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2001 Corvette Z06 by Hib Halverson New Corvette engines usually bring changes to the rest of the powertrain and that had me talking to Bill Zabritski, GM’s Design Release Engineer for manual transmissions and rear axles in performance cars. I’ve known Bill since he worked on the ZF S6-40 development in the mid-’80s for C4 and he’s a passionate believer in the Corvette. Zabritski told me, to accommodate LS6’s increased torque, clutch pressure was increased but pedal effort was held at a comfortable level. All C5 driveshafts are now 6061 aluminum and .315-in. larger in diameter. Driveshaft couplings for Z06 and other stick-shift cars are upgraded from last year. All Z06es have the Tremec six-speed manual transmission. According to Zabritski, for ’01, forward gears in all C5 manuals get sintered carbon synchro rings as a durability improvement. Zabritski also said Z06 uses a specific "M12" version of this transmission having higher ratios in the first three gears (2.97/2.07/1.43 vs 2.66/1.78/1.30) which makes the M12 a wider-ratio trans than the MN6 used in Coupes and Convertibles. With the LS6’s flat torque curve, Zabritski and the Corvette engineers found the Zed-ought-six was quicker with higher ratios in the first three gears.
One problem is C5’s limited transmission and rear axle cooling. This is a function of the car’s front air dam and smooth undercar aerodynamics. M12s get a temperature sensor that turns on a "trans overtemp" light when trans oil gets above 325° F. At that point, as Chevrolet’s press kit says, "...thermal loads become excessive..." and "...the transmission could be damaged if not allowed to cool down." This sensor has been standard on C5 manuals sold in Europe since 1997. The overtemp light reminds us General Motors is still a big company where executives sometimes make compromises. GM is aware of the high transmission and rear axle temperatures sustained by C5s used in motorsports applications, yet, on the Z06, which by Chief Engineer Dave Hill’s own admission, is marketed to the "extreme performance enthusiast"; we get a temperature warning light. What the M12 needs is either a lubricant better suited to high temperatures, such as a synthetic like Red Line or, better yet, a transmission cooler. Apparently, neither are very attractive for factory use, presumably, because of cost, however, an aftermarket transmission cooler, developed for C5 racing applications, is listed in the 2000 GM Performance Parts Catalog. The rear axle assembly used in the Z06 is generally the same unit (3.42:1 ratio with limited slip differential) manufactured by Getrag and used by all C5 manuals since 1997. It has one difference: a shotpeened ring and pinion. Bill Zabritski told me this adds an extra margin of reliability to a powertrain having increased torque output. |
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