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The Smart Tranny

Posted in How To on March 26, 2004
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When speaking about the advancements in new-generation trucks, the conversation typically turns to engines. Terms such as sequential port injection, overhead cams, mass airflow sensor (MAS), all-aluminum construction, speed density system, and so on come to the surface, but the greatest advancement is the way we've made our drivetrains smart.

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Sensors feed information to a computer, which tunes the fuel and timing for maximum performance to a given situation. This has not only increased power per cubic inch with better fuel economy, it's also provided the modern engine a much greater life expectancy.

Just look at the trucks for sale in your local paper. New trucks have more miles because of increased commuting, yet still hold a decent value because they run exceptionally well for the number of miles. Years ago, you couldn't get anything out of a newer vehicle with 100,000 miles on it, except $40 from your local dismantler. Advancements in machining and materials play a part in this increased quality, and coupled with electronic management, the improvements are truly incredible.

What does this have to do with transmissions, you ask? Well, the same technology that has given engines a brain has been incorporated into the newer transmissions. The first true GM electronic tranny didn't come around until 1991. It was a 4L80E and was only equipped on trucks with big-blocks. After GM got its nose bloody with the 700R4 (the first smaller version of an overdrive tranny), the 4L60E replaced it in 1993. The 4LE transmissions have come a long way from the old 350 and 400 turbo hydromantics that coupled the earlier engines to their differentials.

We contacted the guys at Jet Performance Products in Huntington Beach, California, for a peek inside the differences in the new trannies. After talking with the crew and getting lost a few times, it was quite clear that covering all the functions of a transmission would take a year of our entire editorial calendar, and that wouldn't even touch the more complex theory of transmission functions. So we decided to pull out the best parts, highlighting how the basics work and what Jet does to beef up the tranny.

Although we used the 4L80E to show the buildup, the 4L60E uses many of the same electronics with the exception of using the engine rpm for the input speed. Other than that, the upgrade approach is similar.

Basic Transmission Operation
Standard and automatic transmissions serve the same purpose: to change the gear ratio between the engine and differential and keep the engine working in its sweet spot, without lugging or over-revving the engine for a given range of speed. The difference is that an automatic transmission engages and shifts gears without the need of your intervention. It does this through a series of inputs nodes, ranging from the mechanical position of the shift linkage to the engine's rpm. Newer transmissions incorporate electronics that help a computer determine the optimum slip and gear selection for the load at hand.

Hydraulic Fluid
The torque converter attached to the engine spins a hydraulic pump. This pump has the ability to put out about 700 psi, but several different obstacles in the tranny reduce the hydraulic pressure to about 60 psi (normal operating pressure). When the shift linkage is moved, it allows hydraulic fluid to flow and pressurize certain passages within the transmission. This is what engages, slips, or disengages the gears in the transmission.

Clutches and Springs
There are five gears in a four-speed overdrive transmission: First, Second, Third, Overdrive, and Reverse. Each of these gears has a clutch assembly that uses springs to keep the gear disengaged. When hydraulic fluid pressurizes a passage associated with that gear, it pushes against the springs and mashes the clutches together, thereby engaging that gear.

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Each gear in an automatic tranny is not engaged or disengaged by itself. Instead, a combination of gears create a certain gear ratio for the five different output speeds. This is done by way of a planetary gear assembly. A planetary gear assembly is made up of three different types of gears. By controlling these gears in this assembly, you can change its gear ratio and direction. It takes three planetary gear assemblies to make up the five different gears ratios in the 4L80E.

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S = Sun Gear
The sun gear is the center gear in the planetary assembly.

P = Planetary Gears
The planetary gears are the four gears that surround the sun gear.

C = Carrier Gear,br> The carrier gear houses the sun and planetary gears.

Each planetary gear assembly puts out a different gear ratio by simply locking or unlocking the carrier gear, sun gear, or planetary rotation. By locking the carrier only, the sun gear spins with the planetary gears inside the carrier (see illustration 2), which renders a certain gear ratio. Locking the sun gear makes the carrier gear spin the planetary gears, which changes the gear ratio as well as changing the rotation direction (see illustration 3). Leaving both gears unlocked prevents planetary rotation and again changes the gear ratio from either of the other ratios (see illustration 4). With three of these planetary gear assemblies in a transmission, you can see how this quickly becomes confusing.

This is not an exact science; about a million other things are going on within the automatic transmission. Still, this basic tidbit will make it easier to understand how all the performance upgrades affect an automatic tranny.

Computer-controlled transmissions have changed automatics forever. Just as fuel injection has made the engine more powerful, functional, and reliable, the electronic transmission outperforms and outlasts any of its predecessors with a stock engine. With Jet's performance upgrades, the 4L60E will accommodate about 500 hp, and the 4L80E will accommodate about 800 hp. Jet also carries a stand-alone computer to control the tranny in case you want to adapt it to a computer-free engine.


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The Brain Sensors

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