I am noticing that under full throttle launches the shift points seem earlier than before I put larger tires on. From 1st to 2nd it is shifting 500-800 rpm before redline (It did this before). But from 2nd to 3rd it is shifting at least 500 rpm before redline as well - I don't recall it doing this before the larger tires. Not sure about any other gears.
I am wondering if the fact that I haven't had my computer re-calibrated for larger tires is affecting my shift points? Any ideas, thoughts?
Thanks!
Hi dude.
I once experienced the same phenomenon as you, but in the opposite direction. I had a Chev with 4L80E at the dragstrip. This transmission is electronically controlled as well, with WOT shift points as functions of throttle position and vehicle speed.
One time I went with about 2" smaller diameter tires to try and gain a little added effective gearing without a rear end swap. At the track I noticed the shift points were consistently about 300-400 rpm higher, despite not making any changes to the upshift maps in the PCM. I had an engineering contact within GM who, although he was not involved with the Hydramatic division, offered a possible explanation.
The transmission uses a buffered or time averaged version of the vehicle speed sensor output. Any sort of averaging will introduce latency (delay) in the speed signal. With the smaller tires, the machine was ripping through the gears and requiring upshifts much faster. Since the delay remained the same (a programmed function of the averaging), the delay would then become effectively more pronounced to the faster rise rate of the speed signal with the smaller tires. This in turn made the buffered speed seen by the 4L80E a little slower than actual speed during acceleration. The 4L80E waited a split second longer before it upshifted because that split second was needed to allow the 4L80E buffered sensor speed to "catch up" and reach the programmed shift speed, even though the vehicle was actually already well over the shift speed. The delay in upshift timing naturally causes the upshift to occur at a higher rpm.
...so obviously your transmission will shift at different RPM's than it used to. because you have just changed, where in the RPM band, that the load is being felt by the engine...
I can attest to that. Prior to installing the Truspeed, I would tach much higher in the low range until it got up to speed, but after the install, everything seemed to level off back to normal.
__________________
Waiter, how do you prepare your chickens?
Nothing special. We just tell them straight out that they're going to die.
Base: 2001 Tundra - Thunder Grey | SR5 | TRD | 4x4 | HD TRD Coil Springs | Tow Pkg | Factory Spray-on Bedliner | RS3000 Security
Mods: Mickey Thompson Classic II | Bilstein 5100 Shocks | Differential Drop Spacers | 930 Inner CV Boots | ProtechEZLift Limit Straps | Spintech Sportsman XL Muffler | Brembo Front Brake Rotors | Heavy Duty OEM Rear Drums | A.R.E. Z-Series Cap w/ Thule Rack | Stubbs Rock Sliders | Hellwig Anti-Sway Bar | 285/75/16 BFG All Terrain TA KO's | VIAIR 400C Air Compressor w/ 2.5 gal tank | Front clear corner lamps | Odyssey PC1700T battery | Optima Red Top 75/35 battery (backup starter) | Hellroaring 95300A Isolator/Combiner | Truspeed Calibrator
Future: Winch Bumper | UCAs & Coilovers | RCD Lift | 4.30 Gears
I once experienced the same phenomenon as you, but in the opposite direction. I had a Chev with 4L80E at the dragstrip. This transmission is electronically controlled as well, with WOT shift points as functions of throttle position and vehicle speed.
One time I went with about 2" smaller diameter tires to try and gain a little added effective gearing without a rear end swap. At the track I noticed the shift points were consistently about 300-400 rpm higher, despite not making any changes to the upshift maps in the PCM. I had an engineering contact within GM who, although he was not involved with the Hydramatic division, offered a possible explanation.
The transmission uses a buffered or time averaged version of the vehicle speed sensor output. Any sort of averaging will introduce latency (delay) in the speed signal. With the smaller tires, the machine was ripping through the gears and requiring upshifts much faster. Since the delay remained the same (a programmed function of the averaging), the delay would then become effectively more pronounced to the faster rise rate of the speed signal with the smaller tires. This in turn made the buffered speed seen by the 4L80E a little slower than actual speed during acceleration. The 4L80E waited a split second longer before it upshifted because that split second was needed to allow the 4L80E buffered sensor speed to "catch up" and reach the programmed shift speed, even though the vehicle was actually already well over the shift speed. The delay in upshift timing naturally causes the upshift to occur at a higher rpm.
This makes sense - So essentially, the ECM uses the speed signal to predict when to shift? Is that what your saying? And the way it does that is by using time averaging of the speed sensor signal?
So, by the same logic, the shift points would change if the vehicle is going uphill or down, correct? Or pulling a load? Because the vehicle will accelerate at drastically different rates between the scenarios.
I forgot to note in my original post that I WAS in Tow/Haul mode when I noticed this behavior. I haven't noticed any more advancement of shifts when its not in this mode - I was just a little confused at the time because I thought Tow/Haul was supposed to hold the gears longer.
I can attest to that. Prior to installing the Truspeed, I would tach much higher in the low range until it got up to speed, but after the install, everything seemed to level off back to normal.
I'm sorry, I'm not sure I fully understand what you're saying. What do you mean by "tach much higher in the low range?" and "level off back to normal"?
I meant, in the low gears (1st, 2nd..) RPMs would stay higher than they used to, prior to 33" tires, and after the Truspeed, it would shift much cleaner, the way it did with the stock tires. For example, if I was taking off from dead stop, it would almost always hang out a the high RPM in 1st and sometimes 2nd before shifting over. It did this pretty much immediately after the 33's got installed. I learned to just feather the pedal a little bit which forced it to shift over into the next gear, but after the speedo got readjusted back, it started shifting like it used to.
__________________
Waiter, how do you prepare your chickens?
Nothing special. We just tell them straight out that they're going to die.
Base: 2001 Tundra - Thunder Grey | SR5 | TRD | 4x4 | HD TRD Coil Springs | Tow Pkg | Factory Spray-on Bedliner | RS3000 Security
Mods: Mickey Thompson Classic II | Bilstein 5100 Shocks | Differential Drop Spacers | 930 Inner CV Boots | ProtechEZLift Limit Straps | Spintech Sportsman XL Muffler | Brembo Front Brake Rotors | Heavy Duty OEM Rear Drums | A.R.E. Z-Series Cap w/ Thule Rack | Stubbs Rock Sliders | Hellwig Anti-Sway Bar | 285/75/16 BFG All Terrain TA KO's | VIAIR 400C Air Compressor w/ 2.5 gal tank | Front clear corner lamps | Odyssey PC1700T battery | Optima Red Top 75/35 battery (backup starter) | Hellroaring 95300A Isolator/Combiner | Truspeed Calibrator
Future: Winch Bumper | UCAs & Coilovers | RCD Lift | 4.30 Gears
I meant, in the low gears (1st, 2nd..) RPMs would stay higher than they used to, prior to 33" tires, and after the Truspeed, it would shift much cleaner, the way it did with the stock tires. For example, if I was taking off from dead stop, it would almost always hang out a the high RPM in 1st and sometimes 2nd before shifting over. It did this pretty much immediately after the 33's got installed. I learned to just feather the pedal a little bit which forced it to shift over into the next gear, but after the speedo got readjusted back, it started shifting like it used to.
This makes sense - So essentially, the ECM uses the speed signal to predict when to shift? Is that what your saying? And the way it does that is by using time averaging of the speed sensor signal?
Speed and throttle position are the two main parameters the 4L80E uses to do WOT upshifts. The averaging on the speed sensor signal is just to filter any noise or unevenness that might be present in the speed sensor signal.
Quote:
Originally Posted by THUNDERA
So, by the same logic, the shift points would change if the vehicle is going uphill or down, correct? Or pulling a load? Because the vehicle will accelerate at drastically different rates between the scenarios.
I suppose it would, if the theory we discussed is correct. I can't say I ever tried watching the upshifts on steep grades though, so I can't confirm either way.
So if I installed a calibrator like the Hypertech or the Truspeed, does that alter the speed signal to the ECM? Or just to the speedo?
I'm wondering what would happen if I installed one of these and entered 37's in and watched the engine pull all the way to redline for once...that would be nice. Hmmmmm....
Well if you are going to put money into a calibrator, would you consider waiting for one of the tuners to come out instead, then directly alter the shift points with the tuner?
Well if you are going to put money into a calibrator, would you consider waiting for one of the tuners to come out instead, then directly alter the shift points with the tuner?
even the tuners(if or when one gets released) wont be able to physically input your shift points. part of the canned tune in them, normally contains a parameter that holds your shifts longer. but that will probably be the closest thing you will get, to altering your shift points.
So if I installed a calibrator like the Hypertech or the Truspeed, does that alter the speed signal to the ECM? Or just to the speedo?
I'm wondering what would happen if I installed one of these and entered 37's in and watched the engine pull all the way to redline for once...that would be nice. Hmmmmm....
It supplements the signal to the ECU, not just the speedo/odometer. The core signal coming from the speed sensors gets added to or deducted from with these recalibrators, based on the tuning parameters you set, so the ECU starts receiving what it's expecting (based on stock programming) +/- the adjustment value, which in-turn gets used to translate back into the true speed being displayed on the speedo as well as how it's interpreted for shifting.
On a side note, I looked at the Truspeed site and I don't think they have one for the late model 07/08 Tundras yet. Their application guide only goes up through 2005, so you may have to go with the Hypertech.
__________________
Waiter, how do you prepare your chickens?
Nothing special. We just tell them straight out that they're going to die.
Base: 2001 Tundra - Thunder Grey | SR5 | TRD | 4x4 | HD TRD Coil Springs | Tow Pkg | Factory Spray-on Bedliner | RS3000 Security
Mods: Mickey Thompson Classic II | Bilstein 5100 Shocks | Differential Drop Spacers | 930 Inner CV Boots | ProtechEZLift Limit Straps | Spintech Sportsman XL Muffler | Brembo Front Brake Rotors | Heavy Duty OEM Rear Drums | A.R.E. Z-Series Cap w/ Thule Rack | Stubbs Rock Sliders | Hellwig Anti-Sway Bar | 285/75/16 BFG All Terrain TA KO's | VIAIR 400C Air Compressor w/ 2.5 gal tank | Front clear corner lamps | Odyssey PC1700T battery | Optima Red Top 75/35 battery (backup starter) | Hellroaring 95300A Isolator/Combiner | Truspeed Calibrator
Future: Winch Bumper | UCAs & Coilovers | RCD Lift | 4.30 Gears
Re: Larger tires affecting shift points?
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