The vaunted Turbo-Hydramatic TH400 transmission, known more commonly as the Turbo 400, was developed back in the early 1960s — making its debut in 1964 model year Cadillacs — as a three-speed automatic transmission. The transmission was quickly heralded for the number of advantages it delivered, including its impressive strength and the efficiency of its planetary gear design. Those key points, as history has shown, were not lost on the racers of the era, who found the Turbo 400 to be an excellent piece for the strenuous conditions they put vehicles through.
These days, that very same, 50-plus year old design is still used far and wide in drag racing, but more commonly, what was once a three-speed is now simply a two-speed, drawing direct parallels between it and another of General Motors’ famed transmissions: the two-speed Powerglide. While each have their time and place — as we’ll discuss later — the Turbo 400 itself has the heads-up, outlaw drag racing market cornered, with high-tech, modern versions of the original Turbo-Hydramatic roaming the landscape from manufacturers just like Maryland’s ATI Performance Products Inc.
We’ve teamed with ATI to build us a new transmission for our newly-announced Project Evil 8.5 Ford Mustang, which we’ll be campaigning in the Outlaw 8.5 category on the west coast. The car will feature power from a 427 cubic inch small block with a Vortech supercharger, with plenty of horsepower on tap being put to the pavement through an 8.5-inch slick tire.
We sat down with ATI’s JC Beattie Jr. to gain a better understanding of the two-speed Turbo 400 and how it compares head-to-head with the Powerglide in high-powered applications, as we take a guided walk-through of the build-up of our transmission and a discuss how this particular build will benefit our engine and tire combination.
The Turbo 400 At A Glance
Despite all of the advances in engineering and technology in the decades since its launch, the Turbo 400 is still widely considered one of the greatest original production transmissions ever conceived. But it’s as much the operation as the sheer strength that made the Turbo 400 stand out then and now. This was made possible both with the addition of a large multi-plate clutch that could take high doses of input torque, and the cast iron center support for the concentric shafts that mate the clutch to the gear train.
Turbo 400 Background
The production Turbo-Hydramatic benefited greatly in its development from a licensing deal that allowed GM to utilize the Simpson gear set design. The Turbo 400 was the first three-speed transmission with the Simpson gearing, featuring over-running clutches that exhibited first and second gear interaction — in essence, there was no need to simultaneously release and engage the clutch to move from second to third gear.
Battle Royale: The Turbo 400 And The Powerglide
Choosing between the mighty Powerglide and a Turbo 400 is largely a matter of horsepower, weight, and tire type. The difference, from a technical standpoint, and where these factors come into play, is in the strength of the design and the gear options that are available.
“A three-speed helps a lot of guys get down the track, because it doesn’t have the ratio drops that a Powerglide has. So it doesn’t upset the car on the gear change,” Beattie Jr. explains.
The available space inside of a Powerglide limits what low gear you can use and still propel a lot of horsepower. As Beattie Jr. puts it, you simply run out of physical space, because as you go numerically lower in the gear set, the larger the gears become physically. And a point comes where you just run right into the case itself. “There’s just a lot more room in a Turbo 400,” he says.
The Turbo 400, for its part, runs into the same situation, however, the gears are smaller to begin with, so there’s more room to grow. And with so many gear ratio options available on the market to cater to the wide variety of engine combinations, tires, and race distances, this is a good problem to have.
A three-speed helps a lot of guys get down the track, because it doesn’t have the ratio drops that a Powerglide has. So it doesn’t upset the car on the gear change.
This RPM drop is an important point to keep in mind. To perform optimally, engines have to be kept in their power-band all the way down the race track. Any significant deviations will force the engine to labor as it catches back up, and perhaps even unload the tires and suspension. A two-speed Turbo 400 goes 1:1 at the shift point, and this, Beattie Jr. explains, is less of a gear split than any high-horse Powerglide (the lowest drop on a Turbo 400 is 0.28, while the ‘Glide is 0.62). Big-tire racers, in particular, like the close-ratio setup of a three-speed, because the lack of RPM drop allows them to keep the tire wrinkled and the chassis happy in the front half of the run.
The input shaft, however, is oft-considered the weak link in the Powerglide, more so than the gearset.
We can do either one at 3,000 horsepower — the horsepower holding is there, it’s just about the car, the application, and getting down the track.
Two-Speed Or Three?
As alluded to earlier, the Turbo 400 was originally developed as a three-speed, and in non-racing circles, is still known for being as such. But these days, it’s almost a rarity to find a three-speed, aftermarket Turbo 400 in use in heads-up drag racing, as the applications roaming the drag strip these days demand a different configuration. In fact, Beattie Jr. estimates that out of a hundred Outlaw-style cars (be they Pro Mods, Outlaw 10.5, Outlaw Radial, or similar), 99 of them will be running a two-speed Turbo 400, making them “just huge in that world,” as he puts it.
So why the move to a two-speed configuration?
Contrary to common thought, the need for a two- or three-speed configuration isn’t about race distance — eighth- or quarter-mile — but about the tire, track prep, and the horsepower. In fact, some of the top quarter-mile Pro Mod cars run a two-speed 400, but this is done because such powerful cars need a smaller low gear to get that power hooked up at the hit. Ideally they would run a lock up-style unit, as well, but some sanctioning bodies will not allow a lock up automatic to compete with a clutch.
Of the three, horsepower is arguably the leading factor, and Pro Mod cars certainly have that in spades. You see, the entire goal is to match the transmission gear ratio with the rear end ratio to arrive at a happy medium that gets the car off the starting line consistently while still arriving at the appropriate engine RPM and wheel speed at the finish line. Cars that don’t have the power to make use of the 1.58 low gear will commonly go with a three-speed setup, where until recently, a 1.86 was the smallest low gear option available. Radial tire cars in particular, when running on a flypaper-type of racing surface with 3,000-plus horsepower, can’t calm their combinations down enough with a three-speed. So you can see where cars with 4,000 horsepower and up would need — or rather require — a 1.58 low gear, or even lower, with a tight ratio.
“If a guy is quicker with a three-speed, it’s because they either don’t have enough power to make use of a two-speed Turbo 400, or they’re carrying more weight and need the extra starting line ratio,: says Beattie Jr..
Internally, moving from a three to two-speed Turbo 400 means you can remove some of the guts found in the three-speed, including the center support and intermediate clutches. At this point, you’re primarily operating off of the forward and direct clutches and the rear band. The intermediate clutches themselves are the second gear shift in a three-speed, but not in a two-speed. Otherwise, there are some minor valve body changes involved, as well.
If a guy is quicker with a three-speed, it’s because they either don’t have enough power to use a two-speed, or they’re carrying more weight.
“You’re moving the same amount of weight, and it would be like taking the rear gear away,” says Beattie Jr.. “It’s harder to move, so it’s going to flash higher. Just thinking mathematically gear ratio-wise, the more gear the easier it is and the less work the converter has to do.”
So you might be wondering at this point if you can use a three-speed as a two-speed, and Beattie Jr. points out that it can be done with a valve body change with a transbrake for first or second gear, but for safety reasons, ATI prefers not to go that route.
“You really can’t have the best of both worlds, having a three-speed used as two. We can do it, but then it’s not a great three-speed nor is it a great two-speed. It’s never going to be right for both.”
Building A Two-Speed Turbo 400
The transmission being assembled here is built upon ATI’s own T400 Super Case, SFI-certified bellhousing, and roller bearing tail, and features a Moroso “shorty” pan with a pickup tube that places the filter in the rear, allowing for use of a Mopar-style filter. The pan holds Powerglide-like capacity, thus removing some extra weight, and also fits on cars with a crossmember that might be in the way.
Inside is ATI’s Wicked Quick two-speed billet aluminum valve body, and a stock output shaft with a custom 1.57 (2.48/1.48 is stock) low gear ratio. As Beattie Jr. shares in regards to the gearing selection: “When someone can’t put power down, because of the tire rules or the track, the stock gears ratios work for 1,500 horsepower without any troubles. This keeps guys from having to spend a bunch of money on an aftermarket gear set until they really get some power.”
The transmission features a billet aluminum two-speed center support, an aluminum intermediate clutch eliminator (not used in a three-speed), a 36-element sprag on a billet aluminum Severe-Duty direct drum (direct is high-gear) with a steel sleeve that allows for use of Teflon rings and higher line pressure. From there, the build includes a billet steel clutch hub, Vasco intermediate shaft (rated to around 2,500 horsepower) and input shaft (rated to 2,000 horsepower), a complete aluminum pump with a bolt-in 4140 heat-treated stator tube, an adjustable pressure regulator (allowing the pressure to be cranked from 180 to 300-pounds), and Calico-coated gears in the pump with tool steel anti-wear plates.
On a centrifugally-supercharged combination like the one this transmission will sit behind, Beattie Jr. points out first that the driver has to be able to cut a light, so they’ll need engine RPM to do so. Controlling power at the hit can be a little more of a challenge with a blower, so by taking gearing away, you can cut a light and not shock the tire as hard at the hit while the boost is up. Because the car in question will run a radial, which is often dead-hook on a well-prepped track, a two-speed is more suitable. If it were a larger slick, where wheel speed and tire wrinkle are a factor, a three-speed with a 2.20 or 2.30 low gear would be more in line with the combination.
“Without the gear ratio, one doesn’t have to take away as much power, so there’s also not as much time spent manipulating how to put the power back in once the car is off the starting line,” says Beattie Jr.. “A nitrous or turbo car can be more easily controlled in terms of power-adding, but it’s a little harder with a supercharger.”
We’ll be bolting this transmission setup into our Evil 8.5 Mustang later this year, at which point we’ll be able to take all of the knowledge gained here in regards to gearing and how they correlate with the engine and power adder combination to gain some true real-world experience. Of course, with the smallest-of-the-small tires out there under us and a whole lot of power under the hood, we’ll be taking full advantage of everything that a two-speed Turbo 400 has to offer to get us off the starting line. Because with a tire that small, one needs all the help they can get.