Latest Transmission Titbit – Aisin AA80E

Last week I have discussed the GM 8AT and the ZF 9AT, today let’s take a look at the Aisin AA80E, which is used in the Lexus IS350, GS350, LS460, IS-F and the upcoming RC-F.

Unlike other longitude mounted 8AT such as the ZF 8HP and GM 8L90, the Aisin transmission uses compound planetary gear sets. A compound planetary gear set is a planetary gear set that has multiple sun gears or multiple ring gears. There are different types of them, the one we are interested in is called the Ravingeaux gear set, which has two sun gears, two planet carriers and they share one common ring gear.

One Ravingeaux gear set can provide 4 forward gear ratios and also the neutral and reverse.

The Aisin AA80E uses one standard planetary gear, plus one Ravingeaux gear set. When combined together, These two gear sets can provide 6 to 8 forward gear ratios. By the way, there is a name for such combination: it is called a Lepelletier System.

Therefore the AA80E uses two gear sets (strictly speaking there is 3 sets – because the compound set contains 2), and 6 shifting elements – 2 brakes and 4 clutches (no including an extra one-way clutch which prevents the rear planetary carrier from turning counterclockwise in gear #1).

Compared to the ZF 8HP, the ZF uses 4 standard planetary gear sets and 5 shifting elements (2 brakes and 3 clutches).

The advantage of Lepelletier System is: it is more compact and saves weight (the AA80E weights 95kg, 209 lbs). For your reference, the ZF 6-speed transmission 6HP32 weights around 99kg (218 lbs). But this comes with expense, look at the following AA80E shifting chart, you will see at each gear there are 4 clutches/brakes are disengaged.

Disengaged clutches and brakes will still create drag, because there are fluid between the clutch plates and brake pads and it creates fluid drag forces when the plates/pads on each side rotating at a different speed. Compared to the ZF 8HP, it only has 2 clutch/brake disengaged for each gear, therefore the ZF 8HP will has higher operating efficiency.

The Aisin AA80E is rated at 550 Nm torque (406 lb-ft). However the version that Aisin supplies to GM on its Cadillac CTS VSport has higher torque rating, because the CTS VSport engine output torque is 430 lb-ft. The highest trim of ZF 8HP can handle up to 664 lb-ft of torque

At this moment you may have an impression that the Aisin 8AT is inferior to the ZF 8HP, but please remember its first usage is on the 2007 Lexus LS460, the actual design was finalized in 2004; while the first time you see the ZF 8HP (on BMW F01 7-Series) is in late 2008. And also the shifting operation scheme of the Aisin 8AT does provide a better response in some circumstances. For example on highway cruise, if you want to accelerate quickly to pass, you may need the transmission to shift from 8th gear to 5th gear directly, in this situation the Aisin 8AT only need two shifting element to change status, while the ZF 8HP need 4 of them to engage/disengage.