C6, Naked and Exposed:  Corvette Action Center's First Look at the 2005 Corvette - The Sequel: Finally, We Drive It! - Page 8 of 12

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C6 Naked and Exposed - The Sequel: Finally...We Drive It!!! - Page 8 of 12

And, Talk to Some of the People who Built It

by Hib Halverson
Imagery by GM Communications, Richard Prince, U.S. Air Force and Sharkcom
©2005 Shark Communications
No use without permission

Driveline Digest

In our first C6 article, we covered the new 4L65-E automatic transmission in detail but, at the time the January story was prepared, we didn't have a lot of information on the revised, TREMEC T56, six-speed manual so what we gave you was the basics (see C6, Naked and Exposed:  Corvette Action Center's First Look at the 2005 Corvette).

To learn more about the '05 six-speed, recently, we interviewed Bill Zabritski who, since 1978, has been a design engineer for manuals in GM passenger cars. He's one of the few people who's worked in the same area of Corvette development for C3, C4, C5 and C6. Bill told us about changes to the shifter and inside the transmission which make for a noticeable reduction in shift throw, an improvement in shift feel and minimal change in shift effort.

"We had some owners complaining about the shift effort and throw," Zabritski told us, "but you can't just chop-off the shift lever. Yeah, you'll get the shorter throw, but effort goes up, not down. We worked with TREMEC (the transmission supplier) on a lot of other changes besides just the shorter lever."

The shift lever was shortened by 3/4-in. Inside the transmission, TREMEC reduced the distance the synchronizer sleeves move during gear selection. While sleeve movement is less, the teeth on the synchro sleeves must occupy the same location if synchronization is to occur, so the synchro sleeves where widened and the guide plate stops shortened by 0.82 mm to control the reduced movement. The combination of a shorter lever and less synchro travel reduced shift throw by 10%.

As shortening the lever increased shift effort, to bring effort back down; GM and TREMEC made more improvements to the gear selection mechanism inside the transmission.

Click image for larger view This computer-generated drawing of the shift rail, shift forks and synchronizer sleeve shows some of the changes made to reduce shift throw and eliminate the double-bump shift feel. Drawing:  TREMEC	Click image for larger view This computer-generated drawing of the shift mechanism inside the trans. shows Switching to a ball-bearing detent for fore-aft shifter movement, noticeably improves shift feel and incrementally reduces shift effort. Drawing:  TREMEC

"To improve shifter feel we went to a roller detent, to gain fore-aft precision, and a ball detent, for cross-car responsiveness," Zabritski continued. "We're also using linear bearings instead of bushings on the main shift rail. These things taken individually, the differences are not huge; but together, they make for a reduction in effort and an improvement in shift feel."

To improve shift feel, TREMEC went to a "stepped-tooth" configuration for the synchro sleeve teeth. Every fourth tooth is placed forward and incorporates a more acute angle than the other three. Those sharper-angled and longer teeth engage the gear just a little sooner. Previously, the shift effort curve of T56 had two peaks-a "double-bump" as Bill Zabritski says-which gave shifts a "double-detent" feel. The stepped-tooth synchro sleeves eliminate that.

Click image for larger view This computer-generated drawing of the shift rail, shift forks and synchronizer sleeve shows some of the changes made to reduce shift throw and eliminate the double-bump shift feel. Drawing:  TREMEC	Click image for larger view Just a fraction of a second later, the synchronization is complete and the actual gear selection is just starting to take place. This is where the second "bump" of shift feel occurred. For '05, it's gone. Drawing:  TREMEC

What will the C6 manual driver experience with the revised six-speed? As we said earlier, there's no way you'll miss the shift throw reduction. Shift feel is more precise and the double-bump feel is gone. Lastly, when measured, shift effort goes up 3%, but it's virtually impossible to feel that and, judged subjectively, the effort is the same as what it takes to shift my '04 Z06.

The changes Zabritski's group and the people at TREMEC have made addressed many of the legitimate complaints Corvette customers had about shifters. For C5, there are several aftermarket shifter choices, everything from hacks, like sawed-off stock shift levers; to pretty decent solutions, such as the Hurst, and even niche-market items for drag racers, such as the B&M Ripper. With C6's shifter and transmission improvements, the practical need for aftermarket shifters may have been eliminated, except for those hardcore racers who want an extremely short throw and will accept high effort and close-gated gear selection and "mod-freaks" who want to change stuff just for the sake of personalizing their vehicles whether there's a legitimate need or not.

The final six-speed tweak doesn't do anything for shift quality, but it does improve durability of transmissions in Z51s. Because they are intended for severe duty, Z51s with six-speeds have a transmission cooling system. A pump on the front of the trans, driven by the countershaft, sends lubricant through the same cooler in the radiator used by automatics.

Finally, there are some changes in the rear axle. All Corvettes now get the shot-peened ring-and-pinion gearset previously used only on manuals. Bill Zabritski told us that was done because of the LS2's higher torque output. In addition, in a limit-handling move, the limited-slip differential's pre-load has been altered to change the torque flow through the differential in some situations.