Transmission Accomplished – Four options to increase the capacity of a Pontiac Turbo 400

In the early days of the automotive industry, manual-shift transmissions were the norm. Operating a clutch pedal and a shift lever peering up from the floorboard or mounted on the steering column was simply part of the process for venturing onto the road. As technology advanced, convenience equipment such as electric starters and automatic windshield wipers became available to consumers. It should come as no surprise that shiftless transmissions followed soon after.

Designing and producing transmissions that shifted automatically was a challenging and costly endeavor for individual automakers. General Motors wisely created its Hydramatic Division in the ’30s to develop high-quality automatic transmissions for its motor divisions. The first Hydramatic—an extra-cost option available in Cadillac and Oldsmobile models during the late ’30s—used pressurized hydraulic fluid to couple each respective gear to the engine crankshaft, as opposed to the mechanical clutch assembly connected to the flywheel. Soon every GM Division was offering a Hydramatic-produced transmission in its vehicles.

Of all the different Hydramatics produced over the years, none may be as popular as the Turbo-Hydramatic 400 (THM-400 or Turbo 400). Introduced in ’64 as a heavy-duty unit, Pontiac began using the Turbo 400 in ’65 behind the 389s and 421s in the Big-Cars, as well as in ’67 GTOs and Firebird 400s. It could be found behind many 400-to-455ci Pontiacs over the course of its production run.

Used extensively by all GM divisions, Hydramatic phased the Turbo 400 out of production during the late ’80s. With literally millions of them produced in that time, hobbyists and racers of all GM makes still covet the over-engineered design, exploiting its strength and versatility with support from OE and aftermarket parts suppliers alike.

GearStar Performance Transmissions in Akron, Ohio, is one company that specializes in providing custom-built, high-performance automatic transmissions, and the Turbo 400 is among its popular offerings. Owner Zack Farah employs what he considers top-tier technicians and uses only the best domestically manufactured components to create four distinct levels, ranging from an improved stock replacement unit to a super version rated at 1,200 hp and 1,000 lb-ft. While basic rebuild techniques are readily available in any Pontiac service manual, we will instead have GearStar explain to us what improved components make its packages unique.