• GM claims their upcoming C7 Z06 8L90 8-speed automatic shifts faster than the Porsche PDK dual clutch, but can it?

      General Motors caught the attention of enthusiasts by announcing the upcoming C7 Z06 would not only offer an automatic transmission but that it would shift faster than a dual clutch transmission. GM specifically mentions the Porsche PDK dual clutch shift speed as the target. Enthusiasts are debating whether this transmission can beat the Porsche PDK or if this is GM spin to make the automatic no look like a cost cutting measure. So who is right?

      Quite possibly everyone. The 8L90 8-speed is not exactly 'cheap' or solely a cost cutting measure for GM. Cost is obviously a factor and the 8L90 is designed to be compatible with the standard C7 torque tube. GM is killing two birds with one stone by having a new 8-speed that will plug right in where the 6-speed currently offered in the C7 fits. The 8L90 is the same physical size as the 6L80 6-Speed it will replace but also eight pounds lighter thanks to aluminum and magnesium components. A dual clutch that can handle the torque and fit in the same physical space is a tall order that would likely require a lot more work.

      GM could just keep the 6-speed and not do anything at all if they were trying to be cheap but they sure are putting in a lot of effort on the new transmission. The cooling system for example has two inlet ports to provide adequate flow on heavy load and acceleration. This system automatically switches to a single port under lighter load reducing drag for greater efficiency.

      The internals are stout considering the torque rating is 1000nm or 737 lb-ft. It will be able to in stock form support the LT1 and supercharged LT4. It should handle bolt on upgrades to the Z06 before needing to be internally reinforced. A supercharged 6.2 liter V8 is going to put out a quite a bit of torque.

      So, GM is not taking shortcuts here. Is it a cheaper route than developing a dual clutch that can handle the LT4 torque and fit their packaging requirements? Absolutely. Is it a shortcut? No. GM has a team dedicated to this transmissions development and it is essentially custom made for the Z06 before it will go in any other applications. This is a transmission designed with sports car performance in mind.

      That brings us to the question, can it actually shift faster than the Porsche PDK dual clutch transmission? Maybe. The transmission has five clutches. Proponents argue that means it can essentially have five gears pre-selected. However, only two clutches are available at any single time for gear changes. This is not unlike a dual clutch which by definition has two gears engaged. The DCT has the next gear pre-selected and the power flow changes as one clutch engages and the other releases which is limited only by the mechanical speed of the rods. Theoretically the GM 8L90 gearbox can match this aspect of the dual clutch transmission.

      The question becomes how GM is measuring the shift speed. A torque converter transferring rotational power is a bit 'sloppier' so to speak than a clutch until it locks up. Dual clutch transmissions do not use a torque converter with a wet fluid coupling to transfer power to the clutches meaning direct power application. The torque converter however makes torque application fairly smooth which is a benefit for GM in their four-cylinder deactivation fuel saving mode and when cruising. Both use hydraulic pressure to drive the gears and both can start building hydraulic pressure in anticipation of a gearshift.

      When it comes time to make a shift, hydraulic pressure is bled from the engaged clutch and applied to the next clutch. This is the exact same mechanism for the DCT or a wet plate multi-clutch automatic like the 8L90.

      Now, if GM is measuring the shift speed based on the internal gear change speed capability of the clutches, sure, the 8L90 can match a dual clutch as they are mechanically similar and basically doing the same thing. However, are they measuring this through the entire process of a request for a shift, torque application through the converter, and clutches engaging/disengaging to make the gear change? This is not clear and GM has not provided any actual support, numbers, or details on how they are measuring shift speed to back up their 'faster' claim.

      We will have to wait and see some test results to come to a definitive conclusion here but the GM 8L90 is hardly cheap or antiquated technology. There are similarities in its design to a dual wet clutch transmission system but the major point of divergence is the torque converter and the shift rods.

      General Motors Rear Wheel Drive Eight Speed Automatic Transmission
      Technical Paper
      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.
      This article was originally published in forum thread: GM claims their upcoming C7 Z06 8L90 8-speed automatic shifts faster than the Porsche PDK dual clutch, but can it? started by Sticky View original post
      Comments 26 Comments
      1. leveraged sellout's Avatar
        leveraged sellout -
        It would be pretty hard for anything with a TC to match a DCT box in shift speed. Interested to see what happens.
      1. andrew20195's Avatar
        andrew20195 -
        Click here to enlarge Originally Posted by Sticky Click here to enlarge
        Anything else you would like to add? You seem to have a pretty good grip on the subject.

        The difference to me is that the DCT has the shifting rods alternating flow between the clutches so it transfers power from the motor directly to the clutches that way whereas the 8L90 is going through a torque converter that before it locks up will have a degree of slushiness of to it.
        Basically yes, but depending on how sturdy the torque converter clutch is, they can be quite aggressive at locking it before it naturally stalls. The limit is how much punishment the relatively small (compared to a DCT or AMGs wet multiplate clutch) torque converter clutch can handle.

        As I said, it will be interesting to see datalogs and real-world comparisons between this and the PDK.
      1. BlackJetE90OC's Avatar
        BlackJetE90OC -
        Click here to enlarge Originally Posted by TT C6 Click here to enlarge
        WRONG......again. If you believe Corvette engineers. the A8 will be a ZO6 only transmission when it is released and the A6 will live on in the base model and NOT be replaced by the A8. ZO6 has never offered any auto, A6 or otherwise.
        Why do you spew such utter bullsh*t? You don't have a clue what you are talking about.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by BlackJetE90OC Click here to enlarge
        Why do you spew such utter bullsh*t? You don't have a clue what you are talking about.
        Nothing further needs to be said...
      1. V8Bait's Avatar
        V8Bait -
        I like this article better than the random ones that pop up on my Facebook news feed.

        GM knows how to make some pretty awesome antiquated technology. Looks pretty nice for a slushbox. Gonna have to make up a new nickname for them.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by V8Bait Click here to enlarge
        I like this article better than the random ones that pop up on my Facebook news feed.
        Well post it to your feed!