Using shorter drag radials cause any problems?? - Page 7 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 09-14-2005, 10:06 PM   #61
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We will just have to agree to disagree on this one.
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Old 09-14-2005, 10:53 PM   #62
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Firstly, did we determine if the Series I shifts at WOT according to MPH? The Series II definately does.

Secondly, there is another aspect not yet mentioned (I think) and that is acceleration with smaller diameter tires and how that effects the shift.

The Series II PCM commands a shift at a given MPH, say 46 MPH which should be about 5800 RPM, it takes time for the tranny to actually accomplish the shift...maybe 100 RPM difference from the time it'* commanded to the time it'* completed. If you have smaller diameter tires, you effectively have a lower gear ratio (higher numerically) and thus have more torque multiplication. Depending on how much power you have and how good your tranny is, it may take perhaps 300 RPM before the shift is actually completed.

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Old 09-14-2005, 11:40 PM   #63
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Ahh, there it is. That sounds much more likely. Thanks, Foggy.
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Old 09-15-2005, 03:09 PM   #64
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Quote:
Originally Posted by Foghorn
Firstly, did we determine if the Series I shifts at WOT according to MPH? The Series II definately does.

Secondly, there is another aspect not yet mentioned (I think) and that is acceleration with smaller diameter tires and how that effects the shift.

The Series II PCM commands a shift at a given MPH, say 46 MPH which should be about 5800 RPM, it takes time for the tranny to actually accomplish the shift...maybe 100 RPM difference from the time it'* commanded to the time it'* completed. If you have smaller diameter tires, you effectively have a lower gear ratio (higher numerically) and thus have more torque multiplication. Depending on how much power you have and how good your tranny is, it may take perhaps 300 RPM before the shift is actually completed.

Cheers,
Unless my thread'* been hijacked, we're talking about a naturally aspirated Series 2, 1995 vintage.

Just thought of something today...If what you're saying is true, then wouldn't the guys who've went with 245/50/16 tires (as an "upgrade") also already be having some weird shifting characteristics? After all, they are an inch (or more) shorter than the stock size as well...
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