Choosing the right transmission for your car is often a trade-off between durability and power consumption. Fortunately, GM has 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," Stan says. "The Powerglide also yields a lower starting-line gear ratio, allowing small-tire cars to leave the gate without frying the tires."
While very few people are daring enough to tackle rebuilding an automatic transmission themselves, TCI offers complete rebuild kits to get the job done. More importantly, TCI has some sound advice to help avert disaster. "The first thing I would recommend would be to have a transmission manual on the type of transmission you are attempting to rebuild," suggests Stan. Although TCI's kits come with instructions, they explain where the parts go, not the actual process of how to perform the rebuild. "I would also suggest one or two classes in a college vocational program," Stan says. "I've taken four during my time at TCI, just to learn the basics of transmissions and how they work, and our tech guys have all taken one or two courses. It's a necessary evil."
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."
Loose converters are great for launching hard out of the hole, but slippage simply wastes power at the far end of the track. While older GM converters don't incorporate lockup mechanisms, they can still be tightened up near the end of a quarter-mile pass to improve trap speed by 1 or 2 mph. "In order to accomplish this, you need to tighten up the clearance between the turbine and the pump assemblies to 0.060-0.080 inch," explains Stan. "TCI's standard turbine clearance is around 0.080 inch, although in some NHRA Super Stock and Comp Eliminator cars we have run as little as 0.040 inch to reduce slippage at the far end of the track. When you run such tight clearances, the alignment of the input shaft to stator support is critical, and the distance from the converter case and pump to the engine block should be kept to a minimum."
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, since it's no longer receiving a transmission signal."