Automatic transmissions are incredibly complex, but the efforts taken to ensure longer life in big-horsepower applications encompass four key components: the pump, the planetary gearset, the valvebody, and the drums. The pump supplies all the oil into the converter and throughout the transmission. This fluid pressure is critical, as it keeps clutches and bands applied in each gear under load. The planetary gearset multiplies engine torque and, when designed properly, reduces stress on other internal parts in the transmission. The valvebody is responsible for supplying transmission-line pressure and selects the gear the trans operates in. The drums house the clutch and steel combinations, which carry the load in certain gears. "Factory clutch combinations were meant for factory vehicles," explains Stan. "TCI uses modified combinations and friction materials in these clutch packs to allow more horsepower to run through the transmission. We have transmissions that survive in 3,000hp applications, which is possibly due to the rugged composition of our clutch and steel combinations."
Choosing the right transmission for your car is often a trade-off between durability and power consumption. Fortunately, GM offered a wide variety of transmissions over the years, and the Powerglide, the TH350, and the TH400 are among the most popular. According to Stan, in a heavy-load, high-horsepower application, the TH400 would be the best choice, but he also points out that there are lots of hard parts available for the TH350 that would allow using it in similar applications. "In classes like NHRA Stock and Super Stock, a TH350 might be a better choice than a TH400, because it takes less horsepower to operate," says Stan. So where does that leave the venerable Powerglide? "Three-speed transmissions are usually better in vehicles weighing more than 3,000 pounds, but the Powerglide is usually better in vehicles lighter than 3,000 pounds, since they use about half of the horsepower compared with a TH350. The Powerglide also yields a lower starting-line gear ratio, allowing small-tire cars to leave the gate without frying the tires."
Options are often limited for late-model racers looking to beef up their trannys, so many ditch their six-speeds and 4L60Es for old-school slushboxes. However, there are some things to look out for to avoid headaches, and dimensions and fit are the primary concerns. "You can bolt a TH400 to the back of an LS1, but the stock converter configuration won't work," explains Stan. "TCI has designed converters for the TH350 and the TH400 that will bolt directly onto late-model applications. Other things to look out for are tunnel space, speedometer hookup, passing-gear hookup, linkage compatibility, and errors from the PCM, sinceit's no longer receiving a trans-mission signal."
In mega-horsepower race cars, rubber engine and trans mounts won't cut it. Using solid mounts all around may seem like an easy solution, but TCI warns against doing so. "Solid motor mounts are OK, but TCI recommends using rubber or polyurethane mounts on the transmission," suggests Stan. "You want a mount that will absorb some of the shock the drivetrain will experience from hard shifts and chassis twist, as well as possible misalignment of the transmission in the vehicle. Solid transmission mounts/trans misalignment is the number-one cause of case breakage around the bellhousing in TH350s, TH400s, and Powerglides."