This cutaway highlights the...
This cutaway highlights the basic components found in the 4L80-E, 4L85-E, and the 4L85-E SuperMatic transmissions.
Those of us who have long been around cars, especially Pontiacs, can remember when converting from three-on-the-tree to a floor shifter meant we were serious about performance. Then the four-speed manual transmission came into play, known as "four-on-the-floor." Every true hot rodder rowed a four-speed through a Hurst shifter.
As time went on, a sacrilege occurred within Detroit-more and more slush boxes were being factory-installed in musclecars. To many, an automatic transmission didn't belong in a performance car. It was thought to be the domain of a grocery getter, luxury car, or station wagon that moms used to drive the kids to school.
It was quickly realized that with some internal tweaking of the Turbo Hydra-matic (THM) and torque converter, quicker acceleration could be realized with automatic shifting, and it eliminated any human error. A proficient driver with a good manual transmission would most likely be a little faster down the track, but there was always the risk of "blowing" a shift and losing a race. The automatic transmission did have one undeniable downfall, however-it had only three speeds, while the manual gearbox had four forward gears.
The THM 200-4R was the first...
The THM 200-4R was the first GM overdrive. It added a third planetary gear set in the front of the unit to create the overdrive reduction.
The number of gears in a transmission is paramount to its performance. With most transmissions, regardless if they are a three- or four-speed, the top gear is a direct or 1:1 ratio. The lower gears are a higher numerical ratio and act as a multiplier of the engine's torque output. That's why a truck designed to pull a heavy load has such high numerical gear ratios in the transmission and differential.
The downside of steep gearing is that the engine runs out of rpm quickly, and the vehicle's top speed is the direct result of how fast the tires can turn in the least-aggressive transmission gear. Here is where an overdrive trans can help, as it allows for a more aggressive rear gear, while lowering highway rpm by way of the less than 1:1 ratio Overdrive gear.
In addition, the spacing between the gear ratios in the transmission impacts the acceleration of the vehicle. Transmissions with fewer gear ratios, such as a three-speed, need a large drop in the multiplicative advantage when going from Second to Third so that 1:1 can be accomplished. This often makes less torquey engines bog and feel very lazy while diving, especially when trying to accelerate. The Pontiac V-8, however, is known for its extremely high torque output, and this would often mask the severe drop in gear multiplication when shifting either manually or automatically to the direct-drive range.
During the '60s, the Hydra-matic Division of General Motors designed and built what many consider the best automatic transmissions in the world-the THM 350 and THM 400. The 350 was less stout than the 400 and was meant to be placed behind a smaller engine with less torque, while the 400 could handle more than any production engine could put through it.
The THM 700-R4, which was...
The THM 700-R4, which was used in Third-Gen Firebirds, was designed as an overdrive. It wasn't modified from an existing three-speed design and was later renamed 4L60.
Both of these units were known for excellent shift quality, long life, quiet operation, and an extreme level of tunability that allowed the aftermarket to create a true race automatic. The Hydra-matic line was so good that even Rolls-Royce used them in their motorcars. Many drag racers with non-GM vehicles would retrofit a Turbo 400 into their cars. That practice is still going on today.
Despite their great performance, the need for better fuel economy ushered in the era of the automatic overdrive transmission in the early '80s. It allowed for a high numerical rear-gear ratio for quick acceleration, without the penalty of high-engine rpm, as well as the inherent drop in fuel mileage and increase in engine noise and wear when cruising on the highway. The often-desired 3.90:1 rearend could be tamed to a ratio equivalent to 2.92:1 when shifting into Overdrive with a ratio of 0.75:1 (3.90 X 0.75 = 2.925). With this theory, you could drive your Pontiac to the track and not have to sacrifice quarter-mile acceleration for an enjoyable, high-speed ride.