Using shorter drag radials cause any problems?? - Page 4 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 09-14-2005, 04:15 PM   #31
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Quote:
Originally Posted by rjolly87
Quote:
Originally Posted by Rogue
The VSS gets it signal from the rotation of the reluctor ring on the differential. With smaller or larger tires the rotational speed of that ring is off thereby effectiveily changing the final drive ratio.
adding acceleration. but the only clue the car would have is increased acceleration, which it doesnt care about
You have to have acceleration to get to the shift point, otherwise you would remain in the same gear.
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Old 09-14-2005, 04:20 PM   #32
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Quote:
Originally Posted by rjolly87
what rogue is saying, is that the car will shift early <in the rpm band> because of this, which is what we are trying to understand.
OK, I don't understand that either..."IF" the car shifting is solely rpm dependent (like some have expressed), then it isn't going to be an issue. If it is rpm AND mph dependent(like I think rogue said), then I may have some issues...reason being, the transmission won't know who to believe...the VSS or the PCM...

Hopefully, I can just manually shift it and that'll take care of the potential problem...

Whatdya think rogue, what are my chances of being able to manually shift it and eliminate the possibility of weird crap happening??
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Old 09-14-2005, 04:22 PM   #33
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Quote:
Originally Posted by Rogue
The VSS gets it signal from the rotation of the reluctor ring on the differential. With smaller or larger tires the rotational speed of that ring is off thereby effectiveily changing the final drive ratio.
Yes, now how does that change anything besides the speedometer/actual speed relationship with regards to shifting? Does tire size affect how the reluctor ring spins with relationship to tire rotational speed?

Am I incorrect in assuming the VSS only sees the rotational speed of the wheels?

The way I understand the system, the only way for tire size to change anything the PCM sees is if the VSS were in the fenderwell, with its own little wheel of known diameter spinning on the tire tread or on the ground.
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Old 09-14-2005, 04:30 PM   #34
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Quote:
Originally Posted by jwikoff99
Does tire size affect how the reluctor ring spins with relationship to tire rotational speed?

Am I incorrect in assuming the VSS only sees the rotational speed of the wheels?

.
Yes, to both. The VSS (vehicle speed sensor) is the "link" from the transmission/differential, to the speedometer... if the tires are rotating faster, then it has to make the reluctor ring also spin faster, causing an artificial HIGH speedometer reading...it may say 65mph, but I'm "REALLY" only going 53mph...I am pretty sure that'* where rogue'* theory is saying that because the rpm'* are at a certain point, and the mph is also at a certain point, with shorter tires, that "point" has now been changed and the PCM will not shift the transmission properly, or at least at how it does when everything is stock...
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Old 09-14-2005, 04:31 PM   #35
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okay, lets blow this out of proportions, and put little 6 inch tires on <no they wont fit, but im making a point> and set him loose on the track. he takes off, and when the vss reads 50 mph he shifts correct? it is after all at the top of the gear according to the PCM calculations. but what you are saying, is that it will shift lower, at say 3000 rpm, instead of 5800, because that is when it is doing 50mph. if that were the case, and you could magically change the shift points to be at the top of the gear, you could roll 100 mph in 1st gear<according to the vss> which doesnt make sense to me. am i correct in saying that first gear is made taller with smaller tires?

***please note, this is pretty much what i understand you saying, just with differant numbers***
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Old 09-14-2005, 04:36 PM   #36
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What makes the reluctor ring spin?
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Old 09-14-2005, 04:37 PM   #37
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What does the PCM do if you're airborn and you floor it?
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Old 09-14-2005, 04:39 PM   #38
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The VSS calculates speed based on how many times a magnet reads a tooth in the 32 tooth reluctor wheel around the differential. The PCM Calculates speed based on how many times a hit (signal) is made on the reluctor.

When gearing is changed in the car (example 2.93 to 3.29) we used to put a reluctor spoof on, change the amount of teeth to 34. The signal hit rate would be the same time per rotation now with 34 teeth instead of 32.

Without telling the PCM anything has changed it will contintue to calculate speed based on the hit rate of the relecutor. Because the tires are smaller it will spin faster give a false reading which is why your speedometer has an inflated number (doing 75 instead of 72 for example). Because the PCM calculates shift tables based on rpm and vehicle speed an artificially inflated number will cause that calculation to be off.

In rjollys example above the VSS will see 50 mph almost instantly with 6 inch tires due to the rotation of the reluctor spinning so fast and so it will shift way way WAY early.
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Old 09-14-2005, 04:40 PM   #39
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Quote:
Originally Posted by jwikoff99
What makes the reluctor ring spin?
It is attached to the differential.
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Old 09-14-2005, 04:40 PM   #40
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Quote:
Originally Posted by jwikoff99
What does the PCM do if you're airborn and you floor it?
With no load on the tires it will spin them pretty fast and shift through all the gears (sort of like a burnout)
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