"The proof is in the pudding," the old saying goes, and it's all the more appropriate when we're talking slushboxes-in this case GM's ubiquitous four-speed automatic, the TH700-R4. Despite the fact that millions of cars left The General's assembly line with 700s in place, many enthusiasts hold the 700 in light regard for performance applications. On the other hand, a transmission is like any other part of a performance car-it's only as durable as it's built to be. With this in mind, we paid a visit to B&M Racing & Performance's transmission rebuilding facility to have a look at the makings of a high-performance 700-R4.
The 700-R4, featured in various '82-92 GM models, was a product of the OEM's still-ongoing search for better gas mileage, featuring an 0.70:1 overdrive Fourth gear that, in stock form, kicks in early and often. On the other hand, the 3.06:1 First gear and 1.63:1 Second gear found in the 700 are both lower than their counterparts in the TH350 and TH400, providing increased acceleration. The combination of lower First and Second gears and an overdrive cog give the 700-R4 the potential to be a good real-world performance transmission, promising the best of both worlds: acceleration and mileage. The overdrive cog also gives hot rodders the ability to run stiffer rearend gears without unduly affecting fuel mileage at cruising speeds. In its stock form, however, the 700 was never really called upon to handle tons of power. The most it had to deal with was the '92 Corvette's 300hp, 330-lb-ft LT1 power-plant. By the time the 11-year-old tranny had to handle this output, it had been substantially improved from the original version, and therein lies the first key to making the 700-R4 into a performance piece.
All B&M's street/strip 700-R4s are built to '87-and-later specs. By that time, GM had beefed up several areas of its primary slushbox, most notably by changing from a 27-spline input shaft to a 30-spline unit; they also improved lubrication in several ways, including the improvement of the oil pump and including an auxiliary valve body to provide direct lubrication to low gear. For those running pre-'87 units, B&M makes the Super Transkit (PN 70230) for DIYers looking to upgrade the tranny themselves. The manual that comes with this kit provides a comprehensive list of the improvements made to the 700 from 1982 to 1986. We brought B&M a '92 core. We could just as well have given up any '82-92 box, since all cores are totally dismantled, the hard parts thoroughly cleaned and inspected, and upgrades to the later parts made where necessary.
A Painless harness (PN 10206) was chosen since it's designed to fit '67-72 GM trucks. The
Here's the dysfunctional '92-vintage 700-R4 core we turned in. The converter comes off fir
he B&M techs then dismantled our one-gear-only tranny with ruthless efficiency. We'll only