiplying them by .917
Example, if your 1-2 WOT shift speed was 43 mph, it will now be 39.
I hope that makes sense and answers you question.
If you don't care about your speedometer accuracy, you can change gears and tires anyway you like without changing the trans shift tables and the car will shift at the exact same RPM as it did before. Of course once you correct the speedometer, you will need to change both the shift speed tables. IIRC, in HP Tuners, when you change the rear axle ratio, it changes the trans shift tables automatically for you.
In detail:
3.42/3.73 = .917
Therefore, all shift speed table must be reduced by multiplying them by .917
Example, if your 1-2 WOT shift speed was 43 mph, it will now be 39.
I hope that makes sense and answers you question.
Perfect! Thanks so much for the help!
Teching In
Joined: Jun 2012
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If you don't care about your speedometer accuracy, you can change gears and tires anyway you like without changing the trans shift tables and the car will shift at the exact same RPM as it did before. Of course once you correct the speedometer, you will need to change both the shift speed tables. IIRC, in HP Tuners, when you change the rear axle ratio, it changes the trans shift tables automatically for you.
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From: Ann Arbor, MI
The PCM reads RPM from the engine and speed from the VSS at the trans tailshaft. It knows nothing about anything after that.
Technically, the load on the converter will be slightly different which might change its % slippage. I could therefore see a change from e.g. 4.10 to 3.42 causing the engine to hit the rev limiter just before it shifts. However changing from 3.23 to 3.73 should make the converter more efficient.
I suspect the guy that had to get a tune after the rear end change also changed something else, perhaps the converter.
Teching In
Joined: Jun 2012
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Yes, that is correct. I run 26" tires on the street and switch to 28" at the track and the shift RPM does not change.
The PCM reads RPM from the engine and speed from the VSS at the trans tailshaft. It knows nothing about anything after that.
Technically, the load on the converter will be slightly different which might change its % slippage. I could therefore see a change from e.g. 4.10 to 3.42 causing the engine to hit the rev limiter just before it shifts. However changing from 3.23 to 3.73 should make the converter more efficient.
I suspect the guy that had to get a tune after the rear end change also changed something else, perhaps the converter.
The PCM reads RPM from the engine and speed from the VSS at the trans tailshaft. It knows nothing about anything after that.
Technically, the load on the converter will be slightly different which might change its % slippage. I could therefore see a change from e.g. 4.10 to 3.42 causing the engine to hit the rev limiter just before it shifts. However changing from 3.23 to 3.73 should make the converter more efficient.
I suspect the guy that had to get a tune after the rear end change also changed something else, perhaps the converter.
Thanks.
Teching In
Joined: Nov 2013
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From: Fontana, CA
I am having the same problem with my '01 Z28 A4 after getting it partially tuned for the rear end gears. I went from 2.73 to 3.73 and driving it without a tune it shifted just fine other than speedometer showing 105 mph when in reality I was doing 75 via GPS. Once my speedometer was adjusted for the gears swap, I hits the rev limiter before it shifts. Now I need to get it retuned and have the mph table adjusted to lower mph shift points as mention in thread 2 from mrvedit. Hopefully this fixes my problem and its not the tranny that is going out. lol
