Latest Transmission Titbit – GM 8L90

Last time I showed you some interesting facts about the ZF 9HP transmission. Today I am going to talk about the latest GM 8L90 8-speed automatic transmission, which will be used on the 2015 Corvette.

Please do not be confused with the Cadillac CTS 3.6 V6 trim’s 8-speed transmission, that one is not the 8L90. Instead GM sourced it from Aisin and it is a completely different design.

First I would like to show you the structure diagram of the 8L90. The drawing is from the patent GM filed with the US Patent and Trademark Office several years ago. As expected in a patent document, the drawing looks terribly complicated. But this is just for your reference only, you do not need it to read the following content.


Here is some findings:

1. The 8L90 has 4 planetary gear sets (including 1 sun gear, 1 ring gear and 3 planetary pinions), plus 5 shifting elements (3 clutches and 2 brakes). Let Si denote the tooth number of the i-th sun gear, and Ri denote the tooth number of the i-th ring gear. For 8L90:

S1 = 39, R1 = 77;
S2 = 42, R2 = 78;
S3 = 39, R3 = 77;
S4 = 29, R4 = 103;

2. 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.

3.For the first to second gear shifting, the 8L90 places the brakes closed to the output gear set. This significantly lower the rotational inertia the brake need to overcome during the shifting, this also contributes to a faster shifting time.

4. Here is the shift element operation chart for the GM 8L90:


We can see that downshift from 8th to 5th gear will involve releasing one brake and one clutch and engaging another set of brake of clutch (4 actions), this may cost extra shifting time and more noticeable torque delivery unsmoothness.


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