Besides using lighter materials, Goodrich’s team did a lot of mass optimization, putting what they call “metal savers” wherever they could. This means they removed material by drilling, machining or modifying molds and tooling to save metal, and thus mass, wherever it could be done without compromising safety and durability.
By rethinking the age-old ways some of an automatic’s functions are performed, the team also reduced mass. “In the torque-converter area,” explains Goodrich, “we’ve gone to an optimized drive plate as opposed to specific lugs that have been typical in the past. We have a new oil pump that we’ve moved from the centerline axis of the transmission down into the valve body in the bottom pan, which helped reduce the size, the spin loss and the mass. Those are just examples of things we’ve put in place to try to get the mass down. Getting sprung mass out helps the overall performance of the car, so we want to be as light as possible.”
Even with an extreme performance car like the Z06, when Chevy engineers refer to “overall performance of the car” they are talking about fuel economy as much as anything else. The initial fuel-economy target for the Z06 was a 5-percent improvement over the C6 version. The addition of two more gears obviously helps by keeping the engine in a narrower operating range, where fuel consumption can be optimized. “With the 8-speed, you’re able to have smaller steps between the gear ratios,” says Goodrich, “which allows the engine to run more optimally at its most efficient area. Our first gear ratio is 4.56:1, which allows us to lower the numeric axle ratio. So we have great performance with the first gear ratio and improved economy at the same time. When highway cruising in eighth gear, you can cruise along at about 1,450 rpm at 70 mph.”
Besides benefitting from its extra gears, the new transmission also achieves improved fuel economy by virtue of some very clever innovations. “The basic power-flow design that we came up with, which is the configuration of clutches that are on and off in a particular gear, improves economy,” Goodrich points out. “This power flow is such that it has five clutches and four gear sets and in any particular gear you always have three clutches on and two clutches off, and the importance of that is, the fewer clutches you have off, the less parasitic drag you have. So we devised a power flow that allowed us a two-clutch-open configuration in any particular range or conversely a three-clutch engagement for any particular gear, and this was a key enabler for better fuel economy. And we’ll also be using a synthetic transmission oil with this transmission, which again allows us to operate more efficiently with lower losses. Less parasitic loss means better fuel economy.”
A strong, light, fuel-efficient automatic that fits the given space would still not have been suitable for the new Z06 if its shifting performance didn’t set new standards for speed and accuracy. “One of the objectives was to get lightning-quick shifts, particularly when you’re using the paddles and commanding the shifts,” says Goodrich. “They need to be responsive, quick, crisp and clean, and we’ve been able to achieve that with our second-generation transmission controls that GM is employing. We’re at the point where we are getting shift time performance equal to and in some cases better than the DCT that’s in the current Porsche 911.”
DCTs (dual-clutch transmissions) are typically used in super-high-performance cars, and are often looked at as the benchmark for short shift times. Because this type of transmission is manual-based, it does not have a torque converter, and with the resulting very low rotating inertia, it is able to shift very quickly. On the other hand, DCTs are not known for the smoothness of their shifts, especially at low speeds. “Our goal was to get an automatic transmission to shift as quickly, and we’re there,” says Goodrich. “So we have the very quick shift with the convenience and comfort of a true automatic. We’re very proud of what we’re able to achieve right now.”
In addition to the new Hydra-Matic 8L80 automatic, which will be manufactured in GM’s Toledo Transmission plant, the 2015 Z06 will also be available with a 7-speed manual gearbox. Chevrolet feels that it is important to offer this choice, especially since its competitors in the 500-plus-horsepower supercar class are increasingly offering just a single transmission.