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1955 Chevy Transmission Rebuild - The 400 club

The Turbo 400 is a great performance trans-here's how to make it better.

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There are few things in the world of high-performance cars that are truly reliable. One would have to be the GM TH400 automatic. As durable as this three-speed juicebox is, there is still room for improvement. When the Turbo 400 in Greg Smith's 11-second '55 Chevy lost Second gear (see "Speed Tune Your Street Machine," July '06), we took it to consummate automatic-transmission guru John Kilgore for a rehash. Many of Kilgore's ideas are budget based and can be applied to any street-driven or street/strip Turbo 400 trans.

Electric Wot
When the Turbo 400 was new, GM engineers decided to employ an electric "kick-down" function instead of the classic rod or cable to signal wide-open throttle (WOT) to the transmission. The factory system uses an electric trigger installed on the carburetor that sends a 12-volt signal to the transmission. With trans swaps, this simple point is often overlooked. The clue that this circuit is necessary is when the TH400 does not downshift from Third to Second at WOT at a speed of around 40 mph, for example. If you have installed a Turbo 400 in place of a Powerglide or a Turbo 350, you will need to install an electric WOT trigger switch. The easiest and perhaps least expensive way is to install a micro-switch on the carburetor that will complete a circuit only when the linkage achieves WOT. The wiring is simple; run one lead from switched battery power to the micro-switch and then from the switch down to the small male terminal on the driver side of the transmission. Applying 12 volts to the trans increases line pressure and commands WOT upshifts to be dictated by the weights and springs in the governor.

How 'Bout a Refill?
One of the problems with any automatic transmission is the oil level in the pan. Overfilling any automatic is bad because the rotating components will whip the oil into foam, which causes all kinds of problems, since automatics operate on the hydraulic principle of an incompressible fluid. Equally as bad is a low fluid level, especially for Turbo 400s.

Kilgore came up with a very simple solution to this dilemma after witnessing multiple oil-level-related trans problems. Kilgore integrates an oil-fill standpipe into the oil pan's drain plug. Anytime the fluid is drained, the refill procedure is to add ATF until oil begins to dribble out of the tube. With the engine running, shift the trans through Reverse and Drive a couple of times to ensure the converter and valvebody both are properly filled. Check the level again and fill until oil dribbles out the standpipe again. Then the pipe is plugged and you're ready to race.

Burnout Blues
Turbo 350s and 400s are the only automatic transmissions that use a sprag to hold Second gear. The sprag is a one-way roller clutch that is responsible for holding the high-gear drum in Second gear. It is also especially susceptible to failure. When the sprag fails, the trans loses Second gear, which is what happened to Greg Smith's '55 Chevy during our tuning session.

Kilgore reports the burnout procedure is usually the culprit for failed sprags. "Guys hit the throttle hard in First gear during the burnout. This rattles the tires at the same time they shift into Second, and that's what kills the sprag." Kilgore also said, "The sprag does all the work to stop the high-gear drum and accelerate the car, so the key is to minimize the shock load to the sprag." Examining the sprag in Smith's trans revealed it to be off center on the high-gear drum, which placed all the load on roughly four to five of the elements (there are 34 total).

The solution, according to Kilgore and several other transmission people we talked to, is to modify the burnout procedure. In Smith's case, Kilgore modified the Turbo 400 for a manual valvebody and instructed Smith to always do his burnouts in Second gear. This prevents a gear-change hit from hammering the sprag and rolling it over during the burnout. With an automatic valvebody, the best routine would be to gently start the tires spinning and quickly shift into Second gear before using heavy throttle to heat the tires.

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