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I found this nice article that provides more insights into the new GM 8-speed transmission. Looks like the reason why GM claims it is comparable to the Porsche double clutch transmission, is because its unique design of the shift element's location.

According to the article, the GM 8AT is at least has a better design than the ZF 8HP transmission. I am waiting the 2015 model arrive in my local dealership so I can try it out.

All of the three clutches (part 80, 82, 84, circled in red in the above drawing) are placed in front of all of the four planetary gear sets. Compared to the ZF 8HP transmission, the ZF puts all 3 clutches between the 3rd and 4th gear sets. The advantage of putting all clutches in front of the transmission unit, is to let the related valve body assembly more close to the hydraulic pump. This requires less transmission fluid to pass the clutch pistons during engaging/disengaging, therefore the 8L90 design decreases the shifting time. Under daily driving conditions, you should feel the transmission shift more quickly. GM claimed the 8L90 will have comparable shifting performance with the Porsche PDK. We will see how it performs when the 2015 Corvette comes out.
 
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