General transmission info, By MisterP Options

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Here is a well written bit of info by MisterP.


BASICS FOR TRANNY MOD NEWBIES
An automatic transmission changes gears by applying clutches and bands in a programmed sequence. For every gear there is a friction material part in the tranny for that gear; you get "first gear" because the PCM (computer) instructs the transmission to apply the 1st gear clutch, just like you would if driving a manual trans vehicle.

An automatic transmission can be thought of as basically four sections or assemblies working together as a system. It has gears/powertrain, aka "hard parts" that are the shafts and gears which actually carry the power through the transmission. There are also clutches and bands, these are the friction materials designed to "grab" and "hold" a gear and wear out doing so. There is also an intricate hydraulic system that is used to make the clutches and bands move, and lastly it's all held in a big aluminum case attached to the back of the engine.

When the PCM thinks it's time to shift a lot of things happen, beginning with instructing the engine to make less power; this "Torque Management" requires about a quarter-second to take effect. After the engine has weakened and 'flattened-out' the PCM will continue to it's next job, releasing the clutch for the current gear, which takes almost a 1/3-second, and then gently applying the clutch for the next desired gear, adding another 1/3-second. Once the PCM is convinced that the gear it commanded is engaged successfully it will then go ahead and begin ramping-up engine power. And off you go...

The consequence of all this choreography is that:
(1) you get a very smooth, uneventful, luxury barge land yacht kind of shift;
(2) it takes a very long time to shift between gears (.75 to 1 second);
(3) engine momentum/performance is killed by torque management programming;
(4) friction materials in the tranny take a serious beating because the computer is slipping the clutches baaaddd during the shift, just like an inexperienced driver slipping the clutch of a manual trans vehicle in and out of gears.

GM chose to sacrifice friction material life in trade for a non-threatening shift; they got away with this using torque management as a crutch. Without the PCM cutting power between shifts the tranny friction materials would overheat and fail in short order. But even with torque management there is still a lot of friction material wear, and the performance is honestly piss-poor. The 4l60E transmission is actually inadequate for the truck, it was intended for passenger car use with 25% less weight and a smaller engine to boot (less torque) - it is stressed in the SS/VHO and the OEM gets by employing torque management. If you have attempted to overtake another vehicle at WOT and during the 2-3 upshift the truck seems to struggle and take an eternity, that's torque management in action saving the OEM tranny from burning itself up during shifts. So the SS/VHO have a very legitimate longevity, reliability, and performance issue in the shift programming of the as-delivered 4L60E transmission.

THE AFTERMARKET TO THE RESCUE
With careful selection and installation of parts and some PCM re-programming you can upgrade the 4L60E to handle about 450 lb-ft of torque without needing the intervention of torque management; this is A Good Thing. Parts you can find in the aftermarket:

* shift kits - a Shift Improver Kit is a collection of small parts that change the physical characteristics of the transmission's hydraulic system, mainly check valves and springs and passageway size adjustments and such. Changing these hydraulic parts will affect both speed of shifting as well as shift pressures, or how hard the tranny grabs/holds a gear. Also included in the shift kit will be parts for the main hydraulic pump, increasing output pressure/volume and operation at high RPMs. There are several 'intensities' of shift kits, you can purchase a 'shift improver kit' or a 'full race kit' - the difference being that the shift improver kits are calibrated to deliver maximum transmission life, and the racing kits are calibrated to give the absolute fastest possible shift (at the expense of tranny life). Only install a shift improver kit, never a race kit, in your truck.

* servos - in simplest terms, a servo is a hydraulic plunger, or a part that will move when you pump fluid into it. When a servo is "energized" it clamps down on a band or clutch, which in turn grabs the intended gear; when it is de-energized it relaxes and releases its grip on the gear. GM makes an improved 2nd gear servo (used in the Corvette) that makes the 2nd gear friction material clamp down on 2nd gear with much more force eliminating clutch slippage. Also, several aftermarket companies make servos from billet aluminum capable of delivering serious pressures ("billet servos"), making the band or clutch grab its gear with a true death grip.

* Kevlar bands and clutches - bands and clutches (the friction materials) in a OEM transmission are basically high-grade sandpaper glued to metal plates (not making this up here!); specialty companies do make bands and clutches from exotic high strength materials for extreme duty applications.

* High strength shafts, gears, gear cages, etc - a few specialty machining companies make internal hard parts from exotic aerospace-grade materials, and these pieces are not cheap but can make a 4L60E survive at power levels of 650 lb-ft of torque, or towing capacities of 40,000-lbs GVW.

* PCM Tuning software - special computer programs that can be used to change transmission-related settings in the vehicle's onboard PCM, including disabling torque management, adjusting shift pressures, modifying upshift and kickdown points, rev limits, etc. PCM reprogramming is necessary when modifying your transmission because the PCM is an adaptive computer, and any changes it observes in the internal operations of the transmission will be errantly "corrected" over time; meaning, the PCM will soften the vehicle's shift behavior as best as it is able in order to deliver that Cadillac-smooth shift it was originally programmed to deliver, after an learning period the truck will go back to a stock shift!

* Synthetic tranny fluid - outlasts typical ATF and runs cooler but costs a lot more (like 2x more) and delivers a softer shift. If you are using a high-stall converter (not the Trailblazer converter, but 2800-RPM and higher) then you might consider using synthetic transmission fluid and compensating for the lower shift performance by having the pump output increased a small bit. Ford ATF and B&M Trick Shift fluids will deliver a much more pronounced shift, the low-buck GM racing crowd calls Ford ATF 'shift kit in a bottle'. Be sure to discuss what type of fluid you intend to use in your transmission with your shift kit installer; if in doubt continue to use GM Dexron III.

* Large transmission coolers - bigger special purpose radiators for keeping tranny temps under control. Heat kills transmissions FAST, optimum temperature for an automatic transmission is around 150-degrees, and anything above 190 is going to affect the lifespan of the transmission friction materials. High-stall converters and aged/inferior ATF fluids will raise transmission temperatures (installing the Trailblazer converter will increase temps 20-degrees as a rule).


PUTTING ALL THIS TOGETHER:
So, some general suggestions:

(1) you can drive the tranny as-is - if you decide to eliminate the torque management from your PCM without installing a shift improver kit you are doing so at your own risk. As a personal opinion: don't bother adding more than a catback and CAI to the truck as any gains made in more substantial powertrain mods will be wasted in a severely handicapped shift. And high-output powertrain upgrades (FI, NOS) will eventually wreak havoc on a stock-calibrated slushbox.

(2) Entry-level tranny mods - install a shift improver kit, a Corvette 2nd gear servo, and turn off torque management in your PCM. Expect to pay $200-300 for parts, a few hours for installation labor, and $$$ for a specialist to tune the PCM.

It is possible to leave the transmission in the vehicle and still install the shift kit, however the main hydraulic pump will not be reachable meaning that you will not be able to install the hi-rev springs; if you do not expect to rev higher than stock (~6000 RPM which covers most of us) then this is not an issue. With stock friction materials and hard parts this transmission should give reliable service up to 450 lb-ft of torque, go .2-secs faster in the 1/4-mile, and be a whole lot more fun to drive. Good for almost all N/A LQ9 powertrains, many entry-level FI setups including Radix; for those spraying NOS during the shift window you are really stressing your transmissions!

A topic aksed a lot is "how hard does 2nd gear shift?" With a Vette servo installed correctly driving the truck sedately will not give any hint there is a shift kit at all - nobody would be the wiser, except that gear-to-gear changes are fast and no-nonsense. But plant your foot down and you can expect shifts above 2/3rds throttle to be an intense experience, it'll put shock & awe in your unsuspecting passenger. However the shift will not be so hard as to be considered violent, or in risk of damaging the friction materials or tranny internals, again provided the shift kit was installed correctly. Second-gear shift will be "authoritative" at anything more than 1/2-throttle but not outright violent; still, take care not to spill hot McCoffee in your lap.

A common shift-kit issue is that low-speed 2nd gear upshifts (like puttering away from the stoplight) can come delivered with a minor jarring; the phrase for this is 'low-speed banging into second' and there is no risk of harm to the tranny but can get annoying. BManSS has refined a way of adding an additional preload spring into the 2nd gear accumulator which domesticates this behavior. So if you are shied away from installing a shift kit because you are worried about the truck banging into 2nd gear as you are driving through the Walmart parking lot at 5mph, a solution has been found for that.

(3) Add a large capacity transmission fluid cooler - this is a must if using a higher stall torque converter especially in a hotter climate.

(4) Add a billet 4th gear servo - installing an aftermarket billet 4th gear servo will greatly improve upshifting into top gear; when shifting into 4th at WOT this servo will grab 4th about as hard as the OEM tranny grabs 2nd. This is a very welcome improvement for N/A trucks, and totally necessary if you are making FI power in top gear.

(5) Go all out - if you are running high torque loads (turbo, or LQ9 + NOS) then you're gonna need help in the way of improved transmission internals. Good news is that there are currently parts and knowledge available to make the tranny hold together up to 650 lb-ft of torque.

(6) go insane - swap for the much heavier 4L80E, and that's beyond the scope of this essay.