1968 Chevy Nova - Stall Tactics

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 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 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 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 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 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 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 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 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 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 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.