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Old 10-31-2012, 09:28 AM   #9
Devs93
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What are some ways to improve automatic transmission performance?
There are two commonly found 4-speed automatic transmissions: 4L60/700R4 or 4L60E. GM rated these transmissions up to 370 lbs-ft of torque. The heavy-duty 4L80E had a much higher torque capacity rating and was used in 1991 and later GM vehicles, but its use was limited to 454SS OBS trucks only. If you want to really improve the torque capacity of your transmission, you need to rebuild it using a performance rebuild kit that include performance bands and clutch packs from companies like TCI automotive (Pro Super kit) and Jet Performance. This will improve your transmission’s torque capacity up to 470-500 lbs/ft or enough to support over 450 hp. Some high performance heavy duty kits will even support up to 650 hp. Whole transmissions can be also be purchased. Monster Transmission and Performance (http://www.eatmyshift.com/) has 2 levels of upgraded 700R4 and 4L60E transmissions available starting at $1895, no core charge, and free shipping.

There are also simple ways to improve your transmission’s shifting performance and increase the life of the transmission bands by installing a transmission shift kit and installing larger servos from Sonnax (or Jet Performance). A popular upgrade is to install the Corvette (or even larger billet) servo that activates the 2-4 band assembly to guard against slippage while in 2nd gear. It will also help to quicken the 1-2 shift. There is also a 4th gear servo that can be installed that helps to firm up the 3-4 shift and hold the shift band tighter to prevent wear. A shift kit will provide stronger, quicker shifts through out the transmission bands. Either of these upgrades can be perform alone or in conjunction with one another. However, these upgrades will not improve the torque capacity of the transmission, but they will help to reduce wear (slippage) of the transmission bands and may prolong the life of your transmission.

If you have a 4L60E transmission, B&M also makes a product called the Shift Improver Plus+ that splices onto two wires leading from your ECM to the transmission. It has a rocker switch that has two settings on it: firm and extra firm. It serves to electronically increase the line pressure to hold the transmission bands and quicken shifts (similar in function to the TV cable on 700R4 transmissions). I’ve used one on my truck and it does work. This would be a good option for those that occasionally tow or want performance shifts, but not all the time.


What about rear end upgrades? Any good?
We’re talking rear end gears here. This discussion goes hand-in-hand with tire size, because with many truck owners are installing larger 20” or 22” wheels, and it is not unheard of now to have tires that are between 29-32” and much taller than the factory-supplied tires. Take my factory tire for example. It was a 235/75R15 or a 29” tall tire. I’m running a 265/50R20 tire or a 30.5” tall tire and my truck has a 3.42:1 rear end. The addition of these tires to my truck would be equivalent to a similar truck with the stock 29” tires and a 3.25:1 ratio rear end. In order to restore the performance that I had with the stock tires, I would theoretically need a 3.60:1 rear end. Since one isn’t made, the closest one would be a 3.73:1. For 33” tires, I’d theoretically need a 3.90:1 rear end. A good rear end ratio calculator can be found here: http://www.idavette.net/tech/ratioc.htm.


What about rear end differentials? What kinds are there? Do I need one?
Most trucks these days come with mechanically locking differentials, although there are both 2WD and 4WD trucks that have open differentials. Find the RPO codes on your truck (usually in the glovebox) and then check here: http://www.fullsizechevy.com/forums/...d.php?t=164851. The mechanically locking differential is a speed sensitive design that reacts to wheel slip by sensing when one wheel is spinning substantially faster than the other. So most of the time it works as a regular open differential, but as soon as wheel slip happens, going forward or reverse, the locker immediately kicks in. Note axle lockup can only occur at speeds below 20 mph. A simple design and it works well. For some reason, it gets knocked down in favor of the limited-slip differential. GM uses the Eaton G80 mechanical locking differential: http://www.traction.eaton.com/prod2.htm. A better design is available with the Eaton E-locker, which is essentially a mechanical locking differential where the driver has complete control of when and what gets locked.

Another design is the limited-slip differential (http://www.traction.eaton.com/prod3.htm). It uses input torque rather then wheel slip to activate the locker. A common misconception is a limited slip differential provides traction to both wheels all the time. With normal or light throttle applications on dry pavement, only the drive wheel receives torque (or power) - just like an open differential. Detroit Locker makes a clutchless design with no clutch packs to wear out called the Detroit TruTrac (http://www.detroitlocker.com/DT.htm). A great design and should be standard for all limited-slip differentials.

Another new product is the Detroit Electrac (http://www.detroitlocker.com/DE.htm), combining the utility of the TruTrac limited-slip differential with electronic user activation of a full locker controlled by a switch mounted on the dash. The ultimate combination and the best of both worlds.
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