How It Works
Just like a quick stroll through the cruise night parking lot will confirm the dominance of GM overdrive automatics on the street, walking down the staging lanes of any dragstrip will reveal an equally overwhelming number of Powerglides and TH400s. Whether the race cars in question are street/strip machines, Chevys, Fords, Mopars, Top Alcohol dragsters, or 3,000hp Outlaw 10.5 beasts, when it comes to drag racing transmissions it's GM or bust. That's because street transmissions make an excellent foundation for drag race transmissions, and since long before the muscle era GM has dominated the market. In applications that require the ultimate in durability, Powerglides and TH400s have always been the gold standard, but which is best for your application? Furthermore, thanks to the demands of Super Stock racers that bite and claw for every fraction of a horsepower, modern TH350s can now survive behind 900-plus horsepower motors, which as unheard of just 10 years ago. Building the perfect drag automatic is a balancing act among durability, efficiency, and low mass, so to learn more about them we consulted with some of the most well-known trans builders around. Our panel includes Stan Poff of TCI Automotive; Zack Farah of Gearstar; and Rob Sappe, J.C. Beattie Jr., Rick Hults, and Jim Beattie Sr. of ATI. Let's get schooled on what goes into a thoroughbred drag transmission, shall we?
Powerglide Versus Th400
The Powerglide and TH400 have established themselves as the benchmarks of durability in competitive drag racing. Some applications are better suited for a Powerglide, and in others a TH400 is a better choice. For cars that weigh less than 3,200 pounds, launch violently, or are traction limited, the Powerglide is the best choice. In theses types of applications, a Powerglide is the ultimate in consistency as well. On the other hand, for cars that will see any street time at all, a TH400 is a better option. In race cars, compared to a Powerglide a Th400 is better suited for cars with large tires and taller rearend gearing. Heavier cars need three speeds to help get them up and running. Another three-speed option is the TH350. IT's not as strong as a TH400 in terms of ultimate strength, but still very popular for most street/strip applications.
The TH350 is a great compromise between the TH400 and Powerglide. Like the TH400, it has three forward gears, but not as much rotating mass. The trade-off is that it requires a few high-dollar components to get up to snuff with either the Powerglide or TH400, especially if you expect to get a full season of racing out of it without snapping the input shaft or intermediate sprag. In stock trim both the Powerglide and TH400's drums, planetaries, and shafts will take more abuse than the TH350. However, when a TH350 is upgraded with a sprag-type intermediate overrun clutch, a 300M input shaft, a late 700-R4 low roller clutch center support, premium racing frictions and steels, and a shift kit, it can be brought up to par with entry-level Powerglides and TH400s that have been upgraded with premium frictions and steels. The aftermarket offers much more for Powerglides and TH400s to take these units into the 1,000-plus horsepower arena. A Th350 will rarely survive at those horsepower levels.
Performance differences between GM transmissions are usually the most important factors for drag racers, but size, weight, and efficiency are very important considerations as well. A Powerglide and a TH400 do not vary much in width, but length difference will range from 18 inches on a dragster-style Powerglide to 28 inches in a door car. A TH400 width a standard 4-inch extension housing measures 29.5 inches, although there are rare cases where a TH400 could be 34.5 or 38.5 inches. Weight-wise, ATI's lightest Powerglide in a Supercase is 100 pounds, a standard weight Powerglide in a Supercase is 120 pounds, and an OEM-cased Powerglide is roughly 105 pounds. On the other hand, a TH400 can weigh up to 139 pounds with a deep pan. In terms of efficiency, a Powerglide will take roughly 18 hp to turn its internal mass while the TH400 uses roughly 44 hp.
J.C. Beattie Jr.:
Unless you cut good, consistent lights, you're not going to win races. That being the case, several modifications can be made to the transbrake to ensure quick response time and consistent performance. At ATI, we offer both fast-releasing 'brakes in addition to slower 'brakes. Some drivers need to slow down, believe it or not. Griner has long been known for procuring fast-releasing and hard-shifting transbrakes. A few things can be done internally to transmissions for quicker response as well. In a Powerglide, the transbrake is engaged by holding Low gear with the band and Reverse with the clutches. You can work with the Reverse clutch area to get the fluid out of the cavity more quickly, making the Reverse clutches disengage sooner.
Although they were often installed behind six-cylinder engines from the factory, stock Powerglide parts are remarkably robust. pwowerglides that were installed behind V-8s from the factory came equipped with a heavier-duty 1.76:1 ratio planetary set, while Powerglides installed behind four- and six-cylinder engines had a 1.82:1 planetary set. The stock 1.76:1 planetary can hadnle 700-750 hp, while the 1.82:1 ratio planetary is rated at 500-550 hp. In factory Powerglides, the planetary sets were the only real internal difference. The case and all other internal components are essentially the same in terms of strength and durability characteristics. So, all in all, the strongest OE-style Powerglide is good for 750 hp in a 3,000-pound car without a lot of power adders. Of course, with all the aftermarket cases, internals, and valvebody reworks that are available these days, we are holding more than 3,000 hp in 2,600-pound cars.
As a two-speed trans, one of the greatest advantages of running a Powerglide in a drag car is its inherent efficiency. Compared to a TH400, a Powerglide requires a third the amount of horsepower to rotate. Likewise, the Powerglide offers several different First gear ratios to suit a wide range of applications, including 1.76:1, 1.82:1, 1.96:1, 2.0:1, and 2.20:1 ratios. The TH350 and TH400, in comparison, don't have nearly as many gear ratio options. More gear ratios mean more components to spin and counter spin by creating more inertia to overcome. As Newton's Law says, objects at rest stay at rest and objects in motion stay in motion. Along with the inertia and horsepower required to get these clutch drums spinning, there is also parasitic drag exerted by the Second and Third gear clutches, which also increase drag and resistance to acceleration. In a Powerglide, having two gears instead of three greatly reduces these drawbacks. There are also a ton of different aftermarket case options for the Powerglide, as well as just about every type of transbrake setup imaginable.
Rotating Weight Reduction
J.C. Beattie Jr.:
Horsepower that's used to turn a transmission over is horsepower that isn't making it to the rear tires. That said, numerous measures can be taken to remove a significant amount of rotating weight inside a transmission to reduce parasitic power loss by using bearings where needed, you can lower e.t.'s by a tenth of a second. The first areas to focus on for weight reduction are the largest parts with the biggest diameters. This is usually the forward and direct drums. Likewise, you can remove the excess material from the case itself. Hollow and titanium bolts are another option, and other internal parts are often fly cut and milled to reduce mass. As we like to say in the transmission business, to get to a pound in weight reduction you must remove 16 ounces first!
The effects of rotating mass is simple physics. Each and every component, such as the clutch drums, shafts, and planetary gearset, contribute to rotating mass. In the TH400, for instance, the heaviest rotating component is the direct clutch drum. At 12 pounds in cast-steel form, it will last forever. On the other hand, aircraft-grade aluminum drums are available that weigh half as much, but they cost $850 and will only last one or two seasons of racing before spline wear or ring grove damage render them useless. Consequently, efforts to reduce rotating mass are always a compromise. Lightweight parts will free up horsepower, but these components have a limited lifespan. A closer look at a TH400 direct clutch drum reinforces why this is the case. It weighs a substantial 12 pounds, and in First gear, it must counter rotate at 85 percent of engine rpm. After shifting into Second gear, the drum's inertia is stopped by the intermediate clutch / direct drum sprag. That means that if your engine is turning 6,500 rpm, the direct drum is spinning at 5,525 rpm. Not only must it absorb its own rotating mass, but also the mass of the planetary set as well as the horsepower and the inertia of the vehicle when the direct clutch drum is brought to a stop. Reducing parasitic drag by 50 percent requires an expensive aluminum clutch drum with a limited service life. The direct clutch drum represents perhaps 20 percent of the total parasitic drag and is the most readily addressable, and offers the most return in terms of reducing parasitic drag.
Jim Beattie Sr.:
Vasco, 9310, 4340, and 300M are materials commonly used in heavy-duty gearsets and shafts. Each of these metals offers a different degree of strength versus cost. A the bottom of the scale is 4340 carbon steel, which has a tensile strength of 238,000 psi and yield strength of 230,000 psi. The next higher grade of metal is 300M, which is basically 4340 with added silicone content. This results in a tensile strength of 286,000 psi and yield strength of 245,000 psi. It's generally a higher quality and stronger forging than 4340, making it ideal for input shafts. The ultimate in strength is Vasco C350 steel, which has 12 percent cobalt content, 18 percent nickel, and 5 percent molybdenum. This results in a tensile strength of 350,000 psi, and yield strength of 340,000 psi. A common metal used to manufacture gears is 9310, and its sterngth is directly dependent on the type of heat-treated process it's subjected to. It has a tensile strength of 130,000 psi, and yield strength of 155,000 psi. ATI also uses Aermet 100 and AF1410 for input shafts.
Obviously, strength isn't the only consideration. Naturally 4340 steel is the most affordable material, while Aermet and AF1410 are the most expensive. Regular 4340 can be purchased for $0.59 per inch, while 300M jumps to $2 an inch. Vasco, on the other hand, is $18 per inch in a 1.125-inch diameter. Aermet at AF1410 are only available in large mill runs and cost roughly $30,000 per load.
Vasco and 9310 are both of the "vaccumelt" family of metals. There are different grades of these metals, and some are more suitable for certain applications than others. We used 4340 metal when it was popular 15 years ago. Then we graduated to 9310, which is pretty much what pro ring gears in rearends are made from. The next step in progression was moving on to Vasco, which is available in different grades. Some grades are better for gears and other grades are better for input shafts. Today, some of the Vasco materials used in transmissions have been surpassed by other aerospace materials that double or even triple the strength of Vasco.
Jim Beattie Sr.:
Increasing the number of clutch packs and using oversized sprags and bands are common tricks to increase the surface area of a transmission's power-holding components. The catch is that more surface area equates to greater frictional horsepower loss, so there must be a balance between these two factors. From a durability standpoint, increasing the number of clutches is a good thing, provided you have enough clearance for the additional clutches. More clutches equal more surface area that is capable of more holding power, but with the trade-off of increased drag. If you have disposable horsepower, then there is no point of diminishing returns. Just make your transmission as strong as possible. However, if you are running Stock Eliminator then you really don't want to overdo it, and having the correct number of clutches for your horsepower and weight is very important. Of course, oversized or an increased number of elements in a sprag increases its holding power as well, as do oversized bands.
Heat is the big monster lurking in the shadows that will destroy a transmission once the fluid has broken down and lost its viscosity. I recommend always running a temperature gauge to know how hot the transmission is getting. If it is staying between 160 and 180 degrees, change it every 40-50 passes. If at the end of a run you see temperatures ranging in the 200-230 degree mark, measures should be taken to reduce the operating temperature and fluid changes should be performed more frequently. In applications where temperature stays low, thinner fluids can be used. In cars where temperatures run high, thicker fluid needs to be used. Stockers, Super Stockers, and Comp Eliminator racers often use thinner fluids for e.t. purposes. The big horsepower engines equipped with automatics- some of which can reach 3,200-plus horsepower - usually run thicker fluids. TCI Automotive offers its Max Shift transmission fluids in several different weights to better cover all applications
For maximum longevity, a drag transmission should be serviced every week. After the race weekend, the pain should be dropped and inspected on Monday so in the even there was any unusual debris there will be time to get it out and serviced before the next race day. In a Powerglide or a TH350, the filter is a screen and should be removed and cleaned. TH400s use either a felt or screen element type of filter. If it is a felt-type filter, it should be replaced weekly, and new ones can be purchased from a GM dealership. The pan gasket should also be replaced if necessary and torqued to 12-15 lb-ft. I recommend topping the trans off with high-quality brand-name Dexron/Mercon fluid whenever it is serviced.
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