Any way you slice it, 3.08:1 rearend gears just scream absolute wimpiness, especially when they’re bolted inside a fortified Moser 12-bolt rearend. That’s not exactly the kind of thing that impresses the lawn-chair-and-spray-bottle crowd on cruise night, but it’s precisely the setup we opted for in our 1968 Chevy Nova project car. Please keep the granny-mobile jokes to yourselves because there’s good reason for this madness. Last we checked, thanks to the impoverished motoring public, Big Oil execs are frantically trading up for bigger hangers in which to store their rapidly growing fleet of private jets. While gas isn’t cheap, and burning less of the stuff contradicts the very essence of hot rodding, there’s nothing wrong with saving a few bucks at the pump as long as it doesn’t compromise performance. Interestingly, one of the best ways to accomplish this without going broke is by using 50-year-old technology. The wizardry in question is called a variable-pitch torque converter, and adapting it for use in GM transmissions is remarkably straightforward.
" “If you want to change the stall speed of a converter, you change the shape of the stator fins or open up the windows, but changing fin pitch has the most dramatic effect
"
From 1965 to 1967, GM installed switch-pitch torque converters in many of its TH400 transmissions. This arrangement hydraulically alters the angle of the fins on a converter’s stator, thereby increasing or decreasing stall speed by as much as 1,000 rpm on the fly. That’s like being able to swap transmission or rearend gear ratios instantaneously while driving down the road. With the luxury of having a high-stall, switch-pitch converter that magically tightens up at cruising speeds, the net effect is brisk off-the-line acceleration even with a freeway-friendly ring-and-pinion set. For example, in Third gear our TH400-equipped Nova—with its 3.08:1 rearend gears and 255/40-17 rear tires—turns 2,700 rpm on the freeway at 65 mph. Now let’s say we were to upgrade to a 4L80 overdrive that needs shorter 3.73:1 rearend gears to compensate for a tighter, non-variable pitch converter. Even with the 4L80’s 0.75:1 overdrive ratio, the motor would still turn 2,500 rpm at 65 mph. The question is whether or not that 200-rpm difference in cruise rpm justifies the added expense, weight, and complexity of a computer-controlled overdrive, and we just didn’t feel that it was worth it in our Nova.
Like many of the best factory innovations that the Big Three have spawned over the years, rumor has it that GM ceased production of switch-pitch converters due to their high manufacturing costs. This limited production run can make finding parts a challenge, but fortunately shops like Phoenix Transmission Products in Weatherford, Texas, are well stocked with switch-pitch hardware. “A variable-pitch torque converter can be fitted on just about any TH400 transmission,” says Greg Ducato of Phoenix. “The only parts that need to be swapped out for a switch-pitch conversion are the stator, pump, input shaft, and the converter assembly itself. The high-stall mode is activated by an electric solenoid that’s mounted on the pump. From the factory, the solenoid was often activated by the carburetor linkeage, but on hot rod applications you can mount a switch wherever you want.”
Despite our best efforts, we haven’t been able to break a Phoenix-built transmission in any of our in-house project cars, including our ’75 Laguna and ’93 Mustang. Naturally, we didn’t hesitate to give Phoenix a call when it came time to build a fresh TH400 in our project Nova, and we tagged along as Greg put it together. So here’s an up-close look at what it takes to get variably pitched.
" “A lot of people don’t even remember switch-pitch transmissions, but they work great in street and racing applications.” —Greg Ducato
"
| Where the Money Went |
| Item: |
PN: |
Price: |
| Phoenix TH400 |
PTP400SS |
$1,231 |
| Switch-Pitch upgrade |
N/A |
$550 |
| Crossmember |
RCF1-400 |
$219 |
| Kick-down kit |
KD2400HT |
$78 |
| Total: |
|
$2,078 |
The heart of GM’s unique variable-stall...
The heart of GM’s unique variable-stall system is a switch-pitch stator pump (left). Compared to a standard TH400 pump (right), the variable-pitch unit is easily identifiable by a shaft that’s shorter in length and larger in diameter.
With the electric solenoid...
With the electric solenoid energized, and the stator fins in the closed position, torque converter stall speed increases to about 2,800 rpm to maximize off-the-line acceleration. As a car progresses down the road and less torque multiplication is necessary, the solenoid can be switched off. This opens up the fins and increases fluid friction inside the converter, reducing stall speed to roughly 1,800 rpm.
As pressurized fluid exits...
As pressurized fluid exits the feed hole on the pump shaft, it enters an orifice between the babbit bushings located in the center of the stator. The bushings must be carefully aligned to prevent blocking the fluid passage.
Unlike the standard TH400...
Unlike the standard TH400 pump (left), the variable-pitch pump (right) has an electric actuation solenoid and an additional piston. When 12 volts are applied to the solenoid, it energizes the piston, which directs fluid into the variable-pitch circuit in the stator. Retaining clips prevent the solenoid wire from contacting the drum after the transmission has been assembled.
With the electric solenoid...
With the electric solenoid energized, and the stator fins in the closed position, torque converter stall speed increases to about 2,800 rpm to maximize off-the-line acceleration. As a car progresses down the road and less torque multiplication is necessary, the solenoid can be switched off. This opens up the fins and increases fluid friction inside the converter, reducing stall speed to roughly 1,800 rpm.
Switch-pitch transmissions...
Switch-pitch transmissions use a different input shaft than a standard TH400. Compared to the shafts used with standard converters (left), variable-pitch-specific input shafts (right) have a unique orifice placement and sealing ring.