Just like a quick stroll through the cruise night parking lot will confirm the dominance of GM overdrive automatics on the street, walking down the staging lanes of any dragstrip will reveal an equally overwhelming number of Powerglides and TH400s. Whether the race cars in question are street/strip machines, Chevys, Fords, Mopars, Top Alcohol dragsters, or 3,000hp Outlaw 10.5 beasts, when it comes to drag racing transmissions it’s GM or bust. That’s because street transmissions make an excellent foundation for drag race transmissions, and since long before the muscle era GM has dominated the market. In applications that require the ultimate in durability, Powerglides and TH400s have always been the gold standard, but which is best for your application? Furthermore, thanks to the demands of Super Stock racers that bite and claw for every fraction of a horsepower, modern TH350s can now survive behind 900-plus horsepower motors, which was unheard of just 10 years ago. Building the perfect drag automatic is a balancing act among durability, efficiency, and low mass, so to learn more about them we consulted with some of the most well-known trans builders around. Our panel includes Stan Poff of TCI Automotive; Zack Farah of Gearstar; and Rob Sappe, J.C. Beattie Jr., Rick Hults, and Jim Beattie Sr. of ATI. Let’s get schooled on what goes into a thoroughbred drag transmission, shall we?
Powerglide Versus TH400
Rob Sappe: The Powerglide and TH400 have established themselves as the benchmarks of durability in competitive drag racing. Some applications are better suited for a Powerglide, and in others a TH400 is a better choice. For cars that weigh less than 3,200 pounds, launch violently, or are traction limited, the Powerglide is the best choice. In these types of applications, a Powerglide is the ultimate in consistency as well. On the other hand, for cars that will see any street time at all, a TH400 is a better option. In race cars, compared to a Powerglide a TH400 is better suited for cars with large tires and taller rearend gearing. Heavier cars need three speeds to help get them up and running. Another three-speed option is the TH350. It’s not as strong as a TH400 in terms of ultimate strength, but still very popular for most street/strip applications.
Zack Farah: The TH350 is a great compromise between the TH400 and Powerglide. Like the TH400, it has three forward gears, but not as much rotating mass. The trade-off is that it requires a few high-dollar components to get up to snuff with either the Powerglide or TH400, especially if you expect to get a full season of racing out of it without snapping the input shaft or intermediate sprag. In stock trim both the Powerglide and TH400’s drums, planetaries, and shafts will take more abuse than the TH350. However, when a TH350 is upgraded with a sprag-type intermediate overrun clutch, a 300M input shaft, a late 700-R4 low roller clutch center support, premium racing frictions and steels, and a shift kit, it can be brought up to par with entry-level Powerglides and TH400s that have been upgraded with premium frictions and steels. The aftermarket offers much more for Powerglides and TH400s to take these units into the 1,000-plus horsepower arena. A TH350 will rarely survive at those horsepower levels.