By on March 11, 2014

fig1 (1)


For those who don’t share my fascination with diving, a saturation dive is conducted at deep depths for extended periods of time at depths of hundreds of feet. I have read with bemusement the various “deep dives” that have been conducted by various publications into the workings of modern automatic transmissions. These deep dives tend to be more akin to splashing about in the shallow end of a pool. The name Saturation Dive is an engineer’s attempt at humor, namely to convey that this series of articles is much deeper than the stereotypical deep dives.

This time around, we will be taking a detailed look at the ZF 8 speed RWD transmissions. The ZF 8 speed transmission family has been around for a while, so a lot more information tends to be available for it than for the 9 speed. First, a quick rehash of the basics of gears that were discussed in the last saturation dive, for details please refer to the article on the ZF 9 speed.

The simplest gear set consists of 2 parallel gears mounted on 2 parallel shafts. Shown in Fig. 1 is a gear set with a 20 tooth drive gear on the right and a 30 tooth driven gear on the left. For this gear set the speed of the driven gear is 1.5 times lower than the drive gear, and assuming no frictional losses anywhere, the torque on the driven gear is 1.5 times higher. This gear set has a ratio of 1.5:1. This type of a gear set is usually not favorable for packaging since it requires 2 parallel shafts, and there are large separating forces that push the 2 gears apart which means that the bearings supporting the shafts have significant radial loads on them, in addition to an axial load if the gears are helical.

fig2 (1)

A simple planetary arrangement is shown in Fig. 2. A simple planetary gear set has 3 members mounted on concentric shafts, the innermost gear is called a sun gear, the outermost gear is called the ring gear, and there are evenly spaced planetary pinions that mesh with both the sun gear and the ring gear. These pinions are free to spin around their own axes, and ride on the planetary carrier, which is the third concentric member. The radial forces in a planetary gear arrangement cancel out due to symmetry, and therefore the bearings supporting these shafts do not see much, if any radial loads. Since the 3 shafts are concentric, there are significant packaging advantages as well. The planetary gear set can also be scaled up to take higher loads by increasing the number of planetary pinions, packaging permitting. In engineering literature, a “stick diagram” is often used as short-hand to describe planetary gear sets, for the planetary shown in Fig. 2 the stick diagram is shown in Fig. 3.

fig3 (1)

So with that out of the way, let us take a look at the ZF 8hp. There are various CAD renders available from the ZF website, one of them is shown in Fig. 4. There are 4 simple planetary gear sets, and since package space is not at a premium, the gear sets are not nested like they are for the 9 speed. There are 5 shift elements, all of them conventional friction type – no dog clutches required because once again, there is space. 3 of the 5 shift elements are clutches, i.e. they couple rotating shafts together, and 2 are brakes, i.e. they lock the rotating shaft to ground.

Image converted using ifftoany

As we can see from the CAD render the shift element function as follows

  • Gear set 1 and gear set 2 have a common sun gear which can be grounded by brake A
  • Brake B can ground the ring gear of gear set 1
  • Clutch C connects the output of the torque converter to the sun gear of gear set 4
  • Clutch D connects the planetary carrier of gear set 3 to the planetary carrier of gear set 4
  • Clutch E connects the sun gear of gear set 3 to the ring gear of gear set 3

Additionally, the following rigid links are

  • Planetary carrier of gear set 1 is rigidly connected to the ring gear of gear set 4
  • Ring gear of gear set 2 is rigidly connected to the sun gear of gear set 3
  • Planetary carrier of gear set 2 is also connected the output of the torque converter
  • Ring gear of gear set 3 is rigidly connected to the sun gear of gear set 4
  • Planetary carrier of gear set 4 is always the output of the transmission

The stick diagram for the transmission system is shown in Fig. 3. The input is the output shaft of the torque converter, which is not shown in Fig. 5. The torque converter is obviously driven by the engine. The gear tooth counts for the Chrysler applications are as follows

  1. Gear set 1: Sun gear S1 = 48 and Ring gear R1 = 96
  2. Gear set 2: Sun gear S2 = 48 and Ring gear R2 = 96
  3. Gear set 3: Sun gear S4 = 69 and Ring gear R3 = 111
  4. Gear set 4: Sun gear S4 = 23 and Ring gear R4 = 85

For some of the European applications, ZF appears to be using slightly different tooth counts, and therefore slightly different ratios. Now, as usual the gory calculations.

fig5 (1)

First gear

To achieved first gear, both brakes A and B are locked, and the input shaft is connected to the sun gear of gear set 4 by applying clutch C. Since brake B is connected to the ring gear of gear set 1 and brake A is connected to the sun gear of gear set 1, both of these members are connected to ground. This means that the planetary carrier for gear set 1 is stationary as well, and since this carrier is rigidly connected to the ring gear of gear set 4, the ring gear of gear set 4 is therefore grounded. The sun gear of gear set 4 is connected to the input, therefore the gear ratio of first gear is

1st =
 = 4.696     (1)

This ratio is almost identical to the first gear ratio of the 9 speed transmission, but these ratios are achieved through completely different means.

Second gear

The transmission upshifts to second gear by opening clutch C and applying clutch E. The ring gear of gear set 4 continues to be fixed, but the sun gear of gear set 4 is now overdriven with respect to the input shaft because gear set 2 acts as an overdrive (sun grounded, carrier input, ring gear output), while gear set 1 continues to act as an underdrive. The ratio is therefore

2nd =






= 3.130     (2)

Third gear

To achieve third gear brake A is released and clutch C is applied, while brake B and clutch E remain closed. By applying both clutches C and E at the same time, the ring gear and planetary carrier of gear set 2 are spinning at the same speed, which means that the sun gear of gear set 2 also spins at the same speed as the input. Since brake B is grounded and the sun gears for gear set 1 and 2 are rigidly linked together, the carrier of gear set 1 is now underdriven. Since the planetary carrier of gear set 1 is rigidly linked to the ring gear of gear set 4, the kinematic state of gear set 4 is as follows

  • The sun gear is rotating at input speed
  • The ring gear is underdriven with respect to the input speed
  • Therefore gear set 4 acts as a mixer module

After some tedious algebra, the ratio is

3rd =
 = 2.104     (3)

Fourth gear

Upshift to fourth gear is accomplished by released clutch C and applying clutch D. Since clutch D ties the planetary carriers of gear sets 3 and 4 together, the planetary carrier of gear set 3 is now in effect the output. By closing clutches D and E, all 3 elements of gear sets 3 and 4 now spin as a unit at the speed of the output shaft, which means that the ring gear of gear set 2 and the planetary carrier for gear set 1 also spin at the same speed as the output. The input is the planetary carrier of gear set 2, while the ground is the ring gear of gear set 1. If I have lost the B&B while explaining the operation of fourth gear, all I can say for reassurance is that even if I were explaining this at a transmission conference I would have lost a vast majority of my audience. The ratio is

4th = 1 +
 = 1+
 = 1.667     (4)

Fifth gear

If fourth gear calculations were complicated, the fifth gear is (pardon my French) the Piece de resistance of these calculations. The upshift to fifth is accomplished by releasing clutch E and closing clutch C while leaving brake B and clutch D engaged. That sounds simple enough, but now all 4 gear sets are in the mix.

  • Since clutch C is closed, the sun gear of gear set 4, the ring gear of gear set 3 and the planetary carrier of gear set 2 spin at the speed of the input shaft.
  • Since clutch D is applied, the planetary carriers for gear sets 3 and 4 are spinning at the same speed as the output.
  • Ring gear of gear set 1 is grounded through brake B
  • The first constraint is that the sun gears of gear sets 1 and 2 are rigidly linked together
  • The second constraint is that the sun gear of gear set 3 is rigidly linked to the ring gear of gear set 2
  • The third constraint is that the planetary carrier of gear set 1 is rigidly linked to the ring gear of gear set 4

A picture is worth a 1000 words

fig6 (1)

The sun gear for gear sets 1 and 2 is whipping around at 2.15 times the input speed due to the gear constraints. The ring gear of gear set 1 is obligated to spin at 0.72 times the input speed, which sets up an underdrive ratio of

5th =
S1S3(R2+S2)(R4+S4) + S1R2(R4R3−S4S3) + R1S2S3(S4+R4)
S1S3(R2+S2)(R4+S4) + S1R2(R4R3−S4S3) + R1S2S3S4
 = 1.285     (5)

I started this calculation at 10 am, and did not finish it till 4 pm. If there is any interest, I can do a separate write up on the details of this calculation.

Sixth gear

Sixth gear is achieved by locking the 3 clutches C, D, and E together while opening both brakes A and B up. This means all the gears in the transmission spin as a unit, and the ratio is quite simply

6th = 1.000     (6)

Seventh gear

Seventh gear is accomplished by releasing clutch E and engaging brake A. Therefore gear sets 1 and 4 are essentially along for the ride, and gear sets 2 and 3 decide the ratio. The planetary carrier of gear set 2 and the ring gear of gear set 3 are connected to the input shat while the carrier for gear set 3 is connected to the output shaft. Gear set 2 acts as an overdrive in this configuration, with the ring gear spinning at 1.5 times the input speed. Gear set 3 acts as a mixer module since the sun gear is spinning at 1.5 times the input speed but the ring gear is turning at the same speed as the input shaft. The ratio is therefore

7th =



 =  0.839     (7)

Eighth gear

Eighth gear is achieved by releasing clutch C and engaging clutch E. This means that gear sets 1 and 4 are still just along for the ride, and gear set 2 acts as an overdrive, but since clutch E is closed, all 3 members of gear set 3 spins as a unit. The ratio in this case is completely dictated by the ratio of gear set 2

8th =
 = 0.667     (8)

Reverse gear

Reverse is achieved by locking both brakes A and B, and engaging clutch D. This means that the ring gear of gear set 2 is overdriven just like the seventh and eighth gear operation, but since the ring gear for gear set 4 is grounded and the ring gear of gear set 3 is linked to the sun gear of gear set 4, the carriers for gear set 3 and 4 spin backwards due to gear constraints. The ratio is

Rev =



= −
 = −3.297     (9)

Shift feel

All sequential upshifts and downshifts with this design involve just releasing one shift element and engaging another. In engineering literature, this is referred to as a “clutch to clutch” shift. Many skip shifts are also possible in the same way, e.g. a shift from Eighth to Fourth involves releasing brake A and engaging brake B while leaving clutches D and E engaged. This transmission therefore shifts very quickly. If the torque converter is replaced by a launch clutch, this transmission would be equivalent to a dual clutch transmission.

A shift from Reverse to 1st and vice versa is also a clutch to clutch shift. So when you are trying to execute than 7 point turn, this transmission will behave much better than the ZF 9 speed transmission. The ratio spacing is pretty much perfect as well.


A brilliant design, wish I had come up with it. Enough said.


Get the latest TTAC e-Newsletter!

35 Comments on “Saturation Dive: ZF 8-Speed Automatic...”

  • avatar

    Okay, THAT was thorough. Overwhelming to my puny liberal arts brain. But still interesting. Thank you.

    My takeaway: all other things being equal, if I were buying a car with 150K miles on it in 10 years, I would be much more comfortable with the 8-speed over the 9-speed.

  • avatar

    Thanks Timur – another great in depth piece of writing!

  • avatar

    Sounds like you feel that internal clutches are better than dog stops. Is it because their operation is smoother?

    What would make these clutches not function right so as to require their replacement? Friction wear? Software? Solenoids/relays?


    • 0 avatar
      Timur Apakidze


      Friction is usually not the problem. Solenoids tend to go bad before friction material has worm off.

      My preference for friction clutches is twofold – a. smoother shifts and b. proven history of working out in the real world

      • 0 avatar

        Are these autos generally derivative (as a manual would be) of existing designs or more clean sheet?

        Also, would the space limitation of fwd push a compromise to dog clutches in the 9hp or is that a tangential issue more to do with # of gears?


        • 0 avatar
          Timur Apakidze

          These 8 speed and the 9 speed are clean sheet designs. The last ZF RWD transmission had 3 planetary gears, 1 simple and 2 in a compound setup. But have some of the components been carried over? More than likely yes.

  • avatar

    I LOVE MY ZF-8 speed and I LooooooooooooooVE Launch Control!!!

    The only problem I have is the PIECE OF GARBAGE shifter design from the Audi A8’s parts bin. And strangely, it’s even WORSE in the Maserati Ghibli – despite being newer and ridiculously expensive.

  • avatar

    Can these these modern autoboxes with big cog counts be rebuilt? Do the manufacturers make spare parts available? Training?

    The BMW dealer where I live hasn’t opened any transmission, auto or manual, in many years. An internal problem means the whole box is swapped out for a new or remanufactured one. If that’s the way it is with these things I’m guessing they’ll be the ultimate depreciation device. An otherwise decent car won’t be worth the cost.

    • 0 avatar
      DC Bruce

      I don’t know that the situation is much different with other autoboxes in today’s world. I would much prefer to have a defective transmission replaced with one rebuilt at a factory-like facility than at Joe’s Garage.

      Of course, manuals are far simpler . . . but the clutch has to be considered a wear item at about 100,000 miles . . . very much dependent upon the driver, of course. Replacing the clutch in an RWD car is pretty simple; you just drop the transmission. But in a FWD car, I would imagine it’s a more complicated piece of work.

      • 0 avatar
        bumpy ii

        There are some more things you have to do (pull the axles, crossmember, and a transmission mount or three, unhook the shifter and clutch linkage, drain the fluid, hold the engine up with something), but FWD isn’t all that different. I saw some kids change the clutch on a Civic in the Home Deport parking lot one day.

    • 0 avatar

      I don’t know if any automakers support overhauls of the 8HP transmission, but I doubt it. I am familiar with Jaguar and Land Rover’s service of the 8HP; they sell only external seals, oil pan/filter, valve body (mechatronic assembly), fluid and torque converter. They sold a similar list of parts for the predecessor 6HP transmission. However, I know that internal 6HP repair parts such as clutches and seals are available from transmission parts distributors and people are rebuilding them. One interesting strategy employed on both the 6HP and the 8HP is that the transmission control module and solenoids are part of the valve body and cannot be purchased separately. They call this the mechatronic assembly.

      • 0 avatar

        Honestly, besides the added planetary, it seems simpler in design compared to some competing transmissions. Ford example, the GM 6L80E 6-speed uses 5 friction clutches and a sprag clutch, the Chrysler 65-68RFE uses 6 friction clutches and a sprag clutch. The ZF8 uses just 5 friction clutches.

        With Chrysler putting these transmission in everything, I suspect spare parts and rebuild kits will be readily available.

        The only thing that scares me is the integrated valvebody/solenoid pack that must be replaced as a whole. I suspect someone will eventually come out with a kit that allows it to be rebuilt though.

  • avatar

    Wow who would have ever thought auto trans porn would make it big.
    God bless the internet.
    Great article and thanks to TTAC also.

  • avatar

    Somewhere online I read a Jeep test (was it here?) with the 9 speed, and the testers said they never saw 9th engaged.

  • avatar

    D C Bruce I would tend to agree with you for the most part. In our small town we do have the exception, a very small transmission shop that has an excellent reputation. He will take on most everything including the CV transmissions and he is much cheaper than the dealers.

  • avatar

    Impressive (both the tranny and the writeup).

    Any idea on the efficiency of 5th gear? That’s a lot gears to put the power through, and I expect it takes a hit on a few of the others as well.

    In the Toyota hybrid thread I pointed out that the accord hybrid basically replaces all this complication with a hybrid electric system (battery/motor) and just uses a clutch for a transmission. Even if it is slightly more complicated, I am pretty sure it is only getting cheaper.

    • 0 avatar

      Yes, has fifth gear inertia and friction loss been determined?
      And I hope you will be posting an article comparing the Toyota and Honda hybrid systems(hopefully pre and post 2015).

      • 0 avatar
        Timur Apakidze


        The inertia inside an automatic transmission is actually quite low. Engine rotational inertia tends to range from 0.2 to 0.45 kg-m^2. For comparison, the 5th gear inertia is somewhere in the region of 0.04 to 0.08 kg-m^2.

        So while fifth gear operation is complicated, the losses are not that bad. I would estimate the losses at 5th gear and 2000 rpm with 20 hp road load to be no more than 1.5 hp.

        Please remember, even when planetary gear sets are along for the ride, they still contribute to the inertia.

    • 0 avatar
      Brian P

      While 5th gear has a lot going on, the relative speed of the gears to each other is pretty low, except for that sun gear on gear sets 1 and 2.

      Most of the friction in automatic transmissions – aside from torque converter slippage and operating the hydraulic pump – comes from slippage of friction plates in clutches or brakes that are NOT engaged (because the plates are sliding relative to each other). In this transmission there are only ever two clutches or brakes that are not engaged. In 5th, one of the two is the brake for the fast-spinning sun gear. Presumably they’ve paid attention in the design to minimize that friction loss.

      The big friction-reducer here is that once you get past third gear, the ratio steps in all the remaining gears are in the 1.2 to 1.3 range. If the engine is spinning at (say) 2500 rpm and you upshift, it only drops to around 2000 rpm. That’s very easy to do smoothly and quickly even if the torque converter stays locked. The 1-2 and 2-3 steps are around 1.5, and with the gear reduction available, even that is doable with the converter locked without bogging the engine. And 1st gear is low enough that it doesn’t really need further torque-converter multiplication. Net result: torque converter is only needed for starting off from a stop, then it can be locked and stay locked. That’s a big efficency-improver.

  • avatar

    MakesYa just want to go buy a manual 1967 VW Beetle :)

    Nice work! Thanks!

    • 0 avatar

      As a former owner of a ’68 VW bug, I can tell you that virtually all of these cars developed 2nd gear synchronizer problems. Hewever, just because the synchro goes doesn’t mean you can’t drive it. You just need to double clutch when you need to downshift to 2nd.

      The VW was one of few cars with a fully synchronized 4 speed manual transmission at that time. Tempus fugit. A friend had a Dodge Dart with the slant six and and a 3 on the tree with non-synchro first gear.

      An automatic transmission assembles a bit like a set of Russian dolls. You just stack the parts inside the case in the order you are supposed to (well, maybe a bit more complicated than that). A manual needs special jigs and shims to set up the alignment of the parts.

  • avatar
    Ian Anderson

    It is going to take me a while to digest all of the algebra written here, but it will be well worth it. This and your 9 speed article are very good reads for engineering-geared (pun intended) people like me, please keep writing them!

  • avatar

    A question I have with these transmissions is: Is it desirable (meaning does it improve their performance) to minimize the number of clutch/brake functions between gears, or do the controls act fast & simultaneously enough that it doesn’t matter? Another way of asking that is if the only change from one gear to the next is a single clutch/brake engaging, does that shift feel better or occur faster than a shift that requires multiple clutches/brakes to be engaged?

    Another question I have is about programming. If it is ‘easy’ to ‘jump’ multiple ratios (i.e., go straight from 8th to 4th), are these transmissions smart enough to do that? Does their programming force them to travel sequentially through each gear, or (for example) can you stomp on it in 2nd, get up to speed and then immediately shift into 7th, bypassing all the other shifts?

    • 0 avatar
      Timur Apakidze

      Tu answer your first question – yes, if you just need to engage one clutch and disengage another that can be done much quicker and smoother than multiple clutch operations.

      As for programming, on downshifts this transmission seems to be programmed to go from 8 to 4 in one step, for upshifts it sounds like it goes through a couple of gears sequentially before skip shifting to a higher gear. But that is easy to change.

  • avatar

    Excellent article, thanks. Please don’t forget to link to other similar articles such as the 9-speed.

    Editors, I would like these articles to be filed under ‘reviews’ or another simple way to find them in the future.

  • avatar

    How does that ZF compare to the Aisin/Toyota AA80E? I know the ZF guys think theirs is much better but is that true and if so what makes it better?

    • 0 avatar
      Timur Apakidze


      “Better”ness of a design can be a difficult thing to judge objectively. But here is my opinion – the ZF design appears to be capable of higher torque capacity at a lower weight. The ZF design also appears to be a teeny tiny bit more efficient. On the flip side, the Aisin design appears to be adaptable to FWD applications.

      The Aisin design decisions are likely sound as well. You have to bear in mind that things like carryover components, carryover tooling can make big differences in business cases for high capital items like transmissions.

      So while everyone is entitled to their opinion, anyone who thinks that the ZF design is “much better” than another transmission design from the same generation is indulging in a bit of hyperbole.

      This design is a good design, and let us just leave it at that.

  • avatar

    Hi TA

    Thanks so much for this in depth review.

    In some vehicles fitted with this transmission a Launch Control feature is available. I have been debating on another forum how this is achieved (technically) given the presence of a tourque convertor, and what is different during a LC proceedure than just mashing both pedals…

    This article got me thinking that maybe one or more of the clutches may be utilised to perform a clutch-drop type launch as in a DCT/DSG auto trans? Otherwise i cant understand how the LC proceedure allows higher revs pre-launch and not overstress the TC?

    Thanks in advance.

    • 0 avatar
      Timur Apakidze

      I don’t think there is anything special going on inside the transmission for the Launch Control feature beyond going to the highest performance setting for shift times, and perhaps permitting a little higher engine speed at torque converter stall. Launch Control has more to do with Traction Control, Front to rear torque split, etc. It is really not that much different from mashing both pedals except that all other vehicle settings are changed for an optimal launch as well with a single press of the launch control button. Also, the launches are likely going to be more consistent, which helps with bracket racing.

      As far as stress on the torque converter goes, a 2 pedal launch is not a big deal for a torque converter, unless of course you mash both pedals for say 10 minutes. The primary stress caused by a slipping torque converter is thermal and not mechanical.

      ps: Bringing in TTAC authors for your forum debate? You sir are bringing a gun to a knife fight ;)

      • 0 avatar

        Thanks for your informed reply, this answers a few other questions also. Also comforting to know that the TC is fairly safe. I always felt the internet consensus that the TC was at any real risk doing a TC/brake 2-pedal launch was unfounded anyway; with the TCU communicating with the ECU in all modern cars, the engine power would likely be cut prior to any damage to TC/trans…

        I had hoped there might be something more going on technically during LC – to put the TC/automatic-naysayers in their place but alas.

        I still defend the term ‘slush-box’ cannot be fairly used when speaking of the ZF8 (or it’s peers) even when mated with a TC drive, given the priority to lock-up… but the vast ‘a real car has three pedals’ contingent are quite staunch in their view!

  • avatar

    Great in-depth review for an amazing gear box design.
    Can some one post the details of the calculation of the 3rd, 4th and 5th gears.

Read all comments

Back to TopLeave a Reply

You must be logged in to post a comment.

Recent Comments

  • Art Vandelay: Yeah those guys that wrote the Declaration would have never had political discussions online. And yeah,...
  • EBFlex: “But this is the pain we must endure to create a green, more sustainable economy that welcomes all who...
  • Jeff S: @Art–You tell me what Skye is referring to because I don’t see what the hidden meaning is? I...
  • Skippity: Maybe they just got a good deal and their credit scores are fine.
  • Jeff S: @CaddyDaddy–Have a Happy Fourth and God Bless you and your family.

New Car Research

Get a Free Dealer Quote

Who We Are

  • Adam Tonge
  • Bozi Tatarevic
  • Corey Lewis
  • Jo Borras
  • Mark Baruth
  • Ronnie Schreiber