Duck for cover! It's raining planetaries! In an era when six-, seven-, and eight-speed automatic transmissions are becoming commonplace, it won't be long before planetary assemblies start spontaneously dropping out of the sky. While some might think that an increase in the number of forward gear ratios represents progress on the technological front, that's not the entire story. More gears aren't always better. Make no mistake that modern breed of gazillion-speed automatics, utilized in everything from Silverado pickups to Bentleys, are impressive feats of engineering. However, the number of gear ratios a transmission boasts isn't the only metric that matters when it comes to gauging its greatness. Although the venerable GM Powerglide has only two forward speeds inside its case, it has been the gold standard of durability in the racing world since the '50s. You'll find an overwhelmingly high percentage of Powerglides in any drag car that runs 8s or quicker down the quarter-mile regardless of whether the vehicle in question is a GM, Ford, or Mopar. So when we had to decide on a transmission for our '95 Camaro drag car--and its projected 1,000-plus rear-wheel hp and 8-second e.t.'s--the obvious choice was a Powerglide. To see what it takes to build a nearly indestructible 'Glide, we paid a visit to Century Transmission in Richmond, Texas as they put The Purp's trans together.
Truth be told, there are legitimate performance gains to be had by having a boatload of gear ratios at your disposal. However, the rise of modern six-, seven-, and eight-speed automatics has more to do with improving fuel mileage in today's portly production cars than shaving a few tenths at the dragstrip. Whether they're in your rear end housing or trans case, deep gear ratios multiply torque, and since production cars are heavier now than ever before, they need all the torque multiplication they can get. Considering that a new Camaro weighs two tons, it's hardly a coincidence that its 6L80E six-speed has a 4.03:1 First gear ratio. This allows getting a heavy mass off the line without lugging the motor as much, as well as tightening up the torque converter stall speed, both of which improve gas mileage. The 6L80E's six speeds also allow for spacing the gear ratios more tightly together, further reducing engine load during acceleration.
Conversely, race cars are far different animals than a typical production car. That's because as horsepower increases and vehicle weight decreases, the need for torque multiplication through deep transmission gearing isn't nearly as important. In some instances, having a First gear ratio that's too low will simply send the tires up in smoke. The most extreme example is in NHRA Top Fuel and Funny Car. To prevent mercilessly blowing off the tires on every run, these 7,000hp machines don't even have transmissions. Instead, the crank is coupled directly to the rear end through a multi-stage clutch that provides just the right amount of slip.
Granted that The Purp is no Funny Car, but the same concept applies. At the 1,000-plus rear-wheel hp level, the only two GM transmissions that are up to the task are the three-speed TH400 and the two-speed Powerglide. "Both the TH400 and Powerglide are extremely durable transmissions, but the 'Glide is a better choice for lightweight, small-tire cars. Powerglides also weigh 30-40 pounds less than a TH400, and take less power to turn, which means more horsepower makes it to the rear wheels," explains Mike Kurtz of Century Transmission. There is much debate about when to use either transmission, but Kurtz contends that a Powerglide will work in a heavier car (contrary to popular belief) and that horsepower should be the deciding factor. For our Camaro the TH400's 2.48:1 First gear ratio was considered too short. Meanwhile the 'Glide's taller 1.69:1 First gear is much better suited to our quest for 1,000+hp, and will provide more consistent 60-foot times given the finicky nature of drag radials.
Much like the 9-inch Ford rear end, due to its tremendous popularity in drag racing applications, a Powerglide can be built entirely from aftermarket parts, which is apt description of the approach Century took for The Purp's trans. Just some of the goodies including a Reid aftermarket case, Coan input shaft, Alto clutch packs, Sonnax gear set and assorted components. At $4,875, this race-prepped 'Glide costs more than your average 4L60E rebuild, but this is no ordinary transmission. "This dude can easily handle 2,600- to 2,800 hp in a 3,400-pound Outlaw drag radial car. It's pretty much the baddest of the bad," says Mike. After seeing this 'Glide's guts up close, we'd have to agree.