Circle Track and Powerglides

Today's racers not only demand performance, but reliability as well. They've found that combination in the GM Powerglide transmission. With minor modifications, it can go from being a subtle, slip-shifting automatic into a fire-breathing manual shifting monster that lasts. Not only is it used in conjunction with GM engines; the aftermarket offers adapters that allow racing enthusiasts to use it behind Ford and Chrysler power plants, too.

Over the years, weak areas have been located and strengthened. This is due to the racers' persistence for excellence, through trial and error, under the extreme demands required in racing. It is this versatility and reliability that we're going to look at as we dissect this unit. In this issue of Doctor, Doctor, we're going to get into surgical procedures that cover the entire unit. The procedures we'll cover will only pertain to circle track and their use.

The Transmission
This unit has a clutch pack in the lower case for reverse, a band for first gear, and a clutch drum for high gear. You just can't get any simpler than that. But this is also where you can run into problems when building this unit. Because, all too often, the simplest things can be the most frustrating. There are only a couple of weak points in this unit, so don't skimp on them during a rebuild. Okay, let's look at it from front to back.

Torque Converter or Torque Drive Tube?
If you chose to use a torque converter when building a modified Powerglide, there aren't a lot of choices. That's because racers are just looking for engine braking going into the corner, and a cushion (wheel spin) coming out of the corner. To achieve this, rebuilders set up the turbines in the converter to a very close tolerance. This is why the normal stock converter won't hold up to these extreme conditions.

Most racers have decided to eliminate the torque converter and go with a direct drive from the crankshaft to the pump (figure 1a and 1b). What they do is tap into the pressure side of the pump, dumping oil intended for the pressure regulator circuit of the transmission (figure 2). This not only helps reduce heat in the transmission (because torque converters create heat) but gives the racer positive braking in the corners, as well as acceleration out of the turns.

There are many different methods to achieve this, but the plumbing will still be the same. Some racers want to use a pedal, as in a clutch setup, while others use a ball valve mounted close to the driver, to engage and disengage the drive easily (figure 3). When the valve is open, the pump charge pressure is diverted back into the sump or pan. When closed, the pressure runs its normal course through the case to the valve body.

Figure 3

NOTE: Look for an alternate method for dumping pressure at the PR valve in the valve body section of this article.

Input Shafts
The input shaft is a weak area that is often overlooked. And as we learned earlier, the simplest things often cause the largest problems. The stock shaft usually ends up twisting the splines or breaking clean in two. Although they work fine for passenger cars, they simply won't hold up under the stresses of racing.

There are two aftermarket input shafts available that are made of different tensile alloys. Depending on the horsepower rating, you can choose which shaft better suits your needs (figure 4). The most common shaft is made from 4340 billet steel, and can be used with most racing engines... but there is an exception: If you're dealing with extremely high horsepower engines (800 to 1000 HP), you'll want a shaft made from Vasco 300 steel. The Vasco 300 steel shaft requires a bushing at the end of the stator support (turbine side, covered in the pump section). This shaft is as strong as they get. Keep in mind that this shaft is driving two things at the same time: the direct clutch hub and the input sun gear.

Figure 4

High Clutch Drum
This area seems so basic, but if overlooked will give you heartbreak (or should I say clutch hub breaks?) during a race. This part of the transmission consists of a drum with a piston for apply, a multidisc clutch assembly, a drive hub, and a sun gear mounted to the end plate that provides a 1:1 ratio when the clutch is applied.

The drum itself is a very reliable part. For modified racing, the drum of choice is the steel (stock) drum, although the aftermarket has come up with a drum made of aluminum. For this form of racing (circle track) the aluminum drum wears extremely quickly, eliminating reliability during a race.

Inside the drum is the clutch stack, used for locking the input to the output drives, which creates the 1:1 ratio for high gear. Here is where it gets a little tricky: There are many different compounds of clutch plates available on the market. Some claim to have a friction material that allows use of fewer plates while still providing adequate cooling, while others say they provide more strength with more plates used.

The bottom line is: make sure you use a reliable racing plate combination. Different manufacturers of plate linings recommend different types of steel plate be used between fiber plates, such as Kolene or steel. Make sure you take this into consideration, which ever design friction plate you use; it really makes a difference in the life of the plate.

As a rule, when building a performance transmission, allow 0.010" to 0.012" clearance for each friction plate used. So if you're using 5 friction plates, the clearance should be between 0.050" and 0.060".

When the plates lock down, they apply force through the direct hub. This is another critical area where you can't afford to cut costs or skimp. The stock cast hub will break every time in a performance transmission. Here again, there are two aftermarket hubs for this application: A billet steel hub (figure 5a and 5b), which is the strongest by far, and an aluminum hub. The aluminum hub is the lightest, but because it's a softer alloy, it has a higher wear rate than the steel one. These aluminum parts really are only designed for the drag racer, who's going to go through the transmission after every event. Durability isn't the issue in drag racing; only horsepower to the ground for a quarter mile.

Low Band
Since the low band is used only at the start of the race, the band is not really a big issue when building a strong unit. The aftermarket has developed a band with a Kevlar^® lining that is by far the strongest available. Whether you need this style of band depends on the type of circle track racing you do. For instance, if it's a paved, half-mile track, the band doesn't do much work, as the car is in high gear nearly the whole race. In this case, the stock band works just fine. But if you're building a car for a quarter-mile dirt track, that's a different story. In that type of race you may find yourself shifting the trans back into low a number of times. For those tracks consider using the stronger Kevlar^® band (figure 6).

When using a Kevlar^® band, you must correctly prepare the drum surface or the band will burn up before its time. This preparation consists of chucking the drum in a lathe and sanding a crosshatch pattern into the band's contact surface. Don't rough the drum too much: just enough to see the pattern in the surface of the drum -- a Scotchbrite^® pad works great. This will greatly enhance the life of the band.

The band apply piston has also been redesigned in the aftermarket. This new design piston prevents leaks into the direct drum when holding first gear. The original design had one cast iron sealing ring that would leak, due to the higher line pressure in racing applications. The new design piston has two rubber sealing rings that seal off the housing surface completely, to achieve little to no leaks (figure 7). This improves the life of the high clutch pack.

Band adjustment is important. If the band is set too tight it'll cause excessive drag on the high drum. Once again, for a racing application looser is better. Here's how to adjust the band clearance:

* Tighten the adjusting screw to 70 in-lbs
* Back off the adjusting screw 4 turns
* Tighten the jamb nut to 15 ft-lbs

The OE band adjusting pin is made from mild steel that can bend under the stress of racing. This is another area you shouldn't skimp on: Replace the OE pin with an aftermarket pin made from hardened steel (figure 8).

Figure 8

Reverse Clutch Set
Housed in the bottom of the case, the reverse clutch doesn't see much use in racing transmissions. Building this clutch stack to stock is okay, but many racers choose to lighten the static turning force by using only three friction plates. When the vehicle is moving forward, these clutches turn (when not applied) with the reverse ring gear on the planetary set. If the clearance is too tight, extra drag is exerted on the output drive, creating additional heat and reducing rotating horsepower to the wheels. This is caused by the rotating drag between the steel and fiber plates. In the reverse clutch, looser is better: about 0.015" clearance per friction plate used.

Don't overlook the retaining plate and return springs. Stiffer springs and a much stronger spring retainer plate are available in the aftermarket (figure 9). Always use the aftermarket parts to hold the apply piston back during high RPM.

Output Shaft, Planetary Gear Set and Reverse Ring Gear
There are many different output shafts available on the market today, with various gear ratios. For our application, there are really only two to choose from: Both have 27 splines for the yoke to slide on (the same as the 350, 700R4, etc). The two output shafts (figure 10) are:

Of these two, one is more common than the other: Since circle track racing doesn't involve the torsional stresses created in drag racing, the stamped steel (1:82) output shaft is the best for the circle track. Some racers will modify the carrier by machining off the parking gear teeth to lighten the unit, but if you do so, don't forget you have to block the wheels whenever you park the vehicle.

The reverse ring gear is an area where you can do a lot of machining to lighten the unit while maintaining the strength necessary. Figure 11A and

11B shows some areas you can easily machine off without sacrificing any strength. Make sure you don't remove any more metal than is listed and you'll be okay.

Front Pump
The stock pump will work just fine. But that doesn't mean you can't improve on it:

Figure 12

The pin holding the priming valve in the pump often breaks. Prevent this by taping the hole where the valve and spring are located with a _" NPT tap; no drilling is necessary. Install the valve assembly and hold it in place with an Allen head plug. Drill a 3/16" hole through the plug for oil to exhaust. Make sure the priming valve operates smoothly, and you're done.

When eliminating the torque converter you'll need to drill and tap the pump cover (figure 13A and 13B) to add an oil line for dumping off pump charge pressure. The stator support can be machined to accept a bushing at the end of the shaft, so it can be used with the Vasco 300 steel input shaft. If you have this done, send the bushing to the machine shop, and make sure they machine only enough material to cause a 0.0025" crush on the bushing (figure 14). The aftermarket has a replacement stator support shaft you can just press in (figure 15). The aftermarket stator support is much stronger than the factory one, so that's the one to consider if you need this bushing for input shaft support.

Governor
In this racing application, the governor isn't used. There are two ways to eliminate the governor circuit: The first way is to block the feed holes to the governor by tapping the case and installing a threaded plug. When doing this you must still use the governor support at the back of the case. The second way to eliminate the governor is to change the support to a solid output shaft support, available in the aftermarket (figure 16). This support blocks off the holes while providing additional support at the back of the transmission. This is the cleanest and by far the strongest way to support the output shaft.

Figure 16

Valve Body
Converting the Powerglide valve body for manual shifts is easy. Just block the shift valve into the shifted position. All shifts from that point will be from the manual valve. You will also need to make provisions to maintain between 100 and 120 PSI of line pressure at all times. Kits are available in the aftermarket that will provide a manual shift as well as other performance features. It's worth looking into these kits when building this racing unit. Many suppliers will sell you a valve body that has already been modified and is race ready.

Alternative to the Dump Valve
There is an alternate modification you can make at the PR valve that will let you use a clutch pedal, instead of modifying the pump cover for a dump valve. It is called a "direct drive" regulator system: Route a motorcycle cable through a custom bent 3/16" steel brake line, held by a 3/16" compression x 1/8" brass fitting, installed inside the case. Use the stock cable shield from the transmission up to the clutch pedal. This takes a little ingenuity but it can be done (figure 17).

Stepping on the pedal pulls the sliding seat away from the PR valve and reduces PR spring preload until there's no line pressure. This allows infinitely variable pressure regulation for smooth starts, rather than just jumping to 100 PSI on takeoff. It increases engagement control over the dump valve method and is a little cleaner with less plumbing. When using this method, as in any alteration, make sure the pressure is set correctly before racing. Play with the PR springs to achieve this.

Final Notes
Here's a few extra items to consider. Kits are available that allow you to replace all the thrust washers with needle bearings. When doing so, make certain your endplay is around 0.005" to 0.010" total. You can replace the bushing in the direct drum with the Teflon bushing from the front pump in a 4L60 (700R4) transmission.

Use quick-disconnect fittings for the cooler lines at the case, for ease of removal and installation.

Visit your local racetrack and talk with the techs working there. They are usually eager to share tricks that were successful, and share a laugh about those that weren't.

This article is not an all-inclusive build procedure; rather, it's a stepping stone for those who want to build units that will last. Always keep a ledger of things you've done that work. Until next time, keep those transmissions in good working health!

Special thanks to Hughes Performance race Products, Sonnax Industries and TSR Racing Products for their photo contributions for this article. For information about these companies contact: