davepl

If you go to the C7 general forum, the A8 has a bad rep. They've got stickies tracking failures, lots of people with bad experiences, lots of pouting.

But over here in Z land all is quiet about the A8... seemingly indestructible.

Is it just the torque converter that's different, or did they beef ours up in some other way too?

Curious as to why we seem to be OK but they have so many problems.

Wulverine

A bad Tune will kill a transmission no matter how "indestructible" it seems.

davepl

^^^^ No doubt, but are you implying that the Stingray guys get bad tunes at a higher rate than the Z guys, or ???

Always Red Dave

I think what I have read is that the A8 in the stingray allows the Tranny to go a lot more into V4 mode and that's the problem with the torque converter problems.while the A8 Z06 does not go into V4 nearly as much. I had a 2014 A6 Z51 and that tranny was flawless. My wife drives a BMW X3 and you can turn off the V4 mode for good too bad the C7 doesn't have that feature BUT I BET THE C8 WILL HAVE IT WHEN GM BUILDS THAT CORVETTE!

23/C8Z

Dave, I think it's just percentages. there have been failures. i think the pulsating TC issue to quell perceived V4-V8 transition vibrations is causing all the "bad press" regarding the A8..

i believe it is just an inherent design flaw by the engineers which for some reason whether it was: material, construction, final cost penny pinching, or God Forbid, a lack of observation or communication from the beta driver's, etc when the C7 A8 was being tested..

Or they felt it, knew it,but thought it was so imperceptible like a low frequency resonance only present under the "perfect storm" situation, and proceeded with production.

I lean towards the last one, and that would explain why NOW they continue to revise the TC with limited success...

if you think the shudder in some A8's in the C7 are troublesome, look into the vibrations the current gen pickups are having lol... 400k year built, i've driven and road in more than a dozen, owning a from new 16' Sierra SLT A8, and it's so distracting at any speed above 70mph i never take it far.. it's enough to drive you mad. it's a Lease, has been "fixed" twice already and still exhibits the vibration, which is an oscillation, rhythmic very similar to the C7 complaints.

charles7970

I wonder if driving in sport mode would prevent this issue? I don't think the car goes into V4 while in sport or Track mode

NSC5

With the A8 it goes into V4 mode in ANY driver mode. Without a "tune" or a Range module (or equivalent) the only way to avoid V4 mode is to leave the A8 set to manual mode.

A Z06 doesn't go into V4 mode nearly as frequently as a regular Stingray (different LT4 power curve with its lower compression ratio compared to the LT1) and more aero and tire drag mean that the Z06 is in V4 mode under a far narrower range of conditions.

davepl

The automatic will go into V4 in any driving mode.

People have postulated the the LT1 goes into V4 more than the LT4, as mentioned a couple of posts above. But why would that be? You'd think the LT4 would make more torque, and therefore, be able to "hold" V4 mode longer than the LT1. Apparently that's not the case, so it's counter-intuitive for me.

Anyone know why the LT1 would stay in V4 more? I see tire drag and aero, but is there more to it?

NSC5

The LT4 certainly makes more torque under boost but that only happens when there is enough load to stay out of V4 mode anyway. Otherwise it is at a disadvantage due to its lower compression ratio (needed of course since it is designed to operate above atmospheric at high throttle.

V4 mode is only capable of saving some fuel because it slightly reduces the "pumping" losses associated with pulling intake air past a mostly closed throttle plate. Running in V4 mode more closely approximates what you would have with an engine only slightly larger than needed to maintain cruising speed but you still have the friction and reciprocating mass of the 4 "dead" cylinders so V4 mode certainly doesn't provide the same economy you would get from a 3.1L four cylinder replacing the 6.2 but you would have a hell of an economy car.

At one point in the 70s French maker Citreon sold a small very low powered car that was operated at near full throttle but reasonable RPM for highway cruising and it returned excellent mileage for the time. By operating at near full throttle it avoided the pumping loss of trying to pull air past a partially closed throttle plate. I also understand if you needed to merge onto a highway with this car it was best to make merge reservations at least a week in advance. One of my college frat brothers had one while they were living in France for the summer and he wasn't impressed (of course he wasn't the one having to pay for fuel or he might have been).

onfire

Less V4 intrusion and MUCH better cooling for the ZO6 trans. Very apparent at the Corvette plant tour.

Kyflyer

It may be the laws of scale, there are a lot more stingrays and grand sports built than Z06s. although I had an early stingray with the A6 and put over 30,000 miles on it and I did not have any issues with it. The only time I used V4 mode deliberately was on long cruises on the interstates.

C7/Z06 Man

Before the A8 came out for the C7 I read something somewhere about the A8 for the Z06 having different clutch packs to handle the additional HP, calibrations, etc. Anyone else have anything on this or no?

JoesC5

My friend's 7,000 mile 2015 Z06 dumped his A8 and the transmission had to be replaced. I really doubt that his is the "only" A8 that has problems in the Z06, but he was there to take a photo when the tech dropped the transmission's pan. I was going to post the photo, but I can't get photobucket to work today. I have posted the photo before.

He isn't on this forum, as are a lot of C7 owners, so don't believe that what you see on the forum represents the real world out there.

One thing that could contribute to the greater amount of "reported" Stingray A8 problems on this forum is that most A8's are installed in NON-Z06 cars, thus there are fewer Z06's with A8's. I'm not going to look up the numbers, but it wouldn't surprise me to see that four times the number of StingRays are sold vs the Z06 with A8's. That means with the same percentage of failures in the Z06 as the StingRay, we would see four time the number of Stingray A8 problems vs the Z06.

glass slipper

You remembered correctly:

http://gmauthority.com/blog/2014/04/...speed-gearbox/

davepl

That article also calls the 8L90E a "single clutch" transmission. That's either a misnomer or I misunderstand clutch to clutch shifting.

rbartick

Numerous A8 failures have been reported in this section of the forum. For example, here is a thread from last week.

Jsilvester81

my 15 Yukon Denali is in the shop for a failed TC

NSC5

gmauthority is a nice website name but it doesn't always guarantee facts.

But I believe what the author was trying to do in this case was distinguish it from the dual clutch transmissions. But to be correct multi or many clutch would be far closer to correct for these clutch to clutch architecture transmissions as opposed to "single clutch".

The majority of the A8 failures in the C7 general area are the shudder due to torque converter. Of course there are other mechanical failures of the 8L90 like there are with any transmission but these are a very small number and nothing unusual for any transmission (or other major powertrain component); the outlier is the torque converter clutch and I believe that blame can be placed primarily on the V4 mode NVH reduction strategy. Or succinctly in this case the treatment is far worse than the condition since most drivers would prefer a very slight increase in NVH compared to the horrific shudder followed by repair.

gve

I wonder why GM didn't delete the V4 mode on the ZO6 or at least make it optional with a switch, we are already paying the guzzler tax so what's the point of going to V4.

NSC5

I expect it is tied up in CAFE. There are actually three sets of calculations for each vehicle. One provides the EPA "sticker" mileage estimate, a second determines whether or not there will be a gas guzzler tax and how much, and the third is used for CAFE. Each has different requirements and assumptions.