By on December 10, 2013

2014-GM-Eight-Speed-Automatic-Transmission

It’s not known if the leak was intentional or not, but the summary of a paper initially published by the Society of Automotive Engineers, since taken down from the SAE site, says that a new eight speed automatic transmission, given the designation 8L90 by General Motors, will be introduced in the 2015  Corvette, on sale next fall. The all-new 7th generation Corvette is currently offered with GM’s 6L80 six speed automatic and a seven speed manual gearbox. The 8L90 is described as being designed for rear-wheel-drive applications and variants will likely be used in GM’s fullsize pickups and in rear wheel drive Cadillacs.

The 8L90 has about the same overall dimensions as the 6L80, and is said to be able to handle up to 737 lb-ft of torque. It has a shorter first gear for better launch acceleration, an overall ratio spread of 7.0 and three speed sensors for better shift response. Other benefits are said to be better fuel economy, improved performance and a quieter car with improved NVH levels. The current automatic C7 is rated at 28 mpg on the highway and it’s possible that with the new eight-speed it might be able to achieve 30 mpg. Of course, at EPA “highway” speeds, the 455 hp LT1 in the Corvette is more or less loafing along.

Also new for the 2015 model year will be a Z06 version of the new Corvette that will be introduced in about a month at the North American International Auto Show in Detroit. Previous generations of the Z06 were stick only, but now that you can’t buy some Porsche and Ferrari models with a manual transmission, it’s possible that the C7 Z06 may offer the 8L90.

The full SAE paper will be available in April. Fortunately, before the SAE pulled it, someone at corvetteforum.com managed to preserve the text:

General Motors Rear Wheel Drive Eight Speed Automatic Transmission
Technical Paper
2014-04-01
James Michael Hart, Tejinder Singh, William Goodrich

General Motors Rear Wheel Drive Eight Speed Automatic Transmission General Motors shall introduce a new rear wheel drive eight speed automatic transmission, known as the 8L90, in the 2015 Chevrolet Corvette. The rated turbine torque capacity is 1000 Nm. This transmission replaces the venerable 6L80 six speed automatic transmission. The objectives behind creation of this transmission are improved fuel economy, performance, and NVH. Packaging in the existing vehicle architecture and high mileage dependability are the givens. The architecture is required to offer low cost for a rear drive eight speed transmission while meeting the givens and objectives. An eight speed powerflow, invented by General Motors, was selected. This powerflow yields a 7.0 overall ratio spread, enabling improved launch capability because of a deeper first gear ratio and better fuel economy due to lower top gear N/V capability, relative to the 6L80. The eight speed ratios are generated using four simple planetary gearsets, two brake clutches and three rotating clutches. The resultant on-axis transmission architecture utilizes a squashed torque converter, an off-axis pump and four close coupled gearsets. The three rotating clutches have been located forward of the gearsets to minimize the length of oil feeds which provides for enhanced shift response and simplicity of turbine shaft manufacturing. The transmission architecture features a case with integral bell housing for enhanced powertrain stiffness. A unique pump drive design allows for off-axis packaging very low in the transmission. The pump is a binary vane type which effectively allows for two pumps in the packaging size of one. This design and packaging strategy not only enables low parasitic losses and optimum priming capability but also provides for ideal oil routing to the controls system, with the pump located in the valve body itself. The transmission controller is externally mounted, enabling packaging and powertrain integration flexibilities. The controller makes use of three speed sensors which provide for enhanced shift response and accuracy. Utilization of aluminum and magnesium components throughout the transmission yields competitive mass. The dedicated compensator feed circuit, used in GM six speed designs, was supplanted by a lube-fed design in order to simplify oil routing and enhance shift response. Packaging is within that of the GM 6L80 design, allowing for ease of application integration. The overall result is a robust, compact, and cost effective transmission which offers significant fuel economy and performance benefit, over its six speed counterpart, and shall provide an attractive balance of overall metrics in the automatic transmission market.

 

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