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      05-11-2020, 04:12 PM   #1
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DCT Downshifts/Upshifts with Paddles

Just bought my first BMW M4 DCT.

I have 2 questions:

1) When I am going at high speeds and need to downshift (using the paddles) do I need to let go of the gas?

2) When going at high speeds and need to upshift (using the paddles) do I need to let go of the gas as well?

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      05-11-2020, 04:13 PM   #2
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1) No

2) No

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      05-11-2020, 04:17 PM   #3
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Originally Posted by CanAutM3 View Post
1) No

2) No

Nice, thanks and thanks!
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      05-11-2020, 04:38 PM   #4
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I was confused about this as well, it takes a while to get used to it
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      05-12-2020, 11:57 AM   #5
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Quote:
Originally Posted by SixtyMac View Post
Just bought my first BMW M4 DCT.

I have 2 questions:

1) When I am going at high speeds and need to downshift (using the paddles) do I need to let go of the gas?

2) When going at high speeds and need to upshift (using the paddles) do I need to let go of the gas as well?

Cheers!
CanAutM3 posted the correct, but brief, answers to your questions. To expand a little more:

1.) One reason that you would not let off the gas when downshifting is if you need a lower gear for better acceleration, i.e., making a pass around someone on the freeway or a 2-lane road. Here's a neat trick with the DCT if you're downshifting to pass someone: Press harder on the gas pedal and hold the left paddle in. It will automatically downshift to the lowest safe gear without having to pull it 1 or 2 times. Be careful though, as it may shift into a lower gear than you expected. Also, don't do this in slippery conditions!

2.) When going for maximum acceleration and shifting manually, press the gas pedal to the floor and then just upshift with the right paddle. There's no forward momentum loss when upshifting as there is with a manual transmission. Pay close attention to the tachometer and make sure that you shift by redline. If you're doing this from a standing start, you need to shift into 2nd almost immediately or you will hit the rev limiter.
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      05-12-2020, 12:39 PM   #6
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Quote:
Originally Posted by SD ///M4 View Post
Quote:
Originally Posted by SixtyMac View Post
Just bought my first BMW M4 DCT.

I have 2 questions:

1) When I am going at high speeds and need to downshift (using the paddles) do I need to let go of the gas?

2) When going at high speeds and need to upshift (using the paddles) do I need to let go of the gas as well?

Cheers!
CanAutM3 posted the correct, but brief, answers to your questions. To expand a little more:

1.) One reason that you would not let off the gas when downshifting is if you need a lower gear for better acceleration, i.e., making a pass around someone on the freeway or a 2-lane road. Here's a neat trick with the DCT if you're downshifting to pass someone: Press harder on the gas pedal and hold the left paddle in. It will automatically downshift to the lowest safe gear without having to pull it 1 or 2 times. Be careful though, as it may shift into a lower gear than you expected. Also, don't do this in slippery conditions!

2.) When going for maximum acceleration and shifting manually, press the gas pedal to the floor and then just upshift with the right paddle. There's no forward momentum loss when upshifting as there is with a manual transmission. Pay close attention to the tachometer and make sure that you shift by redline. If you're doing this from a standing start, you need to shift into 2nd almost immediately or you will hit the rev limiter.
True but you're playing with fire utilizing the kick down function and spun crank hub. Better off selecting the appropriate gear manually.
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      05-12-2020, 01:21 PM   #7
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Quote:
Originally Posted by M4theWIN1 View Post
True but you're playing with fire utilizing the kick down function and spun crank hub. Better off selecting the appropriate gear manually.
I didn't say that I do this, I was just pointing out one of the "features", fully documented in the owner's manual, of the DCT and how it operates, which is what the OP was asking about.

I drive my DCT as if it were a manual, and I purposely select all gears all the time, upshifting and downshifting as necessary. I also don't have a tune, and I never do "pulls" from a standing start. Anecdotal evidence is that most cars with the SCH issue have tunes >500HP. You make it sound like doing this on the freeway to pass a car occasionally is going to automatically cause the SCH.
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      05-12-2020, 01:44 PM   #8
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Quote:
Originally Posted by M4theWIN1 View Post
True but you're playing with fire utilizing the kick down function and spun crank hub. Better off selecting the appropriate gear manually.
There is no demonstrated correlation between SCH and DCT kickdown...

That being said, I rarely use this feature. I prefer to select gears myself .
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      05-12-2020, 01:55 PM   #9
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Quote:
Originally Posted by SD ///M4 View Post
CanAutM3 posted the correct, but brief, answers to your questions. To expand a little more:

2.) When going for maximum acceleration and shifting manually, press the gas pedal to the floor and then just upshift with the right paddle. There's no forward momentum loss when upshifting as there is with a manual transmission. Pay close attention to the tachometer and make sure that you shift by redline. If you're doing this from a standing start, you need to shift into 2nd almost immediately or you will hit the rev limiter.
To expand even further on this:

There's no interruption in acceleration on a DCT upshift. In the more aggressive Drivelogic modes, there's even a surge in forward acceleration as the DCT recuperates the inertia energy from the engine RPM drop and converts it in forward acceleration. This is the infamous jolt that is being felt on upshifts.

With a 6MT and even an 8AT, you will see passenger heads bob forward on upshifts because acceleration is interrupted. On a DCT upshift, you'll see passenger heads bob rearward from the surge in acceleration .

Being able to do WOT upshifts is one of the key benefits of DCT
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Last edited by CanAutM3; 05-12-2020 at 08:05 PM..
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      05-12-2020, 02:03 PM   #10
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So this aggressive kickdown and SCH everyone is talking about, exactly when is it aggressive? Low gear, high RPM and pedal to the floor or simply a slight downpress to where the car jump down a gear to pull better like from 4th to 3rd?
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      05-12-2020, 02:07 PM   #11
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Quote:
Originally Posted by Tecnniqe View Post
So this aggressive kickdown and SCH everyone is talking about, exactly when is it aggressive? Low gear, high RPM and pedal to the floor or simply a slight downpress to where the car jump down a gear to pull better like from 4th to 3rd?
The higher the gear and the lower the road speed, the more aggressive the kickdown will be.

However, like I said, there is no demonstrated correlation between SCH and the DCT kickfeature. 6MT are just as prone to SCH than DCT and it is mostly tuned cars that are impacted.
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      05-12-2020, 02:11 PM   #12
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Quote:
Originally Posted by CanAutM3 View Post
The higher the gear and the lower the road speed, the more aggressive the kickdown will be.

However, like I said, there is no demonstrated correlation between SCH and the DCT kickfeature. 6MT are just as prone to SCH than DCT and it is mostly tuned cars that are impacted.
There is no demonstrated correlation between anything and SCH, especially since BMW doesn't and won't ever acknowledge it as a manufacturing defect
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      05-12-2020, 02:14 PM   #13
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Quote:
Originally Posted by sna66 View Post
There is no demonstrated correlation between anything and SCH, especially since BMW doesn't and won't ever acknowledge it as a manufacturing defect
Is it really a "manufacturing defect"?
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      05-12-2020, 02:31 PM   #14
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Quote:
Originally Posted by CanAutM3 View Post
Is it really a "manufacturing defect"?
Kinda sounds like it since it seems to rather happen rather quick or not at all in the cases Iíve read about. Maybe itís just random, scary none the less but my issue is with insurance and warranty claims should I apply one of the fixes and something would happen with the drivetrain or it spinning of whatever and them claiming Iíve modified it so itís not covered anymore.
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      05-12-2020, 02:57 PM   #15
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Quote:
Originally Posted by Tecnniqe View Post
Kinda sounds like it since it seems to rather happen rather quick or not at all in the cases Iíve read about. Maybe itís just random, scary none the less but my issue is with insurance and warranty claims should I apply one of the fixes and something would happen with the drivetrain or it spinning of whatever and them claiming Iíve modified it so itís not covered anymore.
Clearly not a "manufacturing" issue, it is how it was designed.

If you look at the crank hub design, it is apparent that it was engineered as a "mechanical fuse", intended to slip in the advent the valvetrain is loaded beyond a certain threshold. If the engineers did not want it to slip, they would have made a splined design.

An aftermarket tune can definitely alter the acceleration characteristics of the engine making it more prone to SCH failures.
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      05-12-2020, 03:27 PM   #16
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Quote:
Originally Posted by CanAutM3 View Post
Clearly not a "manufacturing" issue, it is how it was designed.

If you look at the crank hub design, it is apparent that it was engineered as a "mechanical fuse", intended to slip in the advent the valvetrain is loaded beyond a certain threshold. If the engineers did not want it to slip, they would have made a splined design.

An aftermarket tune can definitely alter the acceleration characteristics of the engine making it more prone to SCH failures.
That is completely wrong information.
An engineer that worked on the S55 came on this board and said it absolutely is not a fuse. It was designed this way to be easier to work on.
This is to paraphrase an engineer that helped design the engine.
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      05-12-2020, 03:31 PM   #17
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Quote:
Originally Posted by behindthen0thing View Post
That is completely wrong information.
An engineer that worked on the S55 came on this board and said it absolutely is not a fuse. It was designed this way to be easier to work on.
This is to paraphrase an engineer that helped design the engine.
Link?
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      05-12-2020, 03:31 PM   #18
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Link?
I'm looking.
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      05-12-2020, 03:43 PM   #19
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Quote:
Originally Posted by CanAutM3 View Post
To expand even further on this:

There's no interruption in acceleration on a DCT upshift. Further, in the more aggressive Drivelogic modes, there's even a surge in forward acceleration as the DCT recuperates the inertia energy from the engine RPM drop and converts it in forward acceleration. This is the infamous jolt that is being felt on upshifts.

With a 6MT and even 8AT, you will see passenger heads bob forward on upshifts because acceleration is interrupted. On a DCT upshift, you'll see passenger head bob rearward from the surge in acceleration .

Being able to do WOT upshifts is one of the key benefits of DCT
This exactly. Such a great feeling to have a WOT upshift throw your head back into the headrest. It's also fun to see the look on a passenger's face when they get to experience that jolt of power.

But, yea... watch the tach close, because all that fun and giant smile are quickly erased by embarrassment when it bounces off the rev limiter and your head bobs forward.
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      05-12-2020, 03:51 PM   #20
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Quote:
Originally Posted by behindthen0thing View Post
That is completely wrong information.
An engineer that worked on the S55 came on this board and said it absolutely is not a fuse. It was designed this way to be easier to work on.
This is to paraphrase an engineer that helped design the engine.
Yes, at some point Boss330 did speculate that the friction assembly was potentially for ease of manufacture rather than for "strength". But that was not a conclusion. A splined design would not be more difficult to assemble.
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      05-12-2020, 03:52 PM   #21
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Quote:
Originally Posted by swagon View Post
Link?
I can't find it. I do remember reading it though.
It might have been somewhere else.

If you think about it there is no reason to design a hub to slip timing.
This will not save your engine from damage in any way whatsoever.
The new M engines don't have this design anymore because it's an unforeseen weakness.

I had crossed virtual paths with an M4 engineer on the internet a few months ago and I asked him why they made the exhaust unequal length.
He said something to the effect of "thats what gives it the aggressive sound!"
Funny, they wanted it to sound this way.
Anyway that's unrelated and an ADD moment.


It's not a fuse, it doesn't protect anything. Any time the hub slips it is an expensive venture. I can't recall one instance of anybody saying they were happy their hub slipped because it avoided damage.
it makes zero sense.
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      05-12-2020, 03:57 PM   #22
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Quote:
Originally Posted by behindthen0thing View Post
I can't find it. I do remember reading it though.
It might have been somewhere else.

If you think about it there is no reason to design a hub to slip timing.
This will not save your engine from damage in any way whatsoever.
The new M engines don't have this design anymore because it's an unforeseen weakness.

I had crossed virtual paths with an M4 engineer on the internet a few months ago and I asked him why they made the exhaust unequal length.
He said something to the effect of "thats what gives it the aggressive sound!"
Funny, they wanted it to sound this way.
Anyway that's unrelated and an ADD moment.


It's not a fuse, it doesn't protect anything. Any time the hub slips it is an expensive venture. I can't recall one instance of anybody saying they were happy their hub slipped because it avoided damage.
it makes zero sense.
Here you go: https://f80.bimmerpost.com/forums/sh...&postcount=188

Most SCH simply result in having to retime the valvetrain. However, some presumed "fixes" have resulted in catastrophic failures. Go figure...

Anyhow, I could be wrong about the fuse aspect, it is something I recall from all the discussions. In the end, on a stock tune, I have zero worries about a SCH.
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