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How do DCTs work?

Old 12-01-2018, 09:28 AM
  #21  
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Originally Posted by Tom73 View Post
So if you are paddle shifting you have to go through all the gears when shifting up or down? In other words you cannot downshift from 8th to 4th without going through 7th and 6th. Or the other, skip from 2nd to 8th without going through the in between gears.
That is correct. It is not a manual transmission and can't skip gears.
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Old 12-01-2018, 10:15 AM
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Originally Posted by Speednet View Post
That is correct. It is not a manual transmission and can't skip gears.
The DCT that Mercedes uses can skip gears.

"[COLOR=left=#333333]Engineers at Daimler’s Stuttgart-Hedelfingen plant – where the new transmission is being manufactured – assert that the multiple downshift function of the 7G-DCT provides faster bursts of speed on tap; whenever necessary, the electronic control system skips individual gears, instead of shifting down through each gear in succession."

Also VW/Audi

"[/COLOR][COLOR=left=#222222]Under "normal", progressive and linear [/COLOR]acceleration[COLOR=left=#222222] and deceleration, the DSG shifts in a [/COLOR]sequential[COLOR=left=#222222] manner; i.e., under acceleration: 1st → 2nd → 3rd → 4th → 5th → 6th, and the same sequence reversed for deceleration. However, the DSG can also skip the normal sequential method, by missing gears, and shift two or more gears."[/COLOR]


Last edited by JoesC5; 12-01-2018 at 04:11 PM.
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Old 12-01-2018, 10:17 AM
  #23  
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Don't be lazy, why wouldn't you just Google this or search youtube instead of cluttering this forum:

https://www.youtube.com/results?sear...=how+dct+works
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Old 12-01-2018, 12:22 PM
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Originally Posted by elegant
Tom73,

I sent you a PM with lots of links and info.
John,
I know that you've done a really nice job of curating some great information on the other site. Thanks for doing that and for sharing!
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Old 12-01-2018, 06:01 PM
  #25  
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I don't care how it works as long as it does work AND I don't have to shift gears
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Old 12-02-2018, 01:43 AM
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An automatic transmission. It's not a manual.
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Old 12-02-2018, 11:24 AM
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I daily drive a Mercedes with a 7 speed DCT. As far as real-world feedback with that trans, it feels like a manual, especially at low speeds. When at a stop and taking your foot off the brake to move, you can feel a lag as the clutch is feathered and sometimes a small amount of ‘lurchiness’. When you are moving and take your foot off the gas to coast, you feel engine breaking just like a manual. At some point around 5 mph you can feel the trans disengage from the motor fully to come to a stop. Otherwise during normal driving it just peacefully cycles through the gears without any drama. It took a little getting used to this trans since my previous dd was an automatic, but I do like this trans over any automatic I’ve driven.

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Old 12-02-2018, 02:31 PM
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Originally Posted by Speednet View Post
That is correct. It is not a manual transmission and can't skip gears.
Incorrect. A DCT can and does skip gears, for instance in kickdown.
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Old 12-02-2018, 02:57 PM
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Originally Posted by Speednet View Post
Just ignore anyone who says it's a manual transmission in any regard. It is not. A DCT is an automatic transmission that is able to shift gears very quickly and precisely because it contains a second clutch that is already engaged to shift the transmission to the next gear.

There is no such thing as a "manual automatic" or other such nonsense being mentioned here. A DCT is simply a different type of automatic transmission.

Put it this way: You know the A8 automatic transmission in the C7? Well, there is absolutely no difference in how you would operate the DCT transmission in a C8. You can either paddle-shift it (like the current A8) or you can allow the transmission to shift itself without using the paddles (like the current A8). NO DIFFERENCE other than the speed and precision in which the shifting is accomplished by the automatic transmission.
There is a big difference between a traditional automatic and a DCT. First, the traditional automatic has a torque converter which provides an indirect connection between the flywheel and the transmission input shaft. The only time there is a direct connection is when the torque converter is locked up during cruise and they do that to improve gas mileage.

A DCT is two separate gear trains connected to the flywheel by clutches that provide a direct connection to the flywheel just like the clutch in an automatic transmission does. With a traditional automatic you can sit with your foot on the brake with the transmission in gear and the engine running and the engine will not stall due to the indirect connection of the torque converter.

As far as shifting the only difference between a traditional manual a traditional auto and the DCT is with the traditional manual the operator has to push the clutch in and out. The paddles are no different than moving a shift lever around. You can locate the shift lever anywhere within reach of the driver and all it does is wiggle around Vs the paddles moving back and forth. Basically, the same action. If you really believe otherwise you are fooling yourself.

Shifting a manual Vs a torque converter automatic has always been a problem since the automatic tends to shift at the wrong time even when shifted manually. We have been able to shift automatics manually since they were first introduced. Take for instance the A4 that was delivered in the C5. The last thing you want is the transmission downshifting just as you start to go full throttle passing the apex of a turn. However, even manually downshifting the transmission didn't guarantee it wouldn't hang up and wait to downshift until the driver started adding throttle. When that happened you would get a sudden change in torque delivered to the rear wheels with the potential for a spin out right at the wrong time. Sometimes it would downshift when the shifter was moved sometimes it wouldn't so it was unpredictable. Driving a manual transmission was easier since the gear changes were predictable.

A DCT lets the driver control the gear changes just like a manual transmission but provides error free up and downshifts which can keep the engine from mechanical over revs. As far as drag racing a DCT will still probably lose to a traditional automatic because it doesn't have that torque converter with its stall speed (increases torque to the rear wheels by providing a gear ratio that varies while the output TC shaft speed catches up with the input shaft speed) which lets the driver bring up the engine rpms and preload the launch.

Bill
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Old 12-02-2018, 03:45 PM
  #30  
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Originally Posted by Sub Driver View Post
Why wouldn't you just google this or watch a video on YouTube? You would get a much quicker and more comprehensive answer.
With pics!

That said and looking at the other responses, here are some other points on why they are used:

A DCT uses two clutches and essentially two very efficient transmissions behind each (all in one box.) One trans is in the next “anticipated” gear so a simple disconnect of one clutch and engagement of the other, you’re in the next gear. Milliseconds. Note the clutches are mounted concentrically - so suggestion to look at pics on net.

Why more efficient? As with any standard trans only two gears are under load at any one time. The few teeth on both efficient spur gears have a “no sliding” involute tooth profile. (You’ll have to google that word as won’t define here or too many folks will fall asleep! )

Automatic transmissions have an inefficient coupling, a torque converter. Each gear consists of a large sun gear and many planitery gears inside that have bearings on each gear shaft, many gear teeth in contact and under load when in that gear - read friction. They also use multiple clutch disks to activate each. That is why they need a trans cooler and under hard use some overheat. The base C7 M7 has no transmission cooler as no inefficient gears and clutch disks.

The efficency is why many, even small, none performance oriented Euopean cars, are using DCTs versus performance benifits in a Ferrari, etc.

Last edited by JerryU; 12-02-2018 at 04:42 PM.
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Old 12-02-2018, 04:08 PM
  #31  
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Originally Posted by Bill Dearborn View Post
There is a big difference between a traditional automatic and a DCT. First, the traditional automatic has a torque converter which provides an indirect connection between the flywheel and the transmission input shaft. The only time there is a direct connection is when the torque converter is locked up during cruise and they do that to improve gas mileage.

A DCT is two separate gear trains connected to the flywheel by clutches that provide a direct connection to the flywheel just like the clutch in an automatic transmission does. With a traditional automatic you can sit with your foot on the brake with the transmission in gear and the engine running and the engine will not stall due to the indirect connection of the torque converter.

As far as shifting the only difference between a traditional manual a traditional auto and the DCT is with the traditional manual the operator has to push the clutch in and out. The paddles are no different than moving a shift lever around. You can locate the shift lever anywhere within reach of the driver and all it does is wiggle around Vs the paddles moving back and forth. Basically, the same action. If you really believe otherwise you are fooling yourself.

Shifting a manual Vs a torque converter automatic has always been a problem since the automatic tends to shift at the wrong time even when shifted manually. We have been able to shift automatics manually since they were first introduced. Take for instance the A4 that was delivered in the C5. The last thing you want is the transmission downshifting just as you start to go full throttle passing the apex of a turn. However, even manually downshifting the transmission didn't guarantee it wouldn't hang up and wait to downshift until the driver started adding throttle. When that happened you would get a sudden change in torque delivered to the rear wheels with the potential for a spin out right at the wrong time. Sometimes it would downshift when the shifter was moved sometimes it wouldn't so it was unpredictable. Driving a manual transmission was easier since the gear changes were predictable.

A DCT lets the driver control the gear changes just like a manual transmission but provides error free up and downshifts which can keep the engine from mechanical over revs. As far as drag racing a DCT will still probably lose to a traditional automatic because it doesn't have that torque converter with its stall speed (increases torque to the rear wheels by providing a gear ratio that varies while the output TC shaft speed catches up with the input shaft speed) which lets the driver bring up the engine rpms and preload the launch.

Bill
Correction to your post. DCT's can have a torque convertor between the flywheel and the DCT's clutches. Acura has such a thing in one of their 4 door sedans..

Also, even though GM does not have a car in production currently that has a DCT, they(GM) have a patent on a DCT with a torque convertor.

Last edited by JoesC5; 12-02-2018 at 04:10 PM.
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Old 12-02-2018, 04:25 PM
  #32  
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^^^
Since I said the same about inefficient torque converters (that's why they are called slush boxes by some- namely some like me! )

There are DCT’s that use two dry concentric clutches. There are also some that use wet clutches, much like each gear in an “automatic trans.” These are not inefficient torque converters and used for smoother engagement. They do generate some heat but not like an unlocked torque converter.

Guess, as you say, an inefficient torque converter can be used before a pair of mechanical clutches or wet clutches. Yield to your knowledge as who else but Accura uses that approach, which is less efficient but no doubt provides a smoother shift.

The use of a torque converter by Accura is interesting.

Last edited by JerryU; 12-02-2018 at 04:56 PM.
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Old 12-02-2018, 05:14 PM
  #33  
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Originally Posted by Bill Dearborn View Post
There is a big difference between a traditional automatic and a DCT. First, the traditional automatic has a torque converter which provides an indirect connection between the flywheel and the transmission input shaft. The only time there is a direct connection is when the torque converter is locked up during cruise and they do that to improve gas mileage.

A DCT is two separate gear trains connected to the flywheel by clutches that provide a direct connection to the flywheel just like the clutch in an automatic transmission does. With a traditional automatic you can sit with your foot on the brake with the transmission in gear and the engine running and the engine will not stall due to the indirect connection of the torque converter.

As far as shifting the only difference between a traditional manual a traditional auto and the DCT is with the traditional manual the operator has to push the clutch in and out. The paddles are no different than moving a shift lever around. You can locate the shift lever anywhere within reach of the driver and all it does is wiggle around Vs the paddles moving back and forth. Basically, the same action. If you really believe otherwise you are fooling yourself.

Shifting a manual Vs a torque converter automatic has always been a problem since the automatic tends to shift at the wrong time even when shifted manually. We have been able to shift automatics manually since they were first introduced. Take for instance the A4 that was delivered in the C5. The last thing you want is the transmission downshifting just as you start to go full throttle passing the apex of a turn. However, even manually downshifting the transmission didn't guarantee it wouldn't hang up and wait to downshift until the driver started adding throttle. When that happened you would get a sudden change in torque delivered to the rear wheels with the potential for a spin out right at the wrong time. Sometimes it would downshift when the shifter was moved sometimes it wouldn't so it was unpredictable. Driving a manual transmission was easier since the gear changes were predictable.

A DCT lets the driver control the gear changes just like a manual transmission but provides error free up and downshifts which can keep the engine from mechanical over revs. As far as drag racing a DCT will still probably lose to a traditional automatic because it doesn't have that torque converter with its stall speed (increases torque to the rear wheels by providing a gear ratio that varies while the output TC shaft speed catches up with the input shaft speed) which lets the driver bring up the engine rpms and preload the launch.

Bill
I've seen you make these arguments before in the forums, and you're just plain wrong. When you say that flipping paddles on a DCT is the same as "moving a shifter around" I know that you're either just a troll or a dolt, but in either case I have no interest in reading further. You go on justifying your love of automatics by continuing to lie to yourself. I really couldn't care less.
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Old 12-02-2018, 05:18 PM
  #34  
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Originally Posted by Bikerjulio View Post
Incorrect. A DCT can and does skip gears, for instance in kickdown.
So you'd like to point out that some DCTs can automatically downshift more than one gear at a time? Thanks for proving my point about DCTs being automatic transmissions.

This is an automatic behavior; not something that you control. Unless your car has more than "+1" and "-1" paddles.

In my real manual transmission I can shift from 4th to 2nd by slotting that gear.
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Old 12-02-2018, 05:22 PM
  #35  
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Originally Posted by JoesC5 View Post
The DCT that Mercedes uses can skip gears.

"[color=left=#333333]Engineers at Daimler’s Stuttgart-Hedelfingen plant – where the new transmission is being manufactured – assert that the multiple downshift function of the 7G-DCT provides faster bursts of speed on tap; whenever necessary, the electronic control system skips individual gears, instead of shifting down through each gear in succession."

Also VW/Audi

"[/color][color=left=#222222]Under "normal", progressive and linear [/color]acceleration[color=left=#222222] and deceleration, the DSG shifts in a [/color]sequential[color=left=#222222] manner; i.e., under acceleration: 1st → 2nd → 3rd → 4th → 5th → 6th, and the same sequence reversed for deceleration. However, the DSG can also skip the normal sequential method, by missing gears, and shift two or more gears."[/color]

As stated in my post right above this, it's an automatic behavior of a DCT to skip gears, which proves my point that they are automatics. You cannot flip a "-2" paddle to downshift from 4th to 2nd. You have to rely on the car automatically doing it for you when it decides to shift (and how to shift). The only thing you do as a driver is to press hard on the accelerator.
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Old 12-02-2018, 06:28 PM
  #36  
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Originally Posted by Tom73 View Post
Can someone help me out here. With all the talk about a DCT in the new C8, I am courious as to how they work. I have never been around one let alone driven one. Sounds to me that they are like a manual shifting automatic but I am sure there is much more to then than that.

Do do they use a torque converter or an automatic clutch of some type? Do you have to shift to neutral when you stop, Do you have to go through each gear when going up/down or can you skip gears?

Is there any advantage other than shifting speed? Any everyday street driving advantage?
I don't know everything about what they will use or do with the C8. I've done quite a bit of driving and coaching (autocross, track time) in VW/Audi cars with their DSG.

The driver interface, from the driver's point of view, it looks like any other automatic. You've got a shift lever with the normal "PRND" markings, and a gate to a "+/-" section. If you move it to the manual mode (slide it over so it moves in the "+/-" section, the paddles on the column are also activated. It seems the same as any other paddle shift automatic.

Operationally, and I'm talking about what's happening inside the transmission, it's much more like a manual transmission with a few key differences. On the Audi/VW system, there is no torque converter. There are just the 2 clutches. The biggest difference is that where there's one pinion/output shaft in a conventional manual gearbox, the DSG (their name for their DCT) has two shafts. One shaft has the odd numbered gears (1,3,5[,7]), and the other shaft has the even numbered gears (2,4,7[,8]). Each shaft has a clutch that is controlled by the computer. The gears on these shafts are very similar to gears in a manual transmission, with dog rings that slide to engage the gears. I'm not sure if there are synchronizers or not, but the operational movements inside are very similar to a manual transmission, with movement controlled by solenoids and hydraulics. The VW/Audi system in the transverse mounted gearbox has a pinion gear on each of the shafts (two pinion gears on the same ring gear). That simplifies things a bit and reduces some of the parts count and gear interfaces. If the ME C8 has the diff in the same case with the gears, they will probably have a similar arrangement.

In normal operation, the computer will select the "next" gear on the opposite shaft, and when the time comes to shift, the current clutch will disengage as the other clutch engages, transferring power nearly instantly from one shaft to the other. As long as you're doing what the computer expects, it all works good. But if you were mildly accelerating in 3rd gear (with it in "manual shift mode"), the computer is going to have 4th gear all engaged and ready to go, waiting on your to tap the "+" paddle or tap the stick in the "+" direction. Lets's say you're approaching a curve in the road, though, so instead, you left foot brake, taking your foot off the throttle pedal at the same time, and hit the "-" paddle (or tap the stick in the "-" direction), the shift might take longer than you expect, and perhaps longer than a modern automatic. First, the thing will have to move the gear forks/sliders around to select 2nd gear on the even shaft, probably having to do some "rev matching" and monkey motion with the electronically controlled throttle and the clutch on the even shaft to get things synced for the shift, before it finally swaps clutches and fully engages 2nd gear.

In short, it's awesome when you do exactly what it expects, and it's a bit weird and quirky when you don't. For autocrossing or track use, if you happen to hit max revs in a gear, the Audi/VW system automatically upshifts (whether you ask for it with the paddle or shifter or not). We had one autocross course last year with some very tight maneuvers out of the start, and the thing shifted to second right at the first point where you had to brake in the really tight stuff. It was doubly annoying. You're trying to slow down as it's dumping high revs onto 2nd gear for you, which means you're fighting the engine with the brakes. On top of that, you're now going through a corner that you want to be in first at the exit, and the thing is in second, so you have to do a downshift with the paddles (or shifter) after slowing enough that it will allow you to go back to first. There can be similar issues under braking near the end of any straight on a track or autocross course. If you go from full throttle to hard braking, at best you're going to face a longer downshift time. It may even ignore your request if you tap the paddle when you're moving too fast for your target gear. You may be able to tune around some of this, depending on the implementation.

To answer your other questions. If you put it in "D", it works like an automatic. When you stop, the thing is smart enough to disengage the clutches for you. When you press the gas, it will engage the clutches slowly and take off. The one difference from a conventional automatic is that it won't "creep" forward if you take your foot off the brake. You have to at least touch the throttle a little bit to make it move.

If you use the manual shift mode, you can go up one with the "+" and down one with the "-". You can use the paddle or the shifter on the VW/Audi, your choice. If you want to shift multiple gears, push it multiple times. If you ask for something impossible (try dropping to 1st at 70mph, for example), the VW/Audi system just ignores you. In "competition" modes, I hope Chevy queues up the requests, so if you request 3rd for example as you're braking for a corner you plan to exit in 3rd, it should get you to 3rd when it's appropriate for your speed, even if you asked for it when it wasn't possible. I wish the VW/Audi did that.
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Old 12-02-2018, 06:42 PM
  #37  
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Originally Posted by Bikerjulio View Post
Incorrect. A DCT can and does skip gears, for instance in kickdown.
Yes, and no.

All current implementations put the gears on alternating shafts, in order from shortest ratios (numerically highest) to tallest. Every shift has to change from one shaft to the other.

Given Chevrolet's history with "CAGS," I fully expect that on a slow to 'normal' start, the their DCT will probably skip from 1st to 4th. It's theoretically possible for a passing "kickdown" to drop from 6th to 3rd, or from 5th to 2nd, but certain skips, that end on the same shaft as the current gear, physically can't happen. For example, it won't ever shift directly from 6th to 4th gear because those gears are on the same shaft.

And I've never seen a paddle or shifter interface for "manual mode" on any DCT/DSG which allowed you to skip on downshifts. There's no way with the manual controls, even if it's physically possible, for you to command the thing to shift directly from 5th gear to 2nd gear.

Last edited by C6_Racer_X; 12-02-2018 at 06:44 PM.
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Old 12-02-2018, 06:57 PM
  #38  
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What I can add to this conversation is that I just started driving a Volkswagen Golf TDI with a DCT. I really like it!

I had almost INSISTED on getting a Golf with a manual transmission but we bought a DCT with far fewer miles at a better price after I was pleasantly suprised on the test drive.

For all practical purposes it's an automatic transmission. I don't know if it's the result of the software that controls it, or what, but it works far better than most slushboxes.

And it's well adapted to the diesel engine as well. Again? Well-tuned Software?

It shifts crisply. It FEELS like what a good driver with a stick would do. I cant think of one situation where it selected the wrong gear, etc.

As far as being unable to shift two gears, DCTs shift so damn fast (300ms) that you could hit the paddle twice and would never know the difference vs skipping a gear.

I enjoy double-clutching a manual transmission going into a corner, I love gutless cars with CVT's that redline the engine while going up the onramp, but I really think this is the best of all if you do NOT want a "true" manual for reasons like stop and go traffic.

Now the best automatic I've ever driven is the 8-speed ZF in the high end BMWs which truly does engine brake (touch the brake, it downshifts a gear and slows you down), but this is close. Damn close.

Last edited by wadenelson; 12-03-2018 at 11:16 AM.
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Old 12-02-2018, 07:07 PM
  #39  
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Ignore the nut jobs that think a DCT is in the same family as torque converter automatics. DCT’s are much closer to traditional manuals.

Here’s the sports cars owner’s road map:

Do you have a boyfriend? —-> own a torque converter automatic or CVT
Do you have a girlfriend? —-> own a manual, DCT, or automated manual

Last edited by lrobe22; 12-02-2018 at 07:48 PM.
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Old 12-02-2018, 08:02 PM
  #40  
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Originally Posted by PWN3D
[left]When at a stop and taking your foot off the brake to move, you can feel a lag as the clutch is feathered and sometimes a small amount of ‘lurchiness’. When you are moving and take your foot off the gas to coast, you feel engine breaking just like a manual. At some point around 5 mph you can feel the trans disengage from the motor fully to come to a stop.
This why some manufactures moved back to regular autos and torque converter coupled DCTs..

Our two Audi's both did what you describe and it was terrible. We are back to BMW with a conventional auto trans. It blows away what Audi offers for a daily driver. On a track I suppose none of this matters though.

Last edited by NY09C6; 12-02-2018 at 08:02 PM.
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