Slush pumps, idiot boxes—the automatic transmission gets called some mean names in racing circles. But people only mock what they don’t understand. Those given to insulting automatic transmissions don’t understand that it works great in many oval track racing applications—especially the General Motors Powerglide. In fact, a Powerglide can weigh less than a standard manual transmission, provide a direct-drive feel with minimal slippage at the torque converter (or none at all with direct drive), and maintain a relatively low rotating mass.
Many Street Stocks, I.M.C.A. Modifieds, and spec series allow (or, in some cases, require) cars and trucks with automatic transmissions. Automatics do have some benefits that are hard to ignore. An automatic tranny can have an initial low cost, be easy to maintain, and provide respectable performance. After all, most short track racers only need a high and low gear.
The most common automatic transmission used in oval racing is, without a doubt, the General Motors Powerglide. We checked with transmission guru Scott Miller, a product specialist at racing transmission–oriented TCI Automotive, to find out how to make the stock Powerglide perform better.
Slippage
Just like any transmission, be it automatic or manual, reducing slippage, friction, and rotating mass usually results in better performance. A manual transmission has a direct connection to the engine, so slippage through a torque converter is not an issue (although a worn-out clutch can allow slippage). Racing automatics have two options between the engine and the transmission: a low-stall torque converter or a pump-driven, direct-drive coupler.
A stock torque converter allows for slippage of 10 to 15 percent. For racing purposes, that much slippage equates to a mushy throttle response and lost horsepower. A low-stall torque converter, which allows two- to five-percent slippage, can eliminate much of the lag between throttle input and actual rear-wheel rotation, essentially making the car significantly more responsive. Often most stock converters are 12 or 13 inches in diameter. A smaller diameter torque converter decreases rotational mass and increases responsiveness. Note that some tracks and sanctioning bodies have rules restricting anything but a stock converter. (That doesn’t mean you can’t improve performance, though.) The problem with smaller torque converters is that they aren’t cheap. Less slippage with a smaller surface area, requires more expensive internal components. A low-stall, 10-inch torque converter can cost $400.
To reduce slippage:
• Use a low-stall torque converter. Low stall means the flash-stall point is somewhere between 1,800 and 2,600 rpm. Heavier cars of 3,000 pounds or more would want to be closer to 1,800-rpm flash-stall speed. Lighter cars less than 3,000 pounds can work better with a torque-converter stall speed rated closer to a 2,200- or 2,600-rpm flash stall. The right torque converter can reduce slippage from about 12 percent with a completely stock converter down to two to five percent. Slippage equals lost power and reduced responsiveness.
• Use a direct-drive setup instead of a torque converter. Direct drive is lighter, eliminates slippage, and is less costly than a torque converter by hundreds of dollars. Some manufacturers even make lightweight “dummy” torque converters for series that require a converter (Note to cheaters: Tech inspectors can spot these relatively easily.) A direct-drive system can cost as little as $140.
• Increase clutch capacity in the high gear drum by adding a clutch drive plate. Many Powerglides were manufactured with four clutch discs, which can handle up to about 350 hp. Above 350 hp, another clutch plate may be necessary to prevent slippage. (Some Powerglides used in six-cylinder vehicles only had three clutch drive plates.) The downside to adding an additional clutch drive plate is that it adds rotational mass. So if you don’t really need it, don’t add it. To reduce the slippage in low gear, a Kevlar-lined low-gear band can be used in place of the stock band. That can result in faster restarts. Note that the operating design of the low gear is different from that of the high gear.
• Change the main-line pressure. Low pressure inside a Powerglide can cause slippage, but high pressure increases drag. So, regulating pressure is an important aspect of using the Powerglide. Not enough main-line pressure will cause slippage and pre-maturely wear out the transmission. But too much pressure will cause drag and also prematurely wear out the pump. TCI recommends a main-line pressure between 140 to 150 psi; however, some racing transmission suppliers recommend main-line pressures up to 170 psi.
Rotating Mass
Reducing rotating mass inside the transmission allows for more horsepower to reach the wheels. Heavy rotating parts require more power to accelerate. This concept applies to all areas of the drivetrain, obviously. If it turns, then lighter is better. The easiest way to reduce rotating mass is to ditch the torque converter and install a direct-drive system. Unfortunately, because of many sanctioning bodies’ rules, that’s not always an option. Also, reducing the weight of the internal components can be expensive. An aluminum high-gear drum can reduce the eight-pound stock weight down to three pounds. A lightweight planetary, or planet carrier gear, can save one pound. A gun-drilled input shaft and an aluminum high-gear hub can each save less than a pound. The reduction in rotating mass would probably be most felt in your wallet. A stock high-gear drum typically costs $60 to $80. An aluminum high gear drum costs about $475. These ultra-lightweight parts are a luxury to most Saturday-night racers. The actual benefit and trade-offs in longevity are difficult to estimate.
Friction
Reducing friction in a Powerglide is one way to let some of the corralled ponies roam free in the green pastures of rear-wheel horsepower. The easiest way to reduce friction is to use a synthetic lubricant. (See the section on synthetic lubricants in this story for more information.) Another way to reduce friction is to use bearings in place of bushings and thrust washers.
Maintenance
Changing the transmission fluid every 150 laps or so is considered a healthy maintenance approach whether you race on asphalt or dirt. When changing the fluid, cut open the filter and look for metal. Metal in the filter is often a sign that the planetary gear needs to be replaced (not good).
Things to do before every race:
• Look for cracks in the rubber mounts, and check to make sure bolts are tight
• Check fluid level
• Lubricate the the spline by the front pump with grease (aircraft-style grease works well)
• Inspect for leaks around seals
When Good Transmissions Go Bad
Growling, grinding, smoking, jumping—those may sound like the symptoms of a bad teenager, but when transmissions have problems internally, they start doing those things, too. A growling sound often indicates that the planetary has a problem and needs to be replaced. Related causes could be normal wear and tear, or it could have been distorted due to the trans- mission overheating.
A distinct grinding sound also may indicate a problem with the planetary, or it could mean the front pump assembly has gone bad. A worn, distorted, or mutilated planetary may cause the car to jump or lurch as well. Smoke coming from the transmission may mean the thrust washers have gone out, or it may indicate slipping clutches. Slipping clutches cause heat, which in turn causes smoke when the fluid goes past its limits.
Synthetic Lubricants
According to TCI Automotive, synthetic transmission lubricants work great in Powerglide transmissions. The benefits include lower friction between gears, better bonding to parts for better overall protection, and less heat buildup. Some people get nervous about using a synthetic transmission fluid in an older automatic transmission, but the only cautionary note we found was to avoid using synthetics in transmissions that haven’t been freshened. Freshening involves replacing the seals, gaskets, bushings, and clutches. Ancient gaskets, worn bushings, and tired clutches may not react positively to the superior qualities of synthetic lubricants. Chances are, a 27- to 38-year-old transmission needs to be freshened anyway before it’s used in a race car.
One drawback: When synthetic transmission fluid leaks onto asphalt, it can be more slippery than leaks from conventional lubricants, and the slick can last longer, too.
Temperature Considerations
The operating temperature of a transmission greatly affects its longevity. In circle track racing, the Powerglide is operating above rpm ranges for which it was designed. Consequently, heat builds up and must be removed. A transmission cooler typi-cally solves this problem. Temperatures in the neighborhood of 180 to 200 degrees are considered safe. The severe danger zone starts at 250 degrees.
Locating a sending unit for a temperature gauge in the transmission pan is the standard way to measure transmission temperature. If there is no gauge in the vehicle, then the temperature of the case can be measured with a pyrometer. The case temperature will be slightly higher than the transmission-fluid temperature. A permanent transmission temperature gauge can help pinpoint potential problems before they occur. Powerglides equipped with a torque converter build more heat than a direct-drive setup. For that reason, a gauge and a transmission cooler are even more important with torque-converter–equipped Powerglides.
Transmission Mounting
Twisting action between the engine and the transmission can lead to cracked bellhousings and cases. In fact, one of the more common problems Powerglide users encounter is cracking cases either around the bellhousing or the rear of the case. This can be prevented by making sure the driveshaft length is correct for the application. If it’s too short, it can bind up the driveline and cause vibration.
Additionally, a lightweight, flexible crossmember with urethane mounts can act as a shock absorber for the transmission. This can help prevent breakage if the transmission is still allowed some movement as the frame rails of the car flex in the corners. This is especially important on rough dirt tracks, which really give the transmission a beating as it bounces up and down. TCI stresses that a solid mount can harm the relatively frail aluminum case. The idea behind using a crossmember is to allow the engine and transmission to move together. We’ve also heard of people that run a crossmember from the rear transmission mount to the top two bellhousing mounts. L-brackets are used to create convenient attachment points on the bellhousing mounts. This additional crossmember can help ensure torsional unity between the engine and transmission. It reduces the chances of a cracked bellhousing or case.
It Can Work for You
The Powerglide was once considered a weak and undesirable transmission for racing until the late ’70s. Drag racers were the first to figure out how to use its strengths and overcome its shortcomings. It slowly gained popularity in the ’80s in circle track racing and by the early ’90s was highly respected and desirable—nearly 20 years after it was discontinued. Advancements in material technology and a respect for its simple design have made the Powerglide one of the most venerable transmissions on ovals and dragstrips everywhere.