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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Rotating Weight Reduction

J.C. Beattie Jr.: Horsepower that’s used to turn a transmission over is horsepower that isn’t making it to the rear tires. That said, numerous measures can be taken to remove a significant amount of rotating weight inside a transmission to reduce parasitic power loss. If you take the weight out, and reduce rotating power loss by using bearings where needed, you can lower e.t.’s by a tenth of a second. The first areas to focus on for weight reduction are the largest parts with the biggest diameters. This is usually the forward and direct drums. Likewise, you can remove excess material from the case itself. Hollow and titanium bolts are another option, and other internal parts are often fly cut and milled to reduce mass. As we like to say in the transmission business, to get to a pound in weight reduction you must remove 16 ounces first!

Zack Farah: The effects of rotating mass is simple physics. Each and every component, such as the clutch drums, shafts, and planetary gearset, contribute to rotating mass. In the TH400, for instance, the heaviest rotating component is the direct clutch drum. At 12 pounds in cast-steel form, it will last forever. On the other hand, aircraft-grade aluminum drums are available that weigh half as much, but they cost $850 and will only last one or two seasons of racing before spline wear or ring grove damage render them useless. Consequently, efforts to reduce rotating mass are always a compromise. Lightweight parts will free up horsepower, but these components have a limited lifespan. A closer look at a TH400 direct clutch drum reinforces why this is the case. It weighs a substantial 12 pounds, and in First gear, it must counter rotate at 85 percent of engine rpm. After shifting into Second gear, the drum’s inertia is stopped by the intermediate clutch/direct drum sprag. That means that if your engine is turning 6,500 rpm, the direct drum is spinning at 5,525 rpm. Not only must it absorb its own rotating mass, but also the mass of the planetary set as well as the horsepower and the inertia of the vehicle when the direct clutch drum is brought to a stop. Reducing parasitic drag by 50 percent requires an expensive aluminum clutch drum with a limited service life. The direct clutch drum represents perhaps 20 percent of the total parasitic drag and is the most readily addressable, and offers the most return in terms of reducing parasitic drag.


Jim Beattie Sr.: Vasco, 9310, 4340, and 300M are materials commonly used in heavy-duty gearsets and shafts. Each of these metals offers a different degree of strength versus cost. At the bottom of the scale is 4340 carbon steel, which has a tensile strength of 238,000 psi and yield strength of 230,000 psi. The next higher grade of metal is 300M, which is basically 4340 with added silicone content. This results in a tensile strength of 286,000 psi, and yield strength of 245,000 psi. It’s generally a higher quality and stronger forging than 4340, making it ideal for input shafts. The ultimate in strength is Vasco C350 steel, which has 12 percent cobalt content, 18 percent nickel, and 5 percent molybdenum. This results in a tensile strength of 350,000 psi, and yield strength of 340,000 psi. A common metal used to manufacture gears is 9310, and its strength is directly dependent on the type of heat-treated process it’s subjected to. It has a tensile strength of 130,000 psi and yield strength of 155,000 psi. ATI also uses Aermet 100 and AF1410 for input shafts.

Obviously, strength isn’t the only consideration. Naturally, 4340 steel is the most affordable material, while Aermet and AF1410 are the most expensive. Regular 4340 can be purchased for $0.58 per inch, while 300M jumps to $2 an inch. Vasco, on the other hand, is $18 per inch in a 1.125-inch diameter. Aermet at AF1410 are only available in large mill runs, and cost roughly $30,000 per load.