Powerglide vs. TH350 vs. TH400
Unlike the Blue Oval camp, GM has blessed Bow Ties with a slew of performance transmission options. So which one's right for you? Although the venerable Powerglide can be built to handle 2,000 hp, it's best suited for lightweight drag cars since it only has two speeds. To make decision making simple, Farah recommends TH400s for big-blocks and TH350s for small-blocks. "The Turbo 400 turbo is 50 percent stronger than the Turbo 350, but may use up to 25 percent more horsepower to turn," he explains. "The Turbo 350 can be manufactured to handle up to 850 hp, which is plenty stout for most small-blocks. The Turbo 400, and the other hand, can be built to handle up to 1,200 hp."
"Slippage is one of the most common types of transmission failure. Gearstar addresses this issue by machining the clutch drums to increase the clutch and steel surface area by as much as 80 percent. Since your clutch and steel surface area is a deceleration surface as well as an acceleration surface, we use a combination of the highest quality frictions materials on the market from Raybestos, Borg-Warner, and Alto Red Eagle. We also use hardened Kolene steels and custom-manufactured, extra-wide carbon fiber bands with Kevlar linings and billet anchors. All our transmissions have at least two shift kits along with extreme-duty springs, sprags, and roller clutches. We custom-machine our pumps and use billet stators to increase flow and maximize pressures."
Swapping in a 700-R4 or a 200-4R trans yields an overdrive ratio in addition to a deeper First gear than a TH350 or TH400. However, some important fitment issues must be addressed. The most important modification that must be performed is to correct the geometry at the throttle-plate of the carburetor or TBI system. "These transmissions do not use a vacuum modulator to adjust line pressure and shift-point rise like the Turbo 350 and 400," says Farah. "Instead, they utilize a TV (throttle valve) cable, which provides the transmission with a torque signal from the engine, so adjusting it properly is critical. The only other modifications required are simply relocating the crossmember and altering the driveshaft length. All the other components required for the transplant are provided with Gearstar's transmission swap packages."
How Automatics Work
Few things in hot rodding are as intimidating as a dissected automatic trans. The parts are many, and assembly requires specialized training, but understanding the role of each can help you make a more informed purchase. The primary components of an automatic are the pump, clutch drums, bands, and planetary gearset. The pump supplies hydraulic pressure to clutch packs, band servos, and valve body control systems. The clutch drums contain the clutch pack and apply piston, which transfers power to the planetary control element. The bands act as a brake clutch and hold elements stationary within the planetary gearset with pressure applied by the servo. "Perhaps the most complex component is the planetary gearset, which consists of a planetary carrier, a sun gear, and a ring gear," explains Farah. "All three gear elements mesh together, and each element can be individually held stationary. By also varying which element transmits input torque from the converter, the planetary set is able to vary gear ratios."