Jim Smart
April 1, 2004

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Another important difference with C4 transmissions is the vacuum modulator, which controls shifts points based on intake manifold vacuum (load and throttle position). Early C4 transmissions have a screw-in vacuum modulator. Later models have a press-in vacuum modulator held in place with a bracket as shown.

Greater Friction
When we improve C4 transmission performance, we do it by getting power to the rear axle reliably. In its most basic form, the C4 has room for additional clutches. The more clutches and plates we can add to a C4, the more bite (less slippage) it's going to have. The forward clutch has room for more clutches and steels. So does the reverse/high clutch. This will greatly improve the transmission of power. Another performance trick is to sand the clutch steels, which offers the same result as refacing a flywheel or turning brake rotors. It roughs up the surface for greater friction and hold.

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Install A Shift Improvement Kit
A B&M Shift Improvement Kit isn't just about racing or blasting from traffic light to traffic light. It is about longer transmission life. In stock form, automatic transmissions have a certain amount of clutch and band "slip" programmed into their valvebodies. This gives a smoother upshift. However, every time there is slippage during the upshift, you are wasting clutch and band friction material. Friction material works its way into the transmission fluid. Think of the fluid as a friction material freeway, carrying this material throughout the transmission. Friction material tends to cut the seals, which allows hydraulic pressure to escape, causing clutches and bands to slip even more. Eventually, the slippage cycle becomes so bad that clutch pistons and control valves stick, or fail to function at all, causing transmission failure. Installing a Shift Improvement Kit isn't just about getting there faster, it's about getting there in the first place.

Cooling It
Through the years, we have consulted with dozens of automatic transmission shops. When we asked them what the greatest reason for transmission failure was, they have all said the same thing-heat. Heat kills automatic transmissions. Ideally, your C4 transmission should operate around 250 degrees F sump temperature. In racing and in towing, automatic transmission sump temperatures can run upwards of 300 degrees F. An external transmission cooler can help get fluid temperature down where it belongs. B&M Racing & Performance has a variety of transmission coolers sized to work in your application. Hint-the larger the better. Install one of these and help your transmission live.

Free From Leaks
Not only do we want our C4 to perform well, we want it to be free from leakage. Too many transmission builders fail to take care of all of the seals during rebuilds. Everything from the front pump seal to shift control seals get overlooked during builds. Whenever your C4 is torn down for a complete rebuild and nose powdering, make sure it gets all new rubber parts, including the shifter shaft seals on the left-hand side. These simple o-ring seals are notorious for leakage when they're overlooked.