Ramapo

I can't find any documentation on A8 rev matching -- all the talk seems to be about M7 rev matching, but...

I have an A8, and the rev matching is there, and I like it, even a lot, but...

Is the matching adjustable? The higher gears don't much matter, actually the rev matching isn't even noticeable, but 4th to 3rd is perfect, as is 2nd to 1st. 3rd to 2nd revs a bit too high. Is there a fix?

Bill Dearborn

When you say it revs too high do you mean it causes a forward surge against the brakes as the transmission downshifts? If it isn't doing that then it isn't revving too high.

Bill

z51rox

I have never heard of a rev matching automatic transmission that is how the torque convertor operates.

Higgs Boson

it's not technically rev matching but what you are talking about with an auto has the same effect, and that is downshifting with 0% pedal input in a way that leaves the RPMs higher in the next gear primed for reacceleration. that's the same thing you do when you rev match/heel toe with a stick, you just have to work the pedals to do so.

otherwise, with 0 pedal and normal decel it just softly goes into the next lower gear around 1000 RPM as you coast/slow to a stop. if you put it in sport or track mode the trans will downshift sooner, maybe around 2000 for example, to keep the revs higher.

this isnt technically rev matching, but rather just a more aggressive/earlier downshift.

edit: the effect also depends on how hard you are accelerating, how fast you come off the throttle, how much brakes you are using, etc so it can determine what your intentions are. It's possible the 3-2 shift occurred during a slightly different driver characteristic exhibition and thereby performed differently (in other words, user error).

Ramapo

If I'm in fourth gear coming to a stop with my foot completely off the pedal and downshift to third, the engine will rev BEFORE it hits third, and it does it perfectly. Then, when I go down to second, it functions the same way, but since it revs a bit too high, yes, there is a slight forward surge, so it's not seamless like the 4-3 shift was. The shift from 2nd to 1st is absolutely seamless, sounding like a perfectly executed 2nd to 1st downshift would with a manual transmission. My only extra revving is between 3rd and 2nd.

Remember, my foot is off the gas for everything described above, and the only purpose is for engine braking when coming to a stop.

Higgs Boson

It's not reving "before" it hits third. If it reved up before it downshifted then you would accelerate since this is an automatic.

I think you may not be accustomed to how the new transmissions operate.

snow

Funny I have driven mine hard and I have never noticed it rev up on down shifts. My AMG does it very loud and cant be missed, it even pops and back fires. I better pay better attention LOL!!!!!!!!!!

Bill Dearborn

He is saying it surges on the 3-2 downshift which would indicate it is revving higher in relationship to the lower gear than the other two downshifts.

Bill

Bill Dearborn

There is only one thing wrong with what is happening and that is that you are using the engine to brake the car. Just not something that is done. If you want to bring the car to a stop use the brakes if you want to slow down a little then take your foot off the throttle. Don't use the transmission/engine to slow the car. A number of things an go wrong when you do that and some of them are very expensive including breaking something in the drivetrain, spinning the car and hitting something plus breaking something in the drivetrain, etc. Sort of like a forward pass in football where there are three things that can happen and two of them are bad. The problem with your scenario there are a number of bad things that can happen and not a single good thing.

Bill

NSC5

Agree with Bill. The only time I downshift my A8 for braking is on mountain grades where it is used more to prevent an increase in speed rather than to slow the vehicle.

Both GMC Duramax diesel/Allison automatic pickups I have owned do brake fairly aggressively via commanded transmission downshifts when tow/haul mode is selected however that behavior is immediately stopped if the rear end loses traction. And the powertrain in those vehicles was designed for this type of usage in heavy towing situations which is not the primary intent with car powertrain setups and programming.

I drive my Z06 in manual mode most of the time but even then I generally let the transmission select the lower ranges automatically as I slow since it does so very smoothly.

falconhulk

Modern auto's downshift and engine brake on declines anyway...AFTER you hit the brakes. I live on a mountain my cars (not the Corvette) do this everyday when the brakes are tapped going downhill with slow people in front. It does not shift up again until the incline has leveled out. That's a lot more stress then slowing to a stop for a light.

Bhandy

mine does the same and the shift speed while in the various modes also determines how fast it rev matches while in paddle mode. In sport and driving freaking hard while manual shifting the auto kicks in the "performance shift" program which keeps the revs higher ever when down shifting. I find it funny how non a8 owners comment on how my 2016 z51 a8 operates. I will say this, the rev match is super fast when that "performance shift" program kicks in but you have to be giving it the beans to maintain that program. PTM modes all rev match, all modes.

Higgs Boson

who's the non-A8 owner in the thread?

RocksZC7

When the A8 is driven in 'Manual' mode, the transmission does perform rev matching when down shifting. It is not the torque converter merely rev matching but the engine actively increasing rpm to match the rpm of the lower gear selected before the actual shift. Depending on the speed and deceleration rate when you down shift, the rev matching may slightly over rev' which will momemtarily change the deceleration rate. This effect is very fast, a fraction of a second. I have over 20k miles on my 2016 Z which is my daily driver. This rev matching feature in manual mode is a kick to drive!

davepl

I don't know much about transmissions but I used to work for Bill Gates and he has almost as much money as some automatic transmisssion shop owners, so I know a little.

There's a "sprag" on the torque converter that is a one-way clutch - when the engine is faster than the transmission, that's fine - the torque converter manages that relationship. But when the transmission is faster than the engine, it drags the engine along.

It's required for engine braking, as otherwise when you were going down a hill the motor would happily return to idle and sit there at 600rpm as you coasted. It'd be weird, so they put that sprag in there to lock the engine to the trans in that scenario.

No active rev matching or anything fancy that I know of, but there's no -reason- they couldn't blip the throttle on downshifts for example - I've just never noticed it.

Higgs Boson

yes, it does blip the throttle but it does it between/in the middle of the shift. however, it is heavily dependent on a correct torque model and even an intake can throw it off and cause it to be a clunky process.

Ramapo

I think I'm somehow not making myself clear here. Any automatic will rev up when downshifting, without gas, just by hitting a lower gear. My A8 revs during the downshift just enough to make a smooth transition. My only (minor) complaint is that the 3-2 downshift isn't seamless -- it revs a bit too high.

And no, I'm not accustomed to how the A8 transmission operates (although I've had my 2015 Z51 3LT A8 for two years and 12,000 miles now) -- all my previous Corvettes were manuals. I also have never driven a paddle shift before -- on any car. I'm thinking that this rev matching thing is unique to Corvettes, but that's why I'm on here looking for advice.

I am being told not to use the paddle to downshift. In drive mode, the paddles cannot be used to upshift, as the automatic will always choose the highest acceptable gear. Does that mean I shouldn't use the paddles at all in drive mode? There's certainly no mention of that in my owner's manual.

Hey, I'm just looking for some advice here. I haven't discussed this with Chevrolet yet -- as I say, it's a minor complaint.

767guy

OP, if your are talking about coasting to a stop with the shift selector in D ( Drive) I think what you are seeing is the A8 downshifting from 5-4, 4-3 with the coincident RPM rise at the downshift point. However, as the car is in its last few MPH just before stopping the downshift is now 3-1, bypassing 2, which will produce a slightly different feel than the previous down shifts. Normal.

Perhaps one of our CF transmission experts will catch your thread and offer some insight.

Bill Dearborn

It doesn't matter whether you have a manual transmission or an automatic transmission you don't use the transmission to slow the car down. The reason being is a sudden downshift with either transmission can break both rear wheels loose. If you don't rev match before the next lower gear when the transmission is through with its shift (either by a driver letting out the clutch or hitting a paddle on an automatic) the engine may not be going fast enough for the speed of the rear wheels. As soon as the gear is engaged something has to give. A lot of times it is the drive train which suddenly speeds up to match the speed of the wheels and if the speeds are high when the downshift is initiated it is the rear wheels giving up the traction they have since the drive train inertia will not let the drive train speed up fast enough. Both scenarios put a lot of stress on the drive train whild the second can put a lot of stress on everything else including the occupants.

Using the transmission to maintain downhill speeds or to reduce downhill speeds is done after the initial downshift or sequence of downshifts is done properly. When I am towing the Vette behind the Tahoe and come to a steep down hill section I will downshift from overdrive to 3rd while I have my foot on the brake. That downshift isn't very abrupt but if I need to drop to 2nd that is a large difference in engine rpm and can really jerk the vehicle. I try to add throttle just as I pull the shifter down and then go back to the brake.

Bill

falconhulk

Again, newer transmissions, including GMC and Chevy, have automatic grade downshifting. It's activated by braking on an incline. No excessive wear occurs.