"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, including Camaros and Corvettes, many enthusiasts hold the 700 in light regard for performance applications. Our own experience seemed to bear out this assessment: our "Mission 305" Camaro (June and Sept. '04) churned out 295 lb-ft of rear-wheel torque, burning up its 700-R4 in the process. And our current '84 Z28 project came with an obviously abused, one-speed only, badly leaking 700 backing its 190-horse, 240 lb-ft 305 H.O. mill. 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--the very 700 we burned up during our Mission 305 engine build and replaced with a T56 ("Trans-Mission 305 Parts 1 and 2," Feb. and Mar. '05). As we've noted, however, we could just as well have given up our '84 Z28's earlier box, since all cores are totally dismantled, the hard parts thoroughly cleaned and inspected, and upgrades to the later parts made where necessary.
Of course, upgrading to better factory components is only part of the story. Besides carefully checking all parts that are reused, replacing them when necessary, and undergoing a meticulous assembly process, B&M 700-R4s are toughened up considerably by adding high-performance B&M Racing Red clutch packs. The 3-4 pack is upped from five discs to seven, and as you'll see below, we'll need the extra holding power if we intend to get any more beans out of our '84 Z28 (which we do, of course). The other key to making the 700 a player is simple: pressure. As the folks at B&M explained to us, increased fluid pressure means the gearshifts happen more quickly and firmly, eliminating clutch slippage that limits performance and causes clutch wear. B&M transmissions are fitted with its Transpak recalibration kit (which can also be bought and installed separately), which increases oil-pump pressure and reworks the transmission valve body to provide the pressure necessary for those firmer, quicker shifts--and more, as shown below.
The result, according to B&M, is a 700-R4 that can handle up to 450 lb-ft of torque--maybe not enough for race-only applications, where Overdrive isn't needed, but certainly good enough to handle an aggressive street/strip machine, which is just what we have in mind. With that, we'll give you an overview of what goes into one of these beefed-up 700s. We've got some other tranny tricks coming up once this new less-slushy slushbox is installed in our Z28, and we'll also do our best to see how well that 450-lb-ft claim stands up.