C4 Transmission Performance Rebuild - Automatic Authority

Band adjustment is critical to improved performance. Mike's Transmissions suggests tightening the intermediate band adjustment screw, then backing off 1-3/4 turns, then tightening the locknut. Tighten the low-reverse band adjustment, back off three full turns, and then tighten the locknut.

This is a cross-section illustration of the intermediate band. Hydraulic pressure is applied to the servo piston, which locks the band around the reverse/high clutch drum. When we opt for a wider intermediate band from B&M, this improves the reverse-high clutch-drum lock-up. Going with the largest servo piston helps lock-up pressure as well.

Most modern automatic transmissions have one band. The C4 has two: the low/reverse band (left) and the intermediate band (right). B&M offers a wider intermediate band which delivers better performance by means of improved friction.

Going Parkin'
If most of us understood how little mass there is in the parking pawl, we would never leave a classic Ford on a hill without the parking brake. This is the parking-pawl assembly, which is what clicks when you put the transmission in Park and the vehicle creeps. This ratchet assembly engages the toothed hub, which is what holds the car on a hill when in Park.

When planning big power gains for your small-block, getting power to the rear wheels becomes paramount. Performance C4s need the largest intermediate band servo possible. The aftermarket offers a wealth of options for those seeking larger intermediate band servos.

There are reproductions of the popular H servo (right), which was used on C4 transmissions behind the 289 High-Performance V-8.

Here's the low-reverse band servo piston being installed on the left-hand side. No modifications are mandated here.

Here's a cross section of the low-reverse band and drum location more toward the rear of the transmission case. Modifications aren't made to this band and servo.

C4 Case Identification
Early C4 transmissions were vented from the main case with a vent tube just above the low-reverse band servo. These get the tailshaft housing without a vent mushroom on top (right). Later-model C4 and C5 transmission cases eliminate the vent tube altogether, opting for a higher mushroom-style vent on top of the tailshaft housing (left).

Venting prevents heat and pressure from blowing the seals and gaskets.

Again, look for the Ford casting numbers on these pieces for more accurate identification.

This is the vacuum modulator that controls upshift firmness and timing. Adjusting the modulator clockwise firms and delays the upshift, while counterclockwise speeds up and softens the upshifts. Make your adjustments one turn at a time, then take a spin to evaluate the upshifts. Do not adjust the vacuum modulator any more than four turns in either direction. C4s came with two types of vacuum modulators. Early versions had a screw-in type; later models have a push-in modulator with a clamp as shown. Weak intake-manifold vacuum caused by a hot cam or torn vacuum hose will adversely affect upshifts.