Using shorter drag radials cause any problems?? - Page 6 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 09-14-2005, 05:06 PM   #51
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Quote:
Originally Posted by Rogue
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)
But wouldn't it be like an infinitesimally small tire? When would it shift? What is it about tire size that can change how quickly the diff spins?

Appearantly there is a key point/relationship between the tire circumferance and reluctor ring. What is it? I don't care if the speedo doesn't reflect actual speed of the car. Quit telling me that.

It'* all about rotational speed.

Example:
Say the engine is at 4000 rpm, first gear, so wheels are spinning at 1000 rpm (I made that number up).

The VSS sees the 1000 rpm because the reluctor ring should be spinning at the same speed as the wheels or something directly propertional to it(right?), translates it as 40 mph.

How does can that change the measured vehicle speed compared to engine speed?
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Old 09-14-2005, 06:08 PM   #52
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From my FSM

Quote:
The vehicle speed sensor (VSS) system is a pulse generator consisting of a speed sensor assembly, located in the case extension, and a toothed vehicle speed sensor rulector wheel, which is pressed onto the findal drive carrier aseembly. As the vehicle drives foreward, the vehicle speed sensor reluctor wheel rotates. This rotation produces a variable AC signal in the pickup coil that is proportiaional to vehicle speed.

The AC signal generated has a frequency and amplitude (voltage). The PCM uses frequency to calculate the vehicle speed. The voltage portion of the signal is used for diagnostic purposes only. The voltage will vary from .5 volts AC at 100rpm to as much as 200 volts AC at 6000 rpm.

This information is then sent to the PCM to indicate how fast the vehicle is traveling and to develop the shift pattern for the transmission.
Quote:
The PCM calculates the gear ratio based on data from the automatic transmission input shaft speed (AT ISS) sensor and the vehicle speed sensor (VSS). The PCM compares the known transmission gear ratio to a calculated ratio for each gear range selected. The VSS output is translated to RPM by the PCM for various uses.
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Old 09-14-2005, 06:10 PM   #53
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Quote:
Originally Posted by jwikoff99
Appearantly there is a key point/relationship between the tire circumferance and reluctor ring. What is it? I don't care if the speedo doesn't reflect actual speed of the car. Quit telling me that.
The speed is calculated by the PCM using various sensors as stated above. If the speedo is off then the PCM is off so there is a corelation. If the PCM is off then the shifts occurs at the wrong time.
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Old 09-14-2005, 06:26 PM   #54
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The only thing that is off is the actual speed of the car.

As long as you don't change the actual final drive ratio (not including tire size) without letting the PCM know about it, I still don't understand what the problem is.
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Old 09-14-2005, 06:34 PM   #55
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Quote:
Originally Posted by jwikoff99
The only thing that is off is the actual speed of the car.
Is that not what this is all about?

With smaller tires the PCM will shift early because it thinks it is faster than it is. If it shifts early you shift at lower rpms resulting in less than optimal track times and acceleration.

With larger tires the PCM will shft later because it thinks its slower than it is. If it shifts later at higher rpms then you run the risk of banging the limiter.

I'm sorry if I can't convey this to you properly however your welcome to try it at the track.
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Old 09-14-2005, 06:48 PM   #56
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We saw this first hand at BFest, BadSSEi got 3/4 down the track and flatened out(acceleration that is). Wouldn't up-shift, but was at rev limit. You could hear something wasn't right (and the burning supercharger oil didn't look good either)
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Old 09-14-2005, 07:45 PM   #57
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so let me get this straight. the computer is actually able to see that the final drive ratio has changed via an inconsistancy in acceleration? it is accelerating faster than it was before? and the computer can actually track that, and compensate for it? are we stuck in the "old school" fashion where the tranny shifts where it wants to, instead of being told to shift by the PCM? is that why we are getting confused?
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Old 09-14-2005, 07:59 PM   #58
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My tires are just a shade smaller than stock. Why did I hit the revlimiter in first at the track when I left it in Perform shift?

I'm beginning to think there is something here that none of us understand. I fail to see how tire diameter could possibly affect shifting rpm. Maybe the light weight or something give the tranny improper feedback. Cause everything to and including the wheels is still spinning in the same manner it was before. The PCM still thinks it'* going however fast and will shift accordingly.
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Old 09-14-2005, 09:22 PM   #59
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Quote:
Originally Posted by jwikoff99
The PCM still thinks it'* going however fast and will shift accordingly.
Exactly!

Look at it this way.

Lets say your doing 72mph at 2000 rpm in third.
When you floor it and it downshifts into second you shift into third at 105 mph which would be around 6200rpm.

Now you change your gear ratio and you now do 72mph at 2400 rpm.
When you floor it and it downshifts into second you shift into third at 105 mph which would be around 6600rpm.

If you didnt tell the PCM you changed the gears but the shift table says shift at 105mph what rpm would you shift at?

What if your limiter was still 6400?

I understand your not changing the actual gears and gearing but by having different tire sizes then what the computer knows skews the gear ratio.
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Old 09-14-2005, 09:47 PM   #60
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mmm...

I'm gonna stick with my story until I actually see otherwise. The PCM will shift at what it thinks is still the same RPM and MPH and throttle and will be normal.

Quote:
With smaller tires the PCM will shift early because it thinks it is faster than it is. If it shifts early you shift at lower rpms resulting in less than optimal track times and acceleration.
Why did I hit my limiter?



Quote:
Lets say your doing 72mph at 2000 rpm in third.
When you floor it and it downshifts into second you shift into third at 105 mph which would be around 6200rpm.

Now you change your gear ratio and you now do 72mph at 2400 rpm.
When you floor it and it downshifts into second you shift into third at 105 mph which would be around 6600rpm.

If you didnt tell the PCM you changed the gears but the shift table says shift at 105mph what rpm would you shift at?
What rpm? It'll shift at 6200 which it thinks is 105 mph. Why would it even think to do otherwise?
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