Wayne Cook
June 30, 2005
Upon disassembly, the transmission is thoroughly cleaned, and the caserepainted. This and most of the other major components (e.g., the frontpump and valvebody) will be reused because, as a rule, they just don'twear out.

When horsepower figures are up and consistent shifting performance is therequirement, it's impossible to beat Ford's tough and dependable C6three-speed automatic transmission. Some folks choose to beef up the C4small-block Cruise-O-Matic for their high-performance transmissionneeds. And while the C4 is a terrific transmission, no amount ofhardening will get it to stand up to the punishment a C6 can endure.

Toget the skinny on all of the latest in C6 performance and durabilityenhancements, we're venturing out to California PerformanceTransmissions (CPT) in Huntington Beach, California. The driving forcebehind CPT is none other than the respected racer, Art Carr. Art is oneof the elder statesmen of our industry and hobby. Art has been workingon automatic transmissions since 1953, and he's been in businessrepairing and modifying them since 1960.

The C6 core is completelydisassembled. Components that will be used again, such as the case,front pump housing, sun gear shell, and valvebody, are cleaned andcarefully inspected. Everything is rebuilt using all new soft parts,such as clutches, bands, and seals. The valvebody gets some secretmodifications for shift improvement and durability. Hard parts, such asnew rings, clips, and bushings, are replaced.

When we make improvementsto the valvebody, we are not only improving shift quality, but we extendtransmission life. In firming up the shift, we reduce clutch and bandslippage, which reduces unnecessary friction material in the fluid thatcauses excessive wear at the seals and hard parts.

When CPT builds a new transmission, improved durability is paramount.The list of durability improvements includes the C6 ring gear housingand rear planetary gear assembly, which are specially modified to acceptnew inner-and-outer bearings. The stock arrangement used only brassthrust washers and bushings. Reduced internal operating friction andimproved longevity is the result of this modification.

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The stock C6 clutch pack contains four clutch frictions. A CPT C6rebuild includes five Raybestos clutch frictions for improvedengagement. To accommodate the additional clutch friction, the pressureplate must be machined down to create the additional space required.

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For a street performance transmission, the stock ratio sun gear shell isemployed. However, for racing applications, the ratios can be modifiedto give us the advantage. The clutch band surface on the stock shell hasno lubrication holes. CPT adds four or five holes to improve lubricationand prolong band service life. Several measures are taken to strengthenthe transmission in the all-important area of the one-two upshift.

CPT pays close attention to the intermediate clutch band. Besides havingimproved lubrication to the band, an improved replacement band is alsoused. This band differs from the stock component in two ways. First, thereplacement intermediate band has a smooth inner surface rather than thechanneled pattern found on the stock band. Second, the new component iswider by a full 20 percent. Both of these elements add up to a moreusable surface area. When these two factors are combined, it's easy tosee why this part contributes to improved holding quality in secondgear.

When the band is installed, CPT likes to set the adjustment a pinchtight because this firms upshifts and prevents premature wear. Whilewe're on the subject of second gear durability, another CPT modificationcomes to mind. A billet servo arm and cover are used in the CPT buildprocess to increase the force of second gear application.

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At the front pump, there's a brass bushing used to prolong service life.A small cut is made to the leading edge of the pump crescent. Thisquiets down the characteristic whining sound the C6 transmission makesat idle. A later model pump stator is used with CPT rebuilds becausethis stator offers improved lubrication qualities. Better lubricationand pump pressure means longer transmission life.

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Another durability improvement California Performance Transmission usesis special Teflon rings on the pump stator shaft, as well as the forwardclutch drum. The original steel rings are prone to excessive wear andhave a way of creating grooves where they aren't wanted. We tossed them.

The stock C6 valvebody is used, but it receives several modifications toreally be effective. Some are in-house secrets that we can't tell youabout. What we can say is CPT uses a larger accumulator spring forfurther improved second gear shift firmness. A larger pressure regulatorspring is used to increase line pressure, which also improves shiftquality and firmness. Further, CPT enlarges certain oil passageways inthe custom valvebody separator plate that improves high-gear shiftfirmness.

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Improvements to a CPT rebuild don't stop here. CPT also makes its ownhigh-performance torque converters in-house. They don't use inexpensivemetric converter covers, but rather they use their own assembly thatprovides a stall speed of about 2,500 rpm, depending on your performanceneeds. Stall speed is where the torque converter begins to transferengine power to the transmission. Choose your stall speed based on whereyour engine makes power. Street applications need a stall speed around1,800-2,500 rpm. Racers need higher stall speeds to get the engine intothe power band before torque multiplication and power transfer happen.

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