This article will explain why people feel the shifting slowness and sluggish characteristics of the Aisin RWD 8-speed transmission, especially in applications that are paired to the Toyota 2UR 5.0L V8 engine.
First, let’s start with some background information.
In 2006, Toyota/Lexus released the world’s first 8AT for RWD-based vehicles, designed and made by Aisin with code name AA80E (for RWD) or AA80F (for AWD application).
The first car model that received this 8AT is the Lexus LS460 sedan. There are multiple variants with different torque ratings in this Aisin 8AT product line:
- AA80E/F: majorly for V8 cars (LX570, LS460, GS460, IS-F, RC-F, GS-F, IS500, 2016+ J200 Land Cruiser), maximum torque rated at 550Nm (406 lb-ft);
- AA81E: used on other low output RWD Toyota/Lexus cars;
- TL80SN: this is a variant Aisin specially made for GM to use in its 2014+ Cadillac CTS VSport sedan. Based on AA80E, Aisin added additional clutch plates and heat treatment to some parts to handle the CTS VSport’s extra torque (430 lb-ft);
- TR80SD: another variant sold to the VW Group and is used in the 1st-gen Audi Q7 and 2nd-gen Porsche Cayenne, torque rating is 800 Nm (590 lb-ft)
On the IS-F/RC-F/GS-F and IS500, the transmission’s torque converter can lock up from the 2nd to 8th gear; In other applications (see this document), it can achieve partial lock up between the 4-8th gears, and full lock up between the 5-8th gears.
Note: AA80E (for V8 engine) and AA81E are almost mechanically identical. They share most of the parts, for example they use exactly the same shift solenoids (see this link for part numbers; USE30 = IS500; GSE31 = IS350 RWD); same (most of) clutch discs (part #35791-50030, 35795-50020, 35605-50020 etc. link); same input sun gear assembly (part #35709-50020, link); same output shaft assembly (part #35073-50010, link); same oil pumps (part #35301-24010, link); same oil strainer assembly (part #35330-50030, link) etc. Major differences are in the software programming, torque convertor (different power capacity) and *maybe* the valve body (the reason for “maybe” is due to: although the whole assembly has different part # between the AA80 and AA81, however I check each of the individual solenoid parts and wiring, they turn out to be identical). Therefore, AA80 and AA81 can be considered architecturally identical. Consequently, all data and conclusions below apply to both variants. In fact, all the excerpted diagrams and data are from Toyota’s SAE paper dedicated to the AA80E. So if you are looking for discussion about the IS500’s transmission, you have found the right place.
Before diving deep into more technical details, it is worth noting that for the RC-F, IS500 and LC500, their engine torque output is rate at the range of 395-398 lb-ft, which only leaves a tiny 8-10 lb-ft of buffer below the transmission’s maximum torque rating. To give you a rough idea of how small this buffer is: for an average adult to open a twist-off cap on a bottle, the torque exerted from the human fingers is around 6 lb-ft. This small buffer makes the transmission to be fragile and vulnerable in high output applications. This also partially explains why the final production version IS500’s transmission (which Lexus deleted the auxiliary transmission oil cooler) easily overheats under intensive driving sessions.
Unlike other longitude mounted 8AT such as the ZF 8HP and GM 8L90 families, 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 used in the Aisin unit is called the Ravigneaux gear set, it has two sun gears, two planet carriers and they share one common ring gear.
One Ravigneaux gear set can provide 4 forward gear ratios and also the neutral and reverse.
The Aisin AA80E uses one standard planetary gear, plus one Ravigneaux gear set. When combined together, These two gear sets can provide 6 to 8 forward gear ratios. Such gear set combination is also called a Lepelletier System.
Therefore, the AA80E uses two gear sets (strictly speaking it has 3 sets – the compound set contains 2), and 6 shifting elements – 2 brakes and 4 clutches (not 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 main advantage of the Lepelletier System is its compactness and R&D cost saving (Aisin’s existing 6AT already uses a similar Lepelletier System design). There is also weight saving benefit back in the early 2000s as well: the AA80E weights at 95kg (209 lbs), compared to the ZF’s 6HP32 weight of 99kg (218 lbs); However, such weight advantage has faded away from today’s standard, for example the later ZF 8HP45 (same input torque capacity) only weights 80kg (176 lbs).
Despite the advantages, the Aisin design brings in multiple serious compromises.
The first concern is efficiency. Look at the following AA80E shifting chart, you will see at each gear there are 4 clutches/brakes are disengaged.
Disengaged clutches/brakes create lots of drag, due to the fact that there are fluid between the clutch plates and brake pads, which creates fluid drag forces when the plates/pads on each side are rotating at a different speed. Compared to the ZF 8HP, at each gear, there are only 2 clutch/brake are disengaged, so the ZF has less drag and higher operating efficiency. In other words, the Aisin unit doubles the clutch drag and friction than the ZF 8HP unit.
When the transmission is not warmed up, the transmission fluid’s viscosity is higher. Since the Aisin unit has way more disengaged clutches than the ZF unit, therefore the fluctuation of its final output caused by the drag symptom is more pronounced. This explains why some IS500 owners reported that the transmission is more jerky and sluggish when the car is cold.
The second drawback is in its shifting feel, characteristics and performance. A quick search on Lexus/Audi/Porsche forums reveals considerable user complaints about the slow shift/sluggish/hesitation symptoms even the vehicle has been fully warmed up, to the extent that some users even get rid of their cars solely because the subpar behavior of this transmission. Those users are not exaggerating the problem.
Look carefully in the above shift diagram, we will see whenever there is a downshift from a gear >= the 6th, to another gear <= the 4th, there is a major load transfer between the C1 and C2 clutch, and the shifting involves two elements to disengage and two elements to engage simultaneously, which triggers a so-call “double clutch-to-clutch” action (note: not to be confused with the DCT mechanism – they are different).
Each of these double clutch-to-clutch downshifts are in fact consists of two shifts behind the scene. With that being said, the control unit needs to orchestrate the 4 elements’ operation properly and try to lower the torque delivery delay and interruption, and give the driver an impression that only 1 shift has occurred. Although there won’t be a distinctive feel of two separate shifts, these two shifts are still overlapping under the hood, so the driver will experience multiple output “swing” and the delay of actions occur during the shifting process. This is the major reason that people get the sluggish/slowness feel on cars equipped with this Aisin 8AT.
More specifically speaking, below is the list of double clutch-to-clutch downshifts for the Aisin 8AT. Unfortunately, many daily use cases that require skip-gear down shifting all fall in this list. For example, down shifting from the 8th-gear is one of the most typical use cases since this is what we usually do when passing another car when the general traffic is moving at 40+mph, or changing from the current lane which has heavy traffic, to the next lane with light traffic and immediately accelerate. So most customers will feel and experience this issue.
- 8 => 4
- 8 => 3
- 8 => 1
- 7 => 4
- 7 => 2
- 7 => 1
- 6 => 3
- 6 => 2
- 6 => 1
If counting in the one-way clutch, basically every gear >= 2nd downshifts to the 1st gear will fall into this similar double-action scenario. If you get used to how other transmissions’ shifting, you will feel the Aisin 8AT behaves “strangely” whenever you bring the car to a complete stop, which shifting from 2nd to 1st is unavoidable.
As a comparison, in the ZF 8HP transmission, most of the skip-gear shifts only require one single clutch-to-clutch operation. In addition, downshifting from the 8th gear to any other gear won’t have such issue at all. For the ZF unit, only these downshifts need double clutch-to-clutch action (8 => 1 is omitted since their allowed speed ranges do not overlap):
- 7 => 4
- 7 => 3
- 6 => 2
- 6 => 1
- 5 => 2
- 4 => 1
As a concrete example for the Aisin 8AT, the following shows what happens after the driver pressing down the accelerator pedal and triggers a 6=>3 gear shift, see how the torque delivery fluctuates during shifting and the time delay as well.
For the shifting lag, we should not be confused this with Lexus’s claim of the 0.1 second shift time for the AA80E. Here the “0.1 second” only refers to the time that the valve body changes the clutches/breaks, it is not the time between you pressing down the gas pedal and the shift completes. In addition, this 0.1s shifting time only applies to one single shifting scenario (the 2=>3 up-shift). There is nothing to brag about because that is the easiest case; most shifting scenario in fact need more time than that. Also, the whole gear shifting process contains way more steps, for example the TCM first needs to compute the power target, and then adjusts the fuel system and injectors to rev up or slow down the engine RPM (to match the target gear), and finally instructs the transmission to engage/disengage the clutches. So there is a time lag exists before the actual shift begins, as shown in the below chart:
Summary: the Aisin AA80 family was state-of-the-art when it came out back in 2006 because it was the only 8-speed choice at that time. The shinning peaked in 2007 when Lexus released the IS-F with the enhanced version of AA80E, which enables full lock up from the 2nd gear, together with new control logics in the TCM to execute a faster shifting than on the LS460 application. However its design has inherit significant drawbacks. At the same time, its competitors (ZF, GM, Ford etc.) showed up in the subsequent years with much better design and performance, which makes the Aisin AA80 transmission to be less attractive. This is the reason why all of its external customers ditched the Aisin unit once alternative options were available. Obviously the AA80E needs a successor, for example the 10AT (AWR10L65) which we have seen on the Lexus LC500, LS500 and LX600 etc.
