Inside Chevy's super-secret shifting software for the '12 Corvette
Unless you've driven auto-trans '11 and '12 Corvettes back-to-back and compared their shifting performance in manual "tap" mode, you might not have known that this year's model is significantly quicker than its immediate predecessor when called upon to execute 1-2, 2-3, and 3-4 shifts.
To learn why, we first searched Chevrolet's part numbers and RPO codes for the two model years, ultimately concluding that no hardware changes had been made to the Vette's new-in-2006 6L80 transmission.
Then, Tom Read of GM Technology Communications, Powertrain mentioned to us that he had attended a media preview for the '12 Camaro ZL1, presented by General Motors' Assistant Chief Engineer for Corvette Automatic Transmissions Rich Mardeusz. One particular slide's bullet point--describing the Camaro automatic's manual mode--caught his attention: "Staged Upshifts--imperceptible shift times."
"That drew my further questioning and investigation of what Staged Upshifts were," Read recalls. "I brought the topic up with Mardeusz during a side conversation at the event, and he told me that it was a new invention already in '12 Corvettes. It was just one of those things that we implemented but didn't have the chance to highlight."
Mardeusz and his team of engineers weren't the only ones who knew that Staged Upshifts existed. So did the United States Patent and Trademark Office, which received a patent application from GM's lawyers (on behalf of a team of engineers) under the heading "Clutch Control for a Transmission." The application's text described "a system and method for controlling a transmission, and more particularly to a system and method for reducing an upshift delay in a transmission." It went on to state that, "while current transmission control systems and methods achieve their intended purpose, there is a need for a new and improved transmission control system and method which exhibit[s] improved upshift performance."
We asked Read if the two men who understand Staged Upshifts best--Mardeusz and Gabe Gibson, lead clutch-control calibration engineer for Corvette transmissions and the system's co-inventor--could downshift long enough to sit down with us and share more details on the latest Corvette's secret upshift software. (Staged Upshifts' other co-inventers, Matt Pruski and Steve Christ, were not able to join us.) What follows are the highlights from our interview. Special thanks to GM's Tom Read for making this story possible.
VETTE magazine: Staged Upshifts is not listed in any '12 Corvette dealer brochures or literature; in fact, we can't find reference to it in any documents released to Chevrolet's dealers or service departments. What is it, and what does it do?
Rich Mardeusz: It is software to command the 6L80's transmission electro-hydraulic control module (TEHCM) to significantly reduce the shift time when you're in tap [mode], to the point that it basically equals or exceeds dual-clutch-transmission (DCT) performance.
VM: Are you saying the Corvette's clutch-to-clutch automatic transmission can now compete against the tap-shift performance of a DCT, simply because you gave it a new instruction set?
RM: Absolutely! We made no hardware changes to the Corvette's transmission and tap-shift system between '11 and '12 to accomplish this.
VM: What Corvettes received this secret upgrade for '12?
RM: All automatic-equipped Corvettes have it.
VM: Take us back to the need for more speed, at least as far as the conception of this software is concerned.
RM: It happened internally at GM a couple of years ago when we were looking at the Corvette's transmission performance and things we could do to improve it. One of the issues we noticed was when you hit the tap button in manual mode, there's a noticeable time delay before the execution of the shift.
Gabe Gibson: We listened to feedback from magazines and customers who asked why DCTs weren't in Corvettes. One of the benefits of the DCT is manual shift times. That's when we started looking into what we could do with the Corvette's existing 6L80 transmission to get us to the levels of a DCT, and, at the same time, improve the shift times in manual mode so much that we can make them almost imperceptible to the owner.
VM: What was causing this delay?
RM: To answer that, let me go through what happens when a Corvette owner hits the tap button. After describing each phase, I'll explain what we did or didn't do about it.
Phase 1:The customer presses the tap upshift paddle. There's a short time delay between the time the button is pushed and the time the transmission-control module (TCM) recognizes the request from the owner. In the Corvette, the communication from the tap button to the TCM is hard wired; it's a direct link. There was no way to increase that speed.
Phase 2:The TCM initiates the shift. When we looked at the Corvette's TCM, we found some minor loops in the computer software that we didn't think were necessary to accomplish the shifts, so we reduced time there--about 50 milliseconds.
Phase 3:The TCM initiates the shift by commanding a clutch fill in order to achieve the upshifted next-gear ratio. Clutch fill is the time required to eliminate the clutch clearance or free pack travel. Instead of filling the oncoming clutch after the shift request occurs, which takes time, the guys at Milford came up with an algorithm where we anticipate that the driver is going to request the shift. Depending upon the performance the driver demands from the vehicle--based on current gear, throttle, torque, and speed--we fill the clutch at different amounts by commanding a variable bleed solenoid. For example, if the driver is doing wide-open throttle (WOT), then we will completely fill the clutch in anticipation of him or her asking for that shift. Doing so basically eliminates the phase completely.
Phase 4:The torque phase comes next. It transfers torque from the current gear to the upshifted gear. We were able to do some time reduction in that area, based on the fact that we knew we were going to do the shift.
Phase 5:Once all the torque is transferred to the upshifted gear, the ratio change will occur. This was an area that had already been optimized.
VM: What gear shifts does Staged Upshifts make faster?
RM: The current system is for the 1-2, 2-3, and 3-4 shifts only.
VM: But not 4-5 and 5-6?
GG: No. With the lower gears, you have a significantly higher vehicle accelerate. In other words, engine rpm is accelerating toward the rev limiter at a much higher rate.
VM: Is Staged Upshifts equally beneficial to all lower-gear shifts?
GG: No. Staged Upshifts is most beneficial for 1-2, then 2-3, then 3-4, in that order.
VM: Are the benefits quantifiable?
GG: Yes. Staged Upshifts achieves approximately a 65 percent reduction in the WOT 1-2 shift time, if you look strictly at the time from when the owner hits the tap button to the time he or she feels something. It went from 350 milliseconds down to about 120 milliseconds.
RM: And the WOT 2-3 shift-time went from 400 milliseconds down to about 200 milliseconds.
VM: Is Staged Upshifts activated in all of the Corvette's driving modes?
RM: It works anytime you're in manual mode. The reason it's not used in Drive mode or Sport mode is because the transmission controller schedules when the upshift happens in this modes; the clutch fills are already happening. In the manual mode, when the owner requests a shift, we need to get all the additional delay out of there so it can happen near-instantaneously.
VM: Finally, how does Staged Upshifts affect Corvette's quarter-mile performance?
GG: Normally we set all of our fastest acceleration runs in Sport mode, letting the transmission shift at the precise time. Staged Upshifts gives Corvette owners the ability--while in manual mode--to shift at the precise time they want to and set the vehicle's fastest times.
Chevrolet has no plans at this time to offer Staged Upshifts as a performance retrofit to automatic '05-'11 C6s. But since the invention is not 6L80-specific, it's likely to be included in the transmissions installed in future Corvettes.