How To Build A Better C4 Transmission

The torque converter is little more than a fluid clutch or coupling that works like a water wheel in an old mill. Fluid is driven by the impeller through the stator to the turbine, which turns the transmission's input shaft. Torque converter architecture is two shell halves welded together with a lot of parts doing the work inside. To rebuild the converter, Troy must cut the welds on a lathe.

With the torque converter split, you can see what does the work. Although this looks very complex, it is quite simple. On the left is the impeller/forward shell, which is tied to the crankshaft. The impeller puts fluid in motion and feeds it to the stator (on right, central hub), which accelerators fluid flow to the turbine/rear shell on the right. The stator does not move, but instead vectors fluid flow to the turbine. Fluid slippage is why we can idle at a traffic light in gear and not stall the engine.

When Troy completes his inspection and replacement of any suspect parts, the two halves are welded back together by this automatic MIG welder. Then the torque converters are pressure tested and dynamic balanced.

Clutch drum contact surfaces are cleaned up on a lathe for a perfect band mating surface and a firm shift, which reduces the risk of burned friction materials from slippage.

All bushings are replaced to improve fit and lube control. Hard parts like hubs, clutch drums, and planet carriers (also known as planetaries) are measured to ensure proper fit and smooth operation. Not all transmission shops do this.

Transmission failure typically comes from a chain of events. As frictions wear or burn out from abuse, friction material goes into the transmission fluid and cuts piston seals, causing poor hydraulic function (slippage and soft shifts), which only makes things worse.

Jaime tells us proper seal and component installation in the first place is crucial to long transmission life. Seals must be generously lubricated with transmission fluid or assembly lube. When components are assembled, Jaime is gentle, ensuring smooth fit. When you are forceful, seals get damaged and could leak.

Clutches (also known as "frictions" in the trade) should always be soaked in transmission fluid before assembly. This allows friction material to expand, which will give your transmission shop a more accurate "feel" when it's time to check clutch spacing.

Clutch spacing (between steels and frictions) must be liberal to ensure proper operation. High-performance applications get an additional number of steels and frictions to increase friction area.

The intermediate servo piston seal rolls on like this. Piston and bore size are important to performance. In a six-cylinder automatic, it is not as crucial. When you're running a lot of power through the C4, you want the largest intermediate servo possible. Opt for TCI Automotive's intermediate servo.

Planet carriers wear out because they're subjected to extreme stresses. This is the reverse planet carrier in cast aluminum with steel gears. The front planet carrier (not pictured) is steel with steel gears. TCI Automotive has a super rugged six-pinion, low-drag front planet carrier (No. 527600) for the C4 if you're running a lot of power.

Not enough transmission shops check the governor for smooth operation. The governor valve piston must slide back and forth smoothly for proper shifts. Depending upon spring pressure, the governor determines when upshifts and downshifts occur. Spring pressure depends on vehicle application.

Transmission function is dependant upon solid band operation. Not all aftermarket bands are created equal. You want the best friction material money can buy. TCI Automotive has intermediate bands for C4 performance applications. We suggest this approach for stock applications as well as for optimum performance. Well-dressed drums and high-performance bands will get you off to a good start even in a stock application.

Servo piston installation must be performed gently and with plenty of seal lubrication.

Do this in haste and you can damage seals, adversely affecting servo function and causing improper band function and friction damage.

You'd be surprised how many transmission shops never replace shifter shaft seals. These seals must be replaced in order to keep fluid where it belongs. Clean up shaft surfaces and use plenty of lubrication.