Picking The Right Automatic Transmission - How It Works

Hard-core drag racing calls for durable, lightweight, and efficient automatic transmissions. The pros explain how to pick the right one

Stephen Kim Jul 18, 2011 0 Comment(s)
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Rob Sappe: Performance differences between GM transmissions are usually the most important factors for drag racers, but size, weight, and efficiency are very important considerations as well. A Powerglide and a TH400 do not vary much in width, but length difference will range from 18 inches on a dragster-style Powerglide to 28 inches in a door car. A TH400 with a standard 4-inch extension housing measures 29.5 inches, although there are rare cases where a TH400 could be 34.5 or 38.5 inches. Weight-wise, ATI’s lightest Powerglide in a Supercase is 100 pounds, a standard weight Powerglide in a Supercase is 120 pounds, and an OEM-cased Powerglide is roughly 105 pounds. On the other hand, a TH400 can weigh up to 139 pounds with a deep pan. In terms of efficiency, a Powerglide will take roughly 18 hp to turn its internal mass while the TH400 uses roughly 44 hp.

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Reaction Time

J.C. Beattie Jr.: Unless you cut good, consistent lights, you’re not going to win races. That being the case, several modifications can be made to the transbrake to ensure quick response time and consistent performance. At ATI, we offer both fast-releasing ’brakes in addition to slower ’brakes. Some drivers need to slow down, believe it or not. Griner has long been known for procuring fast-releasing and hard-shifting transbrakes. A few things can be done internally to transmissions for quicker response as well. In a Powerglide, the transbrake is engaged by holding Low gear with the band and Reverse with the clutches. You can work with the Reverse clutch area to get the fluid out of the cavity more quickly, making the Reverse clutches disengage sooner.

OE Durability

Rick Hults: Although they were often installed behind six-cylinder engines from the factory, stock Powerglide parts are remarkably robust. Powerglides that were installed behind V-8s from the factory came equipped with a heavier-duty 1.76:1 ratio planetary set, while Powerglides installed behind four- and six-cylinder engines had a 1.82:1 planetary set. The stock 1.76:1 planetary can handle 700-750 hp, while the 1.82:1 ratio planetary is rated at 500-550 hp. In factory Powerglides, the planetary sets were the only real internal difference. The case and all other internal components are essentially the same in terms of strength and durability characteristics. So, all in all, the strongest OE-style Powerglide is good for 750 hp in a 3,000-pound car without a lot of power adders. Of course, with all the aftermarket cases, internals, and valvebody reworks that are available these days, we are holding more than 3,000 hp in 2,600-pound cars.

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Powerglide Perks

Zack Farah: As a two-speed trans, one of the greatest advantages of running a Powerglide in a drag car is its inherent efficiency. Compared to a TH400, a Powerglide requires a third the amount of horsepower to rotate. Likewise, the Powerglide offers several different First gear ratios to suit a wide range of applications, including 1.76:1, 1.82:1, 1.96:1, 2.0:1, and 2.20:1 ratios. The TH350 and TH400, in comparison, don’t have nearly as many gear ratio options. More gear ratios mean more components to spin and counter spin by creating more inertia to overcome. As Newton’s Law says, objects at rest stay at rest and objects in motion stay in motion. Along with the inertia and horsepower required to get these clutch drums spinning, there is also parasitic drag exerted by the Second and Third gear clutches, which also increase drag and resistance to acceleration. In a Powerglide, having two gears instead of three greatly reduces these drawbacks. There are also a ton of different aftermarket case options for the Powerglide, as well as just about every type of transbrake setup imaginable.


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