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1968 Chevy Nova - Stall TacticsWith a GM switch-pitch torque converter, you can have both brisk acceleration and relaxed freeway cruising without an overdrive From the September, 2011 issue of Popular Hot Rodding By Stephen Kim Photography by Johnny Hunkins
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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.  Fin pitch, shape, length,...  Fin pitch, shape, length, and fin count all influence stall speed. Fin pitch is the only variable that can be changed without physically modifying the stator fins, which is precisely how the GM system works. As with most torque converters, a standard TH400 stator (left) has fins situated at a fixed angle. Conversely, a variable-pitch stator has hinged fins that can open and close to change the converter’s stall speed.  The shaft on a variable-pitch...  The shaft on a variable-pitch pump features a feed hole that directs trans fluid to the stator assembly. It’s this hydraulic pressure that forces the stator fins to change angles.  Adapting variable-pitch capabilities...  Adapting variable-pitch capabilities to a TH400 requires using an eight-bolt case with an extra fluid orifice. Not all eight-bolt cases have this orifice, and without it, switch-pitch operation is not possible.  The switch-pitch activation...  The switch-pitch activation wire routes downward from the pump, across the valvebody, to an electrical connector on the driver side of the case. There, it joins the connector for the kick-down solenoid.  GM offered variable-pitch...  GM offered variable-pitch TH400 transmissions with either an 11- or a 13-inch torque converter. While both have a “low stall” speed of 1,800 rpm, the 11-inch converter has a “high stall” speed of 2,800 rpm and the 13-inch unit stalls at 2,400 rpm with the fins closed. Phoenix set us up with an 11-inch converter.  Swapping out our old, blown-up...  Swapping out our old, blown-up TH400 for our new replacement trans from Phoenix was very straightforward. It was as easy as unbolting the old exhaust, trans, and driveshaft, then installing the new hardware. After hooking up the trans cooler lines and kick-down cable, the Nova was ready to rock. Thanks to Automotive Excellence in Huntington Beach, California, for helping out with the install. We’ll be bringing you the story on their custom exhaust system soon.  To complement the 523hp Dart-based...  To complement the 523hp Dart-based small-block in our ’68 Nova, Phoenix built us one of its PTP400SS transmissions. This stout TH400 comes equipped with heavy-duty clutches, a Kevlar band, a 34-element sprag, and a 2,200- to 2,600-stall converter. This setup will handle 600 hp, and at $1,231 it’s plenty affordable. Of course, Phoenix optioned our trans out with its variable-pitch option, which costs an extra $550.  Unlike later nonelectronic...  Unlike later nonelectronic GM transmissions that rely on TV cables, the TH400 uses an electric kick-down cable. The cable hooks up to the carb linkage, and when your right foot summons a downshift, the kick-down box sends 12 volts of current to the valvebody. We picked up our electric kick-down kit from Lokar.  If you’re looking for a switch-pitch...  If you’re looking for a switch-pitch TH400 to rebuild, the easiest way to identify one is by the unique stator shaft spline design and fluid orifice. According to Phoenix, variable-pitch TH400s were only built from 1965-’67, and were never offered in a Chevy configuration.  Although we probably could...  Although we probably could have reused the factory crossmember, we picked up this much beefier unit from G Force. Its double-hump design easily cleared our Nova’s exhaust system, and the powdercoated finish looks slick.  There are countless ways to...  There are countless ways to activate the high-stall solenoid with a variable-pitch TH400. For the sake of simplicity, Automotive Excellence hooked up a simple interrupter switch next to the steering wheel. A quick trip around the block confirmed that the switch-pitch setup works exactly as advertised.
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