For decades whenever a drag racer needed an automatic transmission he or she would almost always begin with a GM Powerglide and have it built up to suit their horsepower needs. But lately there has been a shift, and increasingly more racers are moving away from the venerable Powerglide and to transmissions based on GM’s Turbo Hydromatic 400.
The Turbo Hydromatic 400, which is often shortened down to “Turbo 400″ or simply “TH400,” is a three-speed transmission that first began showing up in GM cars and trucks in the middle 60s. In stock form it is one of the stronger transmissions ever produced, but is heavy even for a three speed, and it weighs significantly more than the two-speed Powerglide.
But since we are seeing many race teams adopt the TH400 over a traditional Powerglide we went to ATI Performance Products to find out exactly what is going on. ATI’s JC Beattie Jr, and the rest of the company’s engineering staff gave us quite an education on the strengths and weaknesses of both transmissions, and we think this info can be invaluable for you when choosing a transmission for either an all-out race car, a street/strip fun machine and everything in between.
The two-speed Powerglide is beloved by drag racers because of its light weight, simplicity and durability. Beattie says the strengths of the original OEM Powerglide are easy increased with a new input shaft and raising the line pressure up to 185 pounds. In this form a mostly stock transmission can handle 500 HP in a light bracket car and up to 700 if you have a 1.76 gear set instead of the 1.82
But the downside of the stock Powerglide, besides the fact that they aren’t making them anymore and you can’t even get NOS parts, is that stock clutch hub will fail with any more horsepower. A stock Powerglide also has low line pressure which won’t be adequate for holding the clutches in a racing application and will leave you smoking the transmission. Although they are easy enough to build up, in stock form the Powerglide simply isn’t a performance piece, which is why old-school racers often called it the “slip-and-slide Powerglide.”
Meanwhile, the TH400 has its own strengths and weaknesses. Here is what ATI lists as the transmission’s weak points. “The TH400 tends to be a bit heavier than other three-speed automatics and takes a little more power to turn all the internals and direct drum. Also, when using a trans brake in a stock OEM case, the circuit goes to the rear of the case and then back forward. That means the fluid has a long way to travel making the transbrake slower than a Powerglide. You are also limited to lower line pressure or the case will break at the intermediate lug area.
“But the TH400 also has some great points as a racing transmission. The OEM 2.48 helical-cut gearset is actually a pretty strong unit to begin with and can withstand a substantial amount of abuse before failure. The output shaft is incredibly strong and requires no upgrade in most cases, while the main (or intermediate) and input shaft has its limits and must be replaced at around 800+ horsepower. All in all, for heavier 800 to 1,000 horsepower cars, the TH400 can be used with many of its stock items and be a reliable unit.”
And while ATI may still call the transmissions they sell a Powerglide or a TH400, there really isn’t much left that is actually an original OEM part. For example, for applications up to about 850 horsepower ATI’s engineers say that they can still use a cleaned and inspected case, but that is about it. Every other part gets replaced or modified in some way to improve performance or reliability. The front pump is ported and then precision ground with new pump gears installed. The input shaft is a new piece ATI makes in-house from either 4340, 300M or Vasco. The forward clutch drum is an OEM core in most cases with a new input shaft although alum versions are offered. The high gear clutch (Direct) drum in all racing applications is made of billet aluminum and gets a new 34 element sprag for bracket cars or ATI’s 36 element sprag and in-house made outer-race and clutch hub to hold up to 3000+ HP. This part spins 80% faster than engine speed before it is stopped on the gear change. The lighter weight of the alum drum is actually easier on parts. The gearset is also an OEM part that is extensively modified with new pins and upgraded thrust bushings up to 1000 HP. Most racers with more power than that opt for a 2.10 low gear while less powerful cars get a 2.75 version. The valve body receives extensive machining work to modify the passages in the OEM form or replaced with complete Billet aluminum part.
ATI says the amount of modifications necessary on the TH400 depends on the amount of horsepower it will have to handle. For what ATI calls a “typical” 800 horsepower rated unit, the majority of the hard parts are reconditioned and reused to help save costs for the racer. But once you eclipse the 800 horsepower mark, the first parts to go are usually the input and main (intermediate) shafts along with the direct drum and OEM sprag assembly. What’s interesting is that all the upgraded components ATI uses on its transmissions are all engineered and fabricated in-house. Nearly nothing is shipped in from other manufacturers, and ATI actually sells many of its parts to other transmission builders.
With all the available upgrades, both the Powerglide and the TH400 are capable of withstanding an incredible 3,500 horsepower. ATI has been able to achieve this through constantly upgrading the technology used in designing and manufacturing the input shaft size, gearset material and modifying the transmissions to run a lockup torque converter, among other things. And since both transmissions are capable of handling massive horsepower loads, the real question when it comes to determining which is best for your needs is whether you prefer a two-speed, three-speed or even a 2 speed TH400 transmission.
Typically, heavy vehicles (usually 3,000 pounds or more) and any vehicle that sees time on the street will benefit from using a three-speed transmission. The same is also true if you are racing a high-RPM engine with a narrow powerband. The availability of an extra gear will shorten the rpm drop with each gear change to help keep the engine in its happy zone and not upset the car as much.
Heavier vehicles can benefit from the TH400 because that extra gear allows a lower first gear than you can get with a Powerglide. The mechanical advantage provided by that lower gearing helps the car get up out of the hole that much more quickly. And on a street/strip car the lower first gear helps the car get moving more easily at lower rpm’s. After all, leaving every stoplight at 5,500 rpm on the trans brake is a good way to add to your collection of moving violations.
The greatest advantage of the two-speed Powerglide is the consistency that comes from only having to shift once on your way down the track. Since you only have to shift once, it allows the driver to concentrate on driving the car. Lighter race vehicles– ATI suggests 3300 pounds or less–with the proper gear ratio usually don’t need as much low gear in the transmission. In this situation the lighter, quicker shifting Powerglide is the right choice.
“This is where a Powerglide with a low gear anywhere from a 1.62 to 2.18 can be a good choice,” Beattie says. “This reduction in low gear (lower numerically) also helps a traction-limited vehicle by reducing the mechanical advantage provided to the driveshaft. So you are limiting the torque to the tires which helps keep them from spinning.
“Within the last couple of years, a new way to reduce the ‘low’ gear in a race vehicle even further is by using a two-speed TH400,” Beattie adds. “Two-speed TH400s are built with a custom valve body that only allows the use of the trans brake and second and high gear. The typical second gear in an OEM TH400 gearset is 1.48, but with custom gearsets this ratio can dip as low as 1.31.”
Beattie says that converting a TH400 to a two-speed setup is relatively simple, but there definitely is a right way and a wrong way to do it. The simplest way is to change the manual valve in the valve body to one that doesn’t have low gear. This is a $50 job and can be done very quickly, but this method will also leave you with problems in practically any racing application. We have seen it done although we don’t recommend it.
“The way we do things is not the way someone else is going to think about the same process,” Beattie says. “When looking to use a two speed TH400 you need to have the transmission match the application. Most racers looking for a two speed are looking for a higher ratio– closer to 1 to 1– and to carry this ratio they are going to have a lot of horsepower available. They are also usually limited on tire size, so maintaining traction is critical. The idea of the ratio is to drag out low gear longer to apply more horsepower without worrying about a gear change upsetting the suspension or having too much starting line ratio to begin with.
“The only time we look at the two-speed TH400,” he adds, “is when a racer is looking for a higher first gear than what we make in a Powerglide transmission, which is 1.62:1. Since these transmissions are built for big horsepower levels, they are not entry-level units and are priced accordingly. If you see a two speed TH400 with a price that’s too good to be true, now you know the reason for that.”
ATI has also worked very hard on its lockup converters which eliminate slip and transfer horsepower to the rear wheels with maximum efficiency. Lockup converters have been around for a long time, most OEM units are unlocked by fluid pressure and then locked when the pressure is turned off. However, ATI’s race units are locked up with fluid pressure. ATI’s system has proven that it can successfully hold 3,000 horsepower consistently. It is, in fact, so successful and unique that ATI has been able to patent their ringless pump and input shaft designs that allow the complete sealing of cooler and lock up pressure to the torque converter.
So that’s the basics when it comes to choosing whether a Powerglide or aTH400–in either a three-speed or a two-speed configuration– is right for you and your car or truck. Honestly, there is a lot to consider here, and if you still have questions Beattie recommends giving ATI’s tech help center a call at (877) 298-5039. He says they are more than happy to help racers determine exactly what options they need (and maybe more importantly, don’t need) so that they can enjoy maximum performance without breaking the bank–whether it’s big-money heads up drag racing, bracket racing or just having fun with your hot rod.
ATI Performance Products