GM's 8L90

[supton]

Ward's Auto

Quote:

The 8L90 transmission matches the Corvette’s outgoing 6L80 6-speed unit at 24 ins. (598 mm) long but drops 9 lbs. (4 kg), using advanced lightweight materials such as magnesium and aluminum on a number of parts. Coupled with a unique design, which in an industry-first includes an off-axis oil pump, the work resulted in 24 new patents. Working in tandem with cylinder deactivation, a feature GM calls Active Fuel Management, the 8-speed provides a 3.5% bump in Corvette fuel economy to as much as 29 mpg (8.1 L/100 km) highway, according to EPA estimates. It’s rated at 16 mpg city (14.7 L/100 km). GM needs a transmission with a torque converter to wring out all the benefit of AFM, another reason to avoid a DCT, Juechter says. “That lets us be more aggressive in AFM, and that’s very important because AFM is the single biggest card we can play in terms of fuel economy.”

Needs a torque convertor for for the AFM? I don't get that. Is that because they plan to run the convertor unlocked while using cylinder deactivation? At least under loading that is (instead of downshifting and/or activating more cylinders, bump engine speed slightly by unlocking the convertor). I tend to think of convertor slippage as simply waste, so that strikes me as counter-productive. Or is it to dampen vibration--perhaps at heavier loading the V4 shakes more, and by using the extra mass of the convertor (and perhaps some slippage too) they can dampen it out. ?

 

[javacontour]

Maybe there is enough torque multiplication available so they don't have to activate more cylinders? The saved fuel may be more than the energy lost to waste heat?

 

[dparm]

I'm not sure I understand either. A dual-clutch typically has lower drivetrain losses than a torque converter.

 

[dparm]

Article also says: Juechter also says the Corvette demands certain packaging requirements and the GM 8-speed marries up better to the automaker’s cylinder-deactivation technology, a key fuel-economy feature on THE small-block engines that shuts down half the cylinders during low-load operating conditions. “There’s a very constrained package where the transmission has to live,” he adds. “It is a unique spec.”

 

[Hokiefyd]

Recall that torque converter lockup isn't binary (either on or off), and hasn't been for over a decade (depending on application). Lockup can be 0%, 100%, or quite literally anywhere in between. I suspect that they are tuning certain drivetrain harmonics from the AFM with partial converter lockup. To my knowledge, this wouldn't be possible with a dual clutch transmission, as you can't intermittently "slip" a dual clutch system (or at least not without a lot of wear).

 

[TiredTrucker]

Yet another reason that I would disable the AFM like I did in my 2013 5.3L. I paid for 8 cylinders, and I expect them all to pack the gear. No slouchers. No smoke breaks by cylinders while the others work. Everyone has to work from the time the ignition is turned on till turned off.

 

[supton]

Originally Posted By: Hokiefyd

Recall that torque converter lockup isn't binary (either on or off), and hasn't been for over a decade (depending on application). Lockup can be 0%, 100%, or quite literally anywhere in between. I suspect that they are tuning certain drivetrain harmonics from the AFM with partial converter lockup. To my knowledge, this wouldn't be possible with a dual clutch transmission, as you can't intermittently "slip" a dual clutch system (or at least not without a lot of wear).

I've read of that, and honestly it scares me enough that I want to trade out of my truck at some point. In a manual Trans that would kill a clutch in short order. Using partial lockup to reduce driveline shock, sure; but intentional slip for long periods of time?

 

[Hokiefyd]

Originally Posted By: supton

I've read of that, and honestly it scares me enough that I want to trade out of my truck at some point. In a manual Trans that would kill a clutch in short order. Using partial lockup to reduce driveline shock, sure; but intentional slip for long periods of time?

Well, again, I think that's why they're using a torque converter rather than a dual clutch. A torque converter uses a fluid coupling rather than mechanical friction materials, so a torque converter can sustain a variable rate of lockup. A lot of heat can be generated (transferred to the fluid), but a torque converter doesn't really "wear" like a traditional friction clutch would. In a sense, the "wear" is transferred to the fluid in terms of heat instead of wearing friction material in a conventional clutch. Your truck (and both of my cars, and most automatics) use intentional slip for sustained periods of time already, probably without the user knowing about it. They do this at engine speeds that are low enough where full lockup would put them below their power band, but fully unlocked operation would be wasteful. And I'm not saying that GM would necessarily be running the torque converter at less than 100% of the time, but it could be "slipped" during AFM transitions where a dual clutched setup could not.

 

[440Magnum]

Originally Posted By: supton

Needs a torque convertor for for the AFM? I don't get that. Is that because they plan to run the convertor unlocked while using cylinder deactivation?

My SWAG is that they want the extra parameter (percentage lockup) to use during AFM activation/deactivation in order to smooth out the transition and improve the all-important "customer satisfaction." Lockup schedules are already so complex that you can barely feel a modern automatic shift (or even SEE it on the tach!) during casual driving anymore. Let alone tell whether that drop on the tach was due to a gear change or due to lockup. Its all very complicated now since so many sensors and controls are available, and it really does improve the life of components while at the same time increasing efficiency and performance. I think the dual-dry-clutch transmission is a flash-in-the-pan in the long-term history of cars. It makes nice press, but in the real world it just doesn't provide any advantage and has a lot of downside.

Originally Posted By: Hokiefyd

Recall that torque converter lockup isn't binary (either on or off), and hasn't been for over a decade (depending on application).

1989 was the first mass-market application, to be exact. Not exactly a home-run right out of the box- it was the infamously flaky Chrysler A604 Ultradrive (front-drive). But it had no conventional valve body at all, computer-controlled incremental lock-up, and was the forerunner of every planetary/torque-convertor based automatic found in cars today.

 

[supton]

Originally Posted By: Hokiefyd

Originally Posted By: supton

I've read of that, and honestly it scares me enough that I want to trade out of my truck at some point. In a manual Trans that would kill a clutch in short order. Using partial lockup to reduce driveline shock, sure; but intentional slip for long periods of time?

Well, again, I think that's why they're using a torque converter rather than a dual clutch. A torque converter uses a fluid coupling rather than mechanical friction materials, so a torque converter can sustain a variable rate of lockup. A lot of heat can be generated (transferred to the fluid), but a torque converter doesn't really "wear" like a traditional friction clutch would. In a sense, the "wear" is transferred to the fluid in terms of heat instead of wearing friction material in a conventional clutch. Your truck (and both of my cars, and most automatics) use intentional slip for sustained periods of time already, probably without the user knowing about it. They do this at engine speeds that are low enough where full lockup would put them below their power band, but fully unlocked operation would be wasteful. And I'm not saying that GM would necessarily be running the torque converter at less than 100% of the time, but it could be "slipped" during AFM transitions where a dual clutched setup could not.

But heat buildup is still wasted energy. Maybe it is less loss than a downshift, I guess that is possible. Just strikes me as wasteful. Even if the trans fluid can take that. But I fear the wear on the clutch itself. Lockup off, the "wear" is to the fluid. On, and no wear. Between, and now the clutch is getting eaten up.

 

[Win]

I didn't care much for the AFM on the 6.0L GM V8 back in '08. I could definitely feel it going in and out of AFM, and the engine had a different, somewhat undesirable, feel on AFM. Six years is a long time ago - it would be interesting to drive a modern one and see if it has improved since then.

 

[splinter]

Originally Posted By: Win

..I could definitely feel it going in and out of AFM, and the engine had a different, somewhat undesirable, feel on AFM..

+1 In GM’s LC9/LMG AFM technical summary, it originally claimed the transition from and to V8-V4 mode takes less than 20 milliseconds and “is indiscernible.” Revised releases state the transition “is virtually indiscernible to most drivers.” Feels like a jake brake when it’s engaged in spite of unlocking the 6L80 converter. Nevertheless, once its initial hardware/software issues are ironed out in the field the 8L90 will likely be prove a fine transmission.

 

[SteveSRT8]

Originally Posted By: Hokiefyd

Recall that torque converter lockup isn't binary (either on or off), and hasn't been for over a decade (depending on application). Lockup can be 0%, 100%, or quite literally anywhere in between. I suspect that they are tuning certain drivetrain harmonics from the AFM with partial converter lockup. To my knowledge, this wouldn't be possible with a dual clutch transmission, as you can't intermittently "slip" a dual clutch system (or at least not without a lot of wear).

Spot on. research will tell you that GM has a Patented algorithm that locks and unlocks the TC randomly to even out driveline pulsations. Note also that the reason a slushbox cannot equal a DCT unit in driveline loss is because of the pump drag, not the TC, as the TC is near perfect in lockup as far as transmission of power without waste. 7 or 8 speeds, either way the new Vette just keeps impressing me for the money...

 

[SteveSRT8]

Originally Posted By: splinter

In GM’s LC9/LMG AFM technical summary, it originally claimed the transition from and to V8-V4 mode takes less than 20 milliseconds and “is indiscernible.” Revised releases state the transition “is virtually indiscernible to most drivers.” Feels like a jake brake when it’s engaged in spite of unlocking the 6L80 converter. Nevertheless, once its initial hardware/software issues are ironed out in the field the 8L90 will likely be prove a fine transmission.

The newest fleet van we have is a 2013 Express 3500 6.0 6L90E truck. While the transition to AFM is truly not noticeable, the throttle response gives it away. As you might imagine the throttle programming is drastically changed during 4 cylinder operation. 35k miles and running flawlessly, no oil consumption, nothing...

 

[dailydriver]

Despite how good the AFM systems have become, I still feel it has NO PLACE in a performance car application, save for that it was all but mandated by ever climbing CAFE standards. It is the very first thing I would 'write out' of the ECM on ANY C7 with tuning, IF the EPA/CAFE/CARB/etc. laws have not made that impossible to do. Is even the ZR1 going to have this system mandatory??

 

[supton]

Should just make it a switch on the dash, "ECO mode" or the like.

 

[Hokiefyd]

Originally Posted By: supton

But heat buildup is still wasted energy.

This is certainly true. Energy is consumed in deference to passenger comfort. There have always been trade-offs here, and likely will be for long into the future.

Originally Posted By: supton

But I fear the wear on the clutch itself. Lockup off, the "wear" is to the fluid. On, and no wear. Between, and now the clutch is getting eaten up.

The loads here are quite small, though, and certainly different than slipping a dry clutch. In practice, lock-up mechanisms on modern torque converters just don't seem to wear out.

 

[supton]

I guess so... In my feeble mind, it's wear, and I want to "know" that my trans will make 150k easily. [Never bought this truck to go to 200k+, but sometimes I wonder if I could, hence my paranoid fear.] I guess I'll have to think about that some more. I've been forcing a downshift whenever I notice the convertor unlock, so as to force it to lock up. And run cooler. And avoid the "is it partially locked and thus wearing" fear in my mind. BUT everytime I do that, some clutch/brake/whatever wears instead. If the convertor clutch can take the wear better than the other parts, well, then I guess it's a matter of balancing wear and tear.

 

[Hokiefyd]

Originally Posted By: supton

I guess I'll have to think about that some more. I've been forcing a downshift whenever I notice the convertor unlock, so as to force it to lock up. And run cooler. And avoid the "is it partially locked and thus wearing" fear in my mind. BUT everytime I do that, some clutch/brake/whatever wears instead. If the convertor clutch can take the wear better than the other parts, well, then I guess it's a matter of balancing wear and tear.

Again, I don't think I've ever heard of a modern torque converter in a "consumer grade" application like this wearing out due to partial lock. And if it's unlocking due to load at speed (as your situation suggests), I doubt that there's ANY partial lock. It's probably fully unlocked. The only time my cars partially lock the converter is under light load. And you can certainly feel it in the car (and see it on the tach) when the software gives up and just goes full unlock. It'll cruise around, for example, all day in 5th at 45 mph, at partial lock. It won't fully lock in 5th until 50 mph. But it'll upshift into 5th as early as 40 mph given a light throttle. So there's 10 mph there where the converter is in partial lock and it'll keep the engine speed at around 1500 rpm just cruising along. Slight ups and downs will change the engine speed some, but it tries to maintain about 1500 rpm. If I go to climb a hill, the engine speed might rise to around 1700-1800 rpm. It'll do that for about 3-5 seconds, but if the situation doesn't improve, it goes full unlock and the engine speed will flare to 2200 rpm or so. Similar deal in 3rd gear. The O/D off button actually locks the converter out in 3rd. So I can cruise through the neighborhood, on level ground, at 25 mph in 3rd, and engine speed is around 1300 rpm. It's not at full lock, because engine speed will vary some, but at partial lock. But if I hit the O/D off button while under a slight load, the engine speed will flare to about 1700 rpm. No lock anymore. (It'll also fall down to idle off throttle, with no fuel cut, if I have O/D off. If I leave O/D on and stay in 3rd gear, it will keep the converter in partial lock, cut fuel, and use engine braking until I slow down far enough to where it needs to reapply fuel and unlock the converter to keep the engine at idle. ECM programming is fascinating to me.)

 

[supton]

I know for my trans it won't lock up until 4th (out of six, and 4 is direct drive). Can definately tell, as an amount of throttle/load will raise rpm; really doesn't like pulling below 2k, not unless it achieves lockup. I think I have a really loose convertor in this truck (although at nearly 6,000lb it's probably the nature of the beast). Programming is, err, interesting. A while ago I had this happen: on a hill, in 4/locked, and I wonder if it would go 5. I clicked up, and the convertor unlocked, and... no shift. Clicked down to 4, and it locked up. I repeated this twice more. I can't tell if this is a sign of a problem to come or poor progamming. I've noticed it before, where if I hold 4th longer than it wants, it will unlock and be real lazy shifting about going into 5th. I dunno, it's a truck, I'm fine with clunking into gear, and clunk in/out of lockup.