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.
GM Trans Options
One of the best perks about owning a late-model GM performance machine--besides having enough horsepower to eat Mod motors and poop out Hemis--is the vast selection of outstanding automatic transmissions at our disposal. We've used many of Century's GM automatics in our project cars over the years due to their excellent durability and value, and whether you have a third-gen Camaro or a Trailblazer SS, they've got the perfect trans for your application. For pure race applications such as The Purp, it's tough to beat the Powerglide. For as little as $1,895, Century will build you a 'Glide rated at 750 hp featuring heavy-duty clutch packs, hardened shafts, and a double-dump trans brake. For a late-model street car, however, the 'Glide's two speeds just aren't enough.
Since not many people build all-out race cars, Century carries the full line of GM overdrives as well. No doubt you are familiar with Century's 4L60E and 700R4. Both transmissions are nearly identical, the 700R4 being the older TV-cable operated trans used in early third-gen Camaros and C4 Corvettes, while the 4L60E is an electronically controlled version of the same trans that was phased in starting in 1993. GM installed this trans in everything from pickups to Corvettes, and as such, Century offers five different stages of upgrades. Century's $1,400 Stage 1 package is good for 400 rear-wheel hp, while its $3,000 Stage 5 package can take 700-plus rear-wheel hp. For today's heavy cars, the 4L60E's 3.06:1 First gear ratio makes it a very popular choice.
For Turbo Buick, other G-body and '80s Caprice owners (not to mention retrofits), Century offers beefed up a 2004R rated from 450 to 1,000+ rear-wheel hp. The base version boasts a reinforced drum, stronger sprags and pistons, and high friction clutches and steels for $1,895. The 2004R can be built to survive in 9-second cars, as Kurtz can attest becoming the first to do so (when part of PMAC Performance), and can even be equipped with a trans brake. Its 2.74 First gear makes it a great overal transmission.
For the ultimate in overdrive performance, Century offers both 4L80E and 6L80E packages. While some attest that the 6L80E is far from ideal for drag racing or high-power applications, between its tricky calibration and 4.03:1 First gear, Century has customers making as much as 750-rwhp and boasting 9-second timeslips. Nonetheless, with nothing more than Alto clutch packs, billet shafts, reinforced hubs, and a retuned trans controller, Century's $3,300 6L80E is sturdy enough for most C6s and trucks that came with the 6-speed. Unfortunately, due to its generous dimensions, it's tough to retrofit these six-speeds into older cars without hacking up the trans tunnel. At the top of Century's transmission heap is the 4L80E. Since it's essentially a TH400 with a 0.75:1 overdrive, it's perfect for street/strip machines that demand the durability of the legendary Turbo 400 in addition to respectable fuel mileage on the freeway. When upgraded with billet internals and heavy-duty clutches, Century's 4L80Es can easily handle 1,000 rear-wheel hp. This phenomenal level of durability will set you back about $3,800. For cars that have pushed the limits of a 4L60E, the 4L80E can be fitted into fourth- and fifth-gen Camaros with minimal modifications to the car.