By on April 23, 2014

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On the surface, there are many similarities between the ZF 8HP transmission family and the GM 8L90. Namely both use 4 gear sets, 5 shifting elements (3 clutches and two brakes), off axis pumps, and have roughly the same gear ratio spread at about 7:1 overall spread. The saturation dive is not about dealing with things that are on the surface. To be entirely honest as the details started to emerge on the GM 8L family of transmissions I suspected that it would end up being a ZF licensed design, the ZF 8HP after all is a very good design in my opinion. But the abstract of the paper that we managed to snag before SAE took it down revealed one very important detail – all 3 of the clutches were located in front of the planetary gear sets very much unlike the ZF design.

The amount of information available publicly on the 8L90 is very sparse and is right now limited to the the SAE papers the engineers responsible for the transmission wrote and the patent granted to GM. The folks at SAE want money for the paper, but the patent #8105196 is available as a matter of public record from the USPTO. The structure of this particular saturation dive is therefore going to be a bit different from the past ones. Please refer to the earlier saturation dives for a rehash of the basics of gears.  The drawing from the patent filing is shown below

 

GM 8L90 patent drawing

GM 8L90 patent drawing

The stick diagram

Since the patent diagram is not the easiest thing in the world to read, the stick diagram for the transmission is shown below. The output shaft of the torque converter is the input shaft for this gear arrangement.

Stick diagram for the GM 8L90

Stick diagram for the GM 8L90

 

As we see from the stick diagram, there are 4 simple planetary gear sets. The shift elements function as follows

  • Brake A when activated grounds the sun gears of gear sets 2 and 3
  • Brake B activation grounds the ring gear of gear set 3
  • Clutch C activation ties the input shaft to the sun gear of gear set 4
  • Clutch D activation ties the ring gear of gear set 1 to the sun gear of gear set 4
  • Clutch E activation ties the ring gear of gear set 2 and the syn gear of gear set 1 to the sun gear of gear set

Additionally, the following rigid links exist

  • The input shaft is rigidly connected the planetary carrier of gear set 2
  • The sun gear of gear set 1 and the ring gear of gear set 2 are connected together
  • The sun gears of gear sets 2 and 3 are connected together
  • The planetary carriers of gear sets 1 and 4 are tied together – which in turn is the output shaft of the transmission
  • The planetary carrier of gear set 3 is tied to the ring gear of gear set 4

Clearly while there are similarities, the planetary gear arrangements for the ZF 8HP and the GM 8L90 are actually quite distinct in layout. The stick diagram for the ZF 8HP is shown below.

Stick diagram for the ZF 8HP

Stick diagram for the ZF 8HP

 The similarities between the 2 layouts are enumerated as follows

  1. Rigid links between 2 sun gears, but between gear sets 2 and 3 for the GM design vs. gear sets 1 and 2 for the ZF design
  2. Rigid link between planetary carrier of a gear set and the ring gear of gear set 4, but between gear sets 3 and 4 for the GM design vs. gear sets 1 and 4 for the ZF design
  3. Rigid link between the input shaft and the planetary carrier of gear set 2

The major difference is that the ZF 8HP uses two rigid links between the sun gears and ring gears of adjacent gear sets, the 8L90 uses one such link and has a separate rigid link between the planetary carriers of gear sets 1 and 4. Therefore the GM 8L90 is a unique layout that is different from the ZF 8HP but there are similarities that are quite obvious as well.

First gear

The first gear is achieved by engaging both brakes A and B and clutch C. Engagement of brakes A and B locks the sun gear and the ring gear of gear set 3 to ground, which means that the planetary carrier of gear set 3 is tied to ground as well. Since this planetary carrier is rigidly tied to the ring gear of gear set 4, this means that the ring gear of gear set 4 is stationary. The engagement of clutch C ties the input shaft to the sun gear of gear set 4. This sets up an underdrive gear with a ratio of

(1) 1st   = S4+R4


S4

= 4.5517 

We know that the gear ratio is around 4.55, and also that given the torque capacity requirements GM is more than likely using 4 pinions for higher torque rating similar to the ZF design, therefore (S4+R4) has to be divisible by 4. Also (R4-S4) has to be an even number. A candidate tooth count is S4=29 and R4=103, which leads to a feasible arrangement with 4 37 tooth planetary pinions and a first gear ratio of 4.5517, which is quite close to the known ratio. Since S4+R4 = 132 is divisible both by 4 and 3, for the lighter duty versions of the 8L family it is possible for GM to reduce the number of planetary pinions to 3 for applications that don’t quite require the 1000 Nm the 8L90 is capable of.

For the first gear operation, the GM 8L90 and ZF 8HP are therefore kinematic equivalents, but there is one very significant difference, the GM design places the brakes right next to the output gear set. The reason it matters is because at full torque (1000 Nm) and a 4.55 gear ratio, the 2 brakes are reacting a total of 3550 Nm of torque. The reaction torque in the ZF design has to pass through the outermost tube to the front of the transmission, i.e. approximately a 1.5 kg shaft assuming that the wall thickness is 3 mm, the length is approximately 350 mm and the inner diameter is around 180 mm. Now 1.5 kg does not sound like a lot, the problem is it is at a very large diameter, and rotational inertia is proportional to the square of the diameter. The rotating inertia of the ZF shaft is approximately 0.01 kg-m^2 while the GM design is around a tenth of that because it is at a much smaller diameter.

The input shaft in case of the GM design ends up being the outermost shaft, but the input shaft only carries engine torque while the outermost shaft in case of the ZF design has to carry 3.7 times the engine torque. Also, it appears that the length of the outer shaft in case of the GM design will be shorter than the ZF design.

The lower inertia of the GM design will shave a few precious milliseconds off the shift time, and in a world where the burgerkingring times are important, over a course of a lap the precious milliseconds can add up to a second. As the B&B we all know just how important the burgerkingring times are to market share. On a more serious note, this reduced inertia will also show up as a very small fraction of a mile per gallon for the EPA fuel economy.

Second gear

To shift up to second gear, both brakes A and B stay locked, clutch C is disengaged and E is engaged. Doing so connects the sun gear of gear set 4 to the ring gear of gear set 2. The sun gear of gear set 1 is grounded and the planetary carrier of gear set 2 is connected to the input. Therefore this sets up an overdrive cascaded with an underdrive. The ratio is

(2) 2nd   = (S4+R4)R2


S4(R2+S2)

 =  2.9586 

Knowing the gear ratio, and methods similar to the 1st gear estimation, a feasible and perhaps reasonable estimates for tooth counts is S2=42, R2=78 with an 18 tooth planetary pinion. Since S2+R2 = 120, it is possible to put in 3 or 4 pinions. Once again, the second gear operation is kinematically equivalent to the ZF 8HP. The earlier observations about overall transmission inertia being lower for the GM 8L90 still stand.

Third gear

The shift up to third gear is accomplished by releasing brake A, and engaging clutch E. Therefore shift elements B, C, and E are engaged. By doing so, the ring gear and the planetary carrier of gear set 2 turn at the same speed as the input, and therefore the sun gear of gear set 2 also turns at the input speed. Since this sun gear is linked to the sun gear of gear set 3, and the ring gear of gear set 3 is grounded by brake B, the carrier of gear set 3 is under driven with respect to the input. Therefore gear set 4 acts as a mixer module, with the sun gear rotating at the input speed, the ring gear turning slower than the input, forcing the carrier to turn at a speed that is slower than the input. The ratio is

(3) 3rd   = (S3+R3)(S4+R4)


S4R3+S3S4+S3R4

 =  2.0745  

Knowing the gear ratio, and methods similar to the 1st gear estimation, a feasible and perhaps reasonable estimates for tooth counts is S3=39, R3=77 with an 19 tooth planetary pinion. Since S3+R4 = 116, it is possible to put in  4 pinions. The operation of third gear is somewhat similar to the third gear of the ZF 8HP, this design uses gear sets 2, 3, and 4 while the ZF design uses gear sets 1, 2, and 4. Gear set 4 acts as a mixer module in both cases.

Fourth gear

The 4th gear up shift is achieved by releasing clutch C and engaging clutch D, i.e. shift elements B, D, and E are engaged. Engaging B and D at the same time means that the ring gear and sun gear of gear set 1 spin together with the sun gear of gear set 4. Since the planetary carriers of gear set 1 and 4 are linked together, all 3 members of gear sets 1 and 4 spin together at the output speed along with the ring gear of gear set 2 and the planetary carrier of gear set 3. Since the ring gear of gear set 3 is grounded, this causes the sun gears of gear sets 2 and 3 to be overdriven with respect to the output by a factor of approximately 3. This causes gear set 2 to act as a mixer module, with the planetary carrier as the input and sets up an underdrive gear. The ratio is

(4) 4th   =1 + S2R3


S3(R2+S2)

 =  1.6910 

The fourth gear power flow is significantly different from the ZF 8HP fourth gear power flow.

Fifth gear

Fifth gear is achieved by releasing clutch E and engaging clutch C, i.e. shift elements B, C, and D are engaged. The fifth gear power flow is quite challenging to understand, but I am going to give it the old college try. Since C and D are engaged, the following members turn at the input speed

  1. The ring gear of gear set 1
  2. The planetary carrier of gear set 2
  3. The sun gear of gear set 4

The planetary carriers for gear sets 1 and 4 are rigidly linked together and turn at the output speed. Ring gear of gear set 3 is grounded because brake B is engaged. This sets up the following kinematic states

  • The sun gears of gear sets 2 and 3 are rotating at 2.16 times the input speed
  • Since the ring gear of gear set 3 is grounded, the planetary carrier of gear set 3 is therefore spinning at 0.73 times the input speed
  • Since the ring gear of gear set 4 is connected to the planetary carrier of gear set 3, gear set 4 becomes a mixer module with the sun gear spinning at input speed, the ring gear spinning at 0.73 times the input speed, and the planetary carrier being the output.

The 5th gear ratio is therefore

(5) 5th   = S1S3(R2+S2)(R4+S4) + S3R2(R4R1-S4S1) + R3S2S1(S4+R4)


S1S3(R2+S2)(R4+S4) + S3R2(R4R1x-S4S1) + R3S2S1S4

 =  1.2682 

Knowing the gear ratios, it is possible to back calculate a feasible gear parameters for gear set 1. After a little bit of work, S1 = 39, and R1 = 77 with 19 teeth planetary pinions. Therefore gear sets 1 and 3 appear to be identical in terms of number of gear teeth.

The kinematic state of gear set 4 is very much the same as the kinematic state for gear set 4 of the ZF 8HP, but the way the kinematic state is achieved is different. All 4 gear sets are used to achieve this ratio, albeit in a different manner than the ZF design.

Sixth gear

Sixth  gear is achieved by releasing brake B and engaging clutch E, i.e. the 3 rotating clutches C, D, and E are all engaged but both brakes A and B are open. This means all members of all 4 gear sets turn at the same speed as the input. The ratio is therefore quite simply

(6) 6th   =  1.0000 

Seventh gear

Up shift to seventh gear is accomplished by releasing clutch E and engaging brake A. The engagement of brake A grounds the sun gear of gear set 2, while the engagement of clutches C and D connects the ring gear of gear set 1 to the input shaft. The ring gear of gear set 2 spins approximately 1.5 times faster than the input because the sun gear is grounded, the planetary carrier is the input, and the ring gear is the output. Therefore gear set 1 acts like a mixer module, with the sun gear rotating at approximately 1.5 times the input speed (due to the rigid connection to the ring gear of gear set 1), the ring gear turning at the input speed, and the planetary carrier being the output. The ratio is therefore decided by the ratios of gears sets 1 and 2 alone

(7) 7th   = R2(S1+R1)


R2(S1+R1)+S1S2

 =  0.8467 

Eight gear

Eight gear is achieved by disengaging clutch C and engaging clutch E, i.e. shift elements A, D, and E are engaged. Engaging clutches E and D at the same time causes all 3 members of gear set 1 to rotate at the same speed, and since the planetary carrier of gear set 1 is also the output shaft, this means that the ratio is decided strictly by the gear teeth count of gear set 2. The sun gear of gear set 2 is grounded, the planetary carrier is the input and the ring gear is the output. The ratio is therefore

(8) 8th   = R2


R2+S2

 =  0.6500 

Reverse gear

Reverse gear is achieved by locking both brakes A and B, and engaging clutch D. The sun gear of gears set 2 and the ring gear of gear set 4 are therefore grounded, the sun gear of gear set 1 spins at approximately 1.5 times the input speed (just as it does in 7th and 8th gears). The kinematic constraints imposed by the rigid link between the planetary carriers of gear sets 1 and 4, along with the actuation of clutch D which locks the ring gear of gear set 1 with the sun gear of gear set 4 causes the sun gear of gear set 4 to spin backwards at approximately 1.17 times the input speed, which means that the transmission output spins backwards but 3.908 times slower than the output

(Rev) Reverse  = R2(S1S4-R1R4)


S1S4(S2+R2)

 =  -3.9080 

What have we learned

Based on the available information, this article makes educated guesses at likely gear parameters for the GM 8L90. The likely gear parameters are

  1. Gear set 1: Sun gear S1 = 39, Ring gear R1 = 77
  2. Gear set 2: Sun gear S2 = 42, Ring gear R2 = 78
  3. Gear set 3: Sun gear S3 = 39, Ring gear R3 = 77
  4. Gear set 4: Sun gear S4 = 29, Ring gear R4 = 103

The gear ratio spacing is very good, the transmission feel of the GM 8L90 should be very competitive to the acclaimed ZF 8HP family of transmissions.

There are some obvious similarities between the gear arrangement of the GM 8L90 and the ZF 8HP but there are significant differences as well. These similarities and differences have been explained in this article. The one advantage of locating the clutches close to the hydraulic pump and the valve body is better shifting times since less fluid has to be moved in and out of the clutch pistons to apply and release the clutches. Also, this design is likely very competitive in terms of mass for a given torque capacity, and is better than the ZF 8HP design in terms of rotational inertia.

There are other advances made in this transmission design as well, especially with regards to the hydraulic pump design. This particular transmission features a “cylinder deactivation” of sorts for the pump, when line pressure demands are low (highway cruising) half of the pump can be shut down to achieve higher efficiency while still retaining the pump displacement required to deliver enough flow rate for fast shifts.

Conclusions

This is a very good design, hats off to the engineers at GM. The filing date on the patent is May 1, 2009 therefore props to the management at GM for letting this program move forward through the darkest days of their bankruptcy. This will be considered a seminal design in the history of automatic transmissions.

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42 Comments on “Saturation Dive: The GM 8L90 transmission...”


  • avatar
    tonycd

    Even though this is more detail than I personally have the time or knowledge for, I like that TTAC has it.

    I’m impressed that you’re impressed. It underscores what’s always been true of GM: They have a lot of engineering talent, they just don’t always set it loose from their MBAs’ choke collar.

  • avatar
    eggsalad

    “What have we learned”

    What I have learned is that automatic transmissions are too complex for me to wrap my brain around, so I’ll stick with manuals, still.

  • avatar
    Car Ramrod

    All that for the cost of a manual plus $1200. Sounds like a deal as long as nobody ever has to open it.

  • avatar
    Felis Concolor

    Having recently ordered a custom bicycle fitted with Rohloff’s 3 planetary 14 speed hub changer system I am enjoying these engineering breakdowns and explanations immensely. Thank you very much for taking the time to explain the inner workings in such detail!

    I look forward to this becoming a separate part number, the better to match it to a crate LS9 for a plug-and-play supercar horsepower boost, although the engine’s warranty requirements are going to play hell with most engine bay installations.

    • 0 avatar
      Syke

      I’ll be very curious about your experiences with it. That’s one of those pieces of kit I’ve love to design a bike around. However, finding a place that actually sells them is a prime case of “far and few between”.

      And I’d love to tear one down, despite my practical knowledge being currently limited to Sturmey-Archer five-speeds.

      • 0 avatar
        Felis Concolor

        On the wish list/build sheet, I also added one of Schlumpf’s Speed Drives for front shifter duties; just kick the crank-mounted button to switch the planetaries from direct drive to a 1.6 overdrive ratio. Sadly the recumbent’s frame is much longer than what Gates supplies its Carbondrive belts for, and I fear there isn’t enough support or interest in this particular community to raise enough money up front for them to make the molds necessary for a 2.7 meter belt plus extra for the necessary drive wheels.

        I’m thinking about making a post on the forums about it when the new ride arrives; there are clearly a significant number of pedal-power fans among the B&B here.

        Late last year I discovered a participating branch in the Inter-Library Loan (ILL) program which had a copy of the long out of print “Sturmey Archer Story” history book on its shelves. Sadly my request for a temporary loan was met with a $20 fee simply to send it out for a couple of weeks. I’d love to read that book, but at that price I’d like to have a hard copy of my own to keep around.

      • 0 avatar
        Mike N.

        Harris Cyclery is my go-to place for “eccentric” bike stuff like this:

        http://www.sheldonbrown.com/harris/rohloff-impressions.html

        They don’t currently stock Rohloff, but have similar:

        http://harriscyclery.net/product-list/parts-1400/?rb_ct=1403

  • avatar
    LeMansteve

    Kudos to the thorough technical analysis, but holy cow this is just another reason I’m glad I drive a manual transmission. 1 clutch, simple gear layout. When you need astronomy to describe the inside of your transmission, that’s probably a bad sign!

  • avatar
    gmichaelj

    Thanks again for a good write up and critical analysis.

  • avatar
    tresmonos

    auto trans are voodoo magic. I would love a write up from you regarding TCU’s, even though my current job doesn’t require any understanding from those damned interfaces with solenoids.

    Great read. This is great for people who deal with the vehicle on wheels at a macro level.

    • 0 avatar
      Timur Apakidze

      @tresmonos

      I will add it to my list of things to write about over the course of this year. TCUs these days are just as expensive (if not more expensive) to develop than the transmissions themselves.

      Hopefully some time over the summer I can get to it.

  • avatar
    LeadHead

    How does this design compare to the Aisin 8 speed?

    It should also be noted that Chrysler got the shift time down to 250ms for the 2015 V8 cars somehow.

    • 0 avatar
      goldtownpe

      Another excellent write up.

      Agreed. A comparison to the Aisin would be much appreciated.

      • 0 avatar
        Timur Apakidze

        The Aisin design is actually quite difficult to explain because that layout uses compound planetary gears, not the simple planetary gears of this one.

        If you guys absolutely want a write up, I can do one – it will be even more convoluted than this one.

        • 0 avatar
          mik101

          Please do. :)

        • 0 avatar

          > If you guys absolutely want a write up, I can do one – it will be even more convoluted than this one.

          Just a tip if this happens. The intent in these is good, but frankly the calc/details detracts from the pedagogical experience.

          For example, most readers are just interested in the main diffs between gear types, and a few insights into why one is used in a certain scenario vs another. Completeness is better for a reference/textbook than an interest piece.

          To expand on this, I could read descriptions of how a transmission works (this connected to that), but what’s really useful is context for *why* certain design choices were made. Why is planetary used in all passenger autos instead of auto-sequentials? Do they rev-match to some degree and how? The kind of stuff the mechanically inclined sometimes wonder about but lack the inside background to trivially reason out.

          • 0 avatar
            Timur Apakidze

            @u mad scientist

            To be absolutely honest I am still trying to get the hang of this whole writing thing. I do appreciate your feedback.

            My intent with the saturation dive series of articles is to be reference/textbook type articles. These details are not available elsewhere on the internet and my intent is to provide them to the audience here. To that end, I work very hard to make these articles as factual as I possible and I tend to leave my perspective out of them to a large extent.

            I will be doing a series of less detailed pieces on transmission design (for example why planetary gears) in the near future. So please be patient, give me a few more articles to find the right balance between detail and perspective.

          • 0 avatar
            GiddyHitch

            Stick with the textbook style for these deep (saturation) dives and always show your work! They are fantastic and really flesh out a technical aspect to TTAC that has never really been strong before. I hope to better comprehend more of this article with a few rereads and supplemental research, but that’s better than the car blog fluff that typically gets circulated.

          • 0 avatar

            > Stick with the textbook style for these deep (saturation) dives and always show your work!

            There’s the old joke that for a work with X equations and Y readers, X+1 correlates with Y/2.

            I suspect only transmission engineers will have the context to appreciate the math and those are even less common than those who found the joke funny.

  • avatar
    1998redwagon

    wow. if i had the time i could learn a ton from this article. as it is i clicked on it and read a bit and learned a bit too. fortunately my click counts as much as the click from a transmission geek. just love the detail on this site.

  • avatar
    carguy

    Nice write up.

    GM is making some bold claims about the shifting times of the new 8 speed. We will see soon enough.

  • avatar
    Scott_314

    Excellent. Editor please add to reviews for future reference!

    Can your next article be on a CVT?

    Cheers

  • avatar
    Pig_Iron

    Wait-what? When did transmission schematics start being called “stick diagrams”?

    Apparently I’ve been away too long.

  • avatar

    This is really past my knowledge & interest in transmissions, but glancing over the calcs:

    “approximately a 1.5 kg shaft ..and the inner diameter is around 180 mm. The rotating inertia of the ZF shaft is approximately 0.01 kg-m^2″

    I get:

    1.5*(.18^2)
    Ans = 0.0486

    In any case, I do wonder if there’s some formulaic way to design these things other than heuristics.

    • 0 avatar

      ^ actually my bad, that’s diameter not radius, duh.

      • 0 avatar
        Timur Apakidze

        Brownie points for actually checking the calculations ;)

        With an approximately 185 mm diameter, approximately 300 mm length, and a wild guess at a thickness of 3 mm, and assuming that the outer shaft of the ZF design is made out of aluminum, the weight is approximately 1.5 kg and inertia is approximately 0.01 kg-m^2.

        For reference, a 2.0 L 4 cylinder DOHC engine comes it at 0.16 kg-m^2 or thereabouts.

        And to answer your other questions – both ZF and GM probably have brute force automated methods (i.e. proprietary software) to crunch through thousands of these combinations and to narrow it down to a few dozen concepts for the engineers to evaluate.

  • avatar
    Rday

    Great but let’s hope that this transmission turns out to very reliable. With GM nothing can be taken for granted.

    • 0 avatar
      Zackman

      “With GM nothing can be taken for granted.”

      Ummm… you never heard of Ultradrive and its various incantations? That was Chrysler.

      • 0 avatar
        SC5door

        Until everything else fails around the car. You mention Ultradrive as a jab at Chrysler, yet totally blow off ignition switches, intake gaskets, DexCool, and 5.3L Oil consumption problems along with piston slap on GM’s. Oh and least we forget the grenading transmissions on the LS4 powered cars.

    • 0 avatar
      MBella

      One thing that I don’t think GM has had a problem with over the years was automatic transmissions. They even sold a whole bunch to BMW over the years.

      • 0 avatar
        CJinSD

        Our ’94 325is received its 2nd GM automatic at 30,000 miles and its 3rd at 60,000 miles. We sold it with 92K. Those transmissions had ‘life-time’ fluid and weren’t set up for easy service. Turns out that 30,000 miles was the lifetime of the fluid, just like in any other transmission. The difference was the life span of the transmissions.

  • avatar
    Zackman

    Sheesh… I still can’t figure out how a RWD differential works!

    • 0 avatar
      tankinbeans

      Seconded. I almost kind of understand how a manual transmission works and can almost follow it, but this is a little – well a lot – above my knowledge.

      I know others with the knowledge and background will understand this and be able to piece it together in their meathunk, but I’m just going to finish my response by saying TL:DR.

      I read some, but it went over my head.

  • avatar
    johnny_5.0

    Dear GM,

    Please spread this goodness throughout your lineup much more quickly than the glacial pace at which Chrysler has been spreading the ZF 8HP.

    Thanks,

    Everyone

    PS: I still remember reading reviews of 2011 MY Challengers/Chargers with words like ‘the 8 speed auto is likely to come for model year 2012′. Four years later is better than never I suppose.

    • 0 avatar
      redav

      Does anyone know the plan of where they’ll implement this transmission? I’ve heard the Corvette & Camaro, and likely trucks. I assume it will find its way into Cadillacs. I don’t think I’ve heard anything about timetables, though.

      What types of variations would it be feasible to make? Do they ever release lightweight or heavy-duty versions of transmissions for different applications? What type of limitations might there be?

  • avatar
    lozz

    Dear E-I-C p.t.,

    More articles of this in-depth quality, please. Also, ones on other aspects of modern vehicle mechanical design.(suspension?)

    Like many others I find that much of this is way over my head, but when you finish reading a few small light-bulbs are starting to spark up in your brain.

    That’s what it’s all about.

    I second the calls for an in-depth account of CVT.

    I have avidly read your own in-depth descriptions of race design and practice, also, and would welcome more of the same.

  • avatar

    light and casual reading when compared to GM’s incentive matrix.

  • avatar
    DC Bruce

    Heavy going for the technically illiterate (like I am ), but worthwhile. Thanks for the effort & keep more coming.

    Eventually, even English majors might learn something.

  • avatar
    vettezl1

    Lexus LFA tranny is the way to go. Add torque converter for Caddys and girly Vettes.

  • avatar
    Jun K

    V good article. I especially liked the logical deduction of number of teeth on the planetary gearsets. One suggestion to the author, when discussing gear ratios, it would be good if you could mark in red the powerflow from engine to output shaft. The power flow would be easy to visualize for the rapid reader.
    Regarding 8l90 itself I think there are going to be times during shifts where there would be sudden loss in power at wheels. ex gear 1 to gear 2 shift. Imagine at low temps when fluid from C hasnt yet drained out and E is engaged, I think planetary gear set 2 would get locked and for a fraction of sec the driver would feel loss in power. Havent checked the other gear sets in detail, but just looking at the layout, am sure shift calibration is going to be really really tricky as number of planetary gear sets increase.


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