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700 Deluxe

CHP Takes an Inside Look at Building a High-Performance 700-R4

John Nelson Jul 25, 2005

"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.


Here's the dysfunctional '92-vintage 700-R4 core we turned in. The converter comes off first and is sent to B&M's converter manufacturing department, where it will be cut open and reborn as a piece similar to the HoleShot 2400 unit we grabbed for our Z28. It'll get a furnace-brazed turbine and impeller, an alloy steel turbine hub, and heavy-duty needle thrust bearings before being balanced and tested.

The B&M techs then dismantled our one-gear-only tranny with ruthless efficiency. We'll only be showing you a portion of the process to make a few points. With the trans pan, filter, and the torque converter lockup control wiring removed, the main valve body is quickly pulled off. Note the auxiliary valve body (arrow), found on '87-and-later units, which provides lubrication directly to the lower gears--all B&M 700s, regardless of build year, come with this feature.

Once the 1-2 accumulator housing (foreground) is removed, the separator plate can also be removed to reveal the 3-4 accumulator housing (arrow). These assemblies consist of pistons and springs that act as shock absorbers when the respective shifts occur. We're looking for firmer and quicker shifts, so these springs will be replaced with spacers, as you'll see below.

After the transmission oil pump is unbolted from the case, a little creative prying sets it free. All oil pumps are rebuilt to new condition and modified to produce greater fluid pressure.

With the pump out of the way, it's time for our 700 to spill its guts. Taking a good grip on the input shaft, the B&M tech pulled the housing and shaft assembly free from the case. Note the reverse input clutch housing and drum assembly (arrow) and its slightly toasted appearance--more on that later.

With the input shaft removed, along with the housing and drum assembly, we get to the crux of the matter. Three sets of clutches (3-4, forward, and overrun--or coast--clutches), each with it's own piston and spring set for application and release, reside within the housing part of the housing and shaft assembly. After removing a large retaining ring, the 3-4 clutches are ready to be pulled out.

Yikes? Who started the fire? Now we know why The Bitchin' Camaro ended up sans Third and Fourth gear. These severely burnt pieces show that the stock clutch plates were slipping like mad, unable to cope with 295 lb-ft of torque. The forward clutches were only slightly better, though the overrun clutches looked OK.

Jumping ahead, the techs have already gone through the input housing and shaft assembly, installing new, heavy-duty B&M Racing Red clutch plates throughout, along with new spring assemblies, seals, bearings, washers, and retaining rings in the appropriate places. Hard parts, such as pistons, rings, and housings, are steam-cleaned and inspected. Later-model upgrades are installed where necessary. In this shot, we're looking at the new 3-4 clutch assembly. B&M upgrades this pack to a seven-disc assembly--two more than stock--for greater holding power.

The outer surface of the reverse input clutch housing and drum assembly is what the 2-4 band assembly wraps around--when applied while in Second and Fourth gears, the band is supposed to stop the drum. The pieces from our core are on the right; the band itself it black and toasty, while the drum is black and actually grooved, more graphic evidence of our overmatched 700's suffering. The clean pieces on the right will replace this debris.

We've skipped ahead a bit here--our empty case and all its metallic innards have been pressure-washed and the case painted, so it's time for reassembly. This illustrates one of the upgrades B&M incorporates into its 700-R4s. The later-model planetary gearset on the left (called a reaction carrier assembly) features an oil deflector (arrow) to provide the cogs inside with more lubrication. Here, this assembly is being joined to the internal reaction gear support, which has been fitted to the output shaft.

The works then went into the very clean and shiny case, which was being supported in a vertical position an upside-down tailshaft housing. The low and reverse clutches were placed around this assembly; the requisite piston and springs had already been installed in the lower end of the case.

The reaction sun gear is slid into place, landing right in the middle of the previously shown planetary assembly. (Planets need a sun, right?) The reaction sun gear shell goes into place after that, and a thrust washer and retaining ring hold the works in place. Except for the tailshaft assembly, with its speedo gears and governor (which signals road speed to the valve body), the rear portion of this 700 is done.

Before continuing, the techs pointed out another upgrade found on '87-and-later. This reaction carrier shaft is considerably thicker than its earlier counterparts, enabling it to handle more torque. It has already been joined to the input internal gear and is about to be mated to another planetary gearset, the input carrier assembly. This assembly went into the case shaft first, with a bushing in between it and the reaction sun gear.

After inserting the input sun gear and its accompanying bushing, the reassembled housing and shaft assembly--containing the previously discussed hi-po clutch sets and the input shaft-- along with the reverse input clutch housing and drum assembly was inserted into the case.

A new 2-4 band assembly was then put into place. To guard against future slippage and its resultant damage, the servo that activates this band is upgraded, as you'll see.

As we mentioned earlier, all oil-pump assemblies are rebuilt and modified for optimum performance and greater fluid pressure, which will help provide firmer and quicker shifts. This boost in pressure even helps the tranny work while in reverse, acting to eliminate any shuddering. The new pressure regulator spring shown above is key.

The pump rebuild was comprehensive, right down to replacing the worn vanes in our unit (which definitely showed wear) with brand-new pieces.


B&M Racing & Performance Products
Chatsworth, CA 91311

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