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      01-09-2013, 08:42 PM   #1
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M drive rev match

I am new to my M3 so I thought I would ask the experts. I use my car as a DD. I drive around it auto mode, normal, comfort.

Question why does some times the car just downshifts like a normal auto car and other times when it is downshifting it rev matches like it does when I am in S mode racing around.

It seems like it just does it randomly as well. I have tried to get it to do it and I can't and then it just does it other times.

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      01-09-2013, 09:21 PM   #2
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I kind of chalked it up to based on the speed I was decelerating. But sometimes it's just random based on your pedal inputs.
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      01-09-2013, 09:26 PM   #3
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the weirdest is when youre in S1 and it rev matches downshifts
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      01-09-2013, 09:40 PM   #4
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Doesn't every automatic transmission rev match downshift every time? Isn't that the whole point?

Are you saying that sometimes it downshifts slowly while other times it does more briskly?
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      01-09-2013, 09:56 PM   #5
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Quote:
Originally Posted by gatorfast View Post
Doesn't every automatic transmission rev match downshift every time? Isn't that the whole point?

Are you saying that sometimes it downshifts slowly while other times it does more briskly?
I am saying that most times to just downshifts normally like my Lexus and other times when it downshifts the rpm's rev way up. I am sorry I am not explaining it well.
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      01-10-2013, 03:28 AM   #6
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I noticed it tends to rev-match if you downshift when your foot is on the gas pedal / if you're accelerating slightly, or maybe if you've been driving aggressively. If you're just coasting / braking to a stop, it doesn't.
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      01-10-2013, 05:04 AM   #7
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I know exactly what you mean. It really depends on how fast you decelerate
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      01-10-2013, 06:07 AM   #8
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Quote:
Originally Posted by thecakeisalie View Post
I noticed it tends to rev-match if you downshift when your foot is on the gas pedal / if you're accelerating slightly, or maybe if you've been driving aggressively. If you're just coasting / braking to a stop, it doesn't.
^This.

My conclusion is the same. DCT will rev match if your foot is on the throttle even in S1, S2 or S3. I actually use this feature as I usualy use S2 on my DD. If I want a quick downshift while braking, I blip the throttle with the heel and toe technique and I get a nice and quick rev matched downshift .

This behavior of the DCT seems to be part of the software fix for the DCT lag issue, as the car did not behave that way when I originaly got it in 2008.
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      01-10-2013, 08:06 AM   #9
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Quote:
Originally Posted by CanAutM3 View Post
If I want a quick downshift while braking, I blip the throttle with the heel and toe technique and I get a nice and quick rev matched downshift .
.
ok now im lost. Heel toe with DCT? What is the point of that? Why would you want one foot hitting the gas and brake at the same time? If you want the car to downshift, simply use the paddles or shift lever.
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      01-10-2013, 09:47 AM   #10
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Quote:
Originally Posted by CPWINCH View Post
I know exactly what you mean. It really depends on how fast you decelerate
That's true, match revs is only necessary while deaccelarating rapidly.

Quote:
Originally Posted by M3forMe View Post
I am saying that most times to just downshifts normally like my Lexus and other times when it downshifts the rpm's rev way up. I am sorry I am not explaining it well.
I will only match revs when necessary, such as hard braking, while making downshifts.

Quote:
Originally Posted by CanAutM3 View Post
^This.

My conclusion is the same. DCT will rev match if your foot is on the throttle even in S1, S2 or S3. I actually use this feature as I usualy use S2 on my DD. If I want a quick downshift while braking, I blip the throttle with the heel and toe technique and I get a nice and quick rev matched downshift .

This behavior of the DCT seems to be part of the software fix for the DCT lag issue, as the car did not behave that way when I originaly got it in 2008.
Not necessary to heel/toe in a DCT. The transmission will match revs for you when it is needed, such as rapidily deaccelarating. Just use to paddles to downshift, while braking hard and transmission will match revs by blipping throttle on down shifts.
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Last edited by ortho281; 01-10-2013 at 01:39 PM.
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      01-10-2013, 11:52 AM   #11
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Does the DCT really ride the clutch plate during downshifts? I figured every downshift would be rev matched to extend the life of those oh so expensive wear items...
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      01-10-2013, 12:14 PM   #12
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It always rev matches in S4-S6. In S1-S3 it depends on driving conditions AFAIK
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      01-10-2013, 12:42 PM   #13
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Quote:
Originally Posted by gatorfast View Post
ok now im lost. Heel toe with DCT? What is the point of that? Why would you want one foot hitting the gas and brake at the same time? If you want the car to downshift, simply use the paddles or shift lever.
Answer: To get a faster rev matched downshift in S1, S2 or S3 drivelogic modes.

It is not something I do often but it is useful in some specific occasions.

There is a significant delay from when you pull the paddle to when the downshift happens in the lower Drivelogic modes and the downshift itself is pretty slow. No need for heel and toe in S4 to S6 modes .

Last edited by CanAutM3; 01-10-2013 at 07:24 PM.
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      01-10-2013, 12:46 PM   #14
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Quote:
Originally Posted by ortho281 View Post
Not necessary to heal/toe in a DCT. The transmission will match revs for you when it is needed, such as rapidily deaccelarating. Just use to paddles to downshift, while braking hard and transmission will match revs by blipping throttle on down shifts.
I don't need to heal, I am perfectly healthy

Even if I brake hard, my DCT will not rev match in S1 or S2. The only way I can get it to rev match in those modes is to blip the throttle .
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      01-10-2013, 12:49 PM   #15
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Quote:
Originally Posted by cposk View Post
Does the DCT really ride the clutch plate during downshifts? I figured every downshift would be rev matched to extend the life of those oh so expensive wear items...
Since DCT has two clutches, it is actually the two clutches slipping simultaneously as one engages and the other disengages. When it performs a rev match, the two clutches are shortly disengaged while the throttle is blipped.

The wear on downshifts is nothing compared to the wear on full throttle upshifts.

Last edited by CanAutM3; 01-10-2013 at 07:26 PM.
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      01-10-2013, 01:39 PM   #16
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Quote:
Originally Posted by CanAutM3 View Post
I don't need to heal, I am perfectly healthy
Sorry, just caught that. heel/toe
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      01-10-2013, 10:46 PM   #17
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Quote:
Originally Posted by kevin///mlee
It always rev matches in S4-S6. In S1-S3 it depends on driving conditions AFAIK
I noticed the exact same thing and did a bunch of test to confirm the same results.
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      01-12-2013, 11:43 PM   #18
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Quote:
Originally Posted by CanAutM3 View Post
Since DCT has two clutches, it is actually the two clutches slipping simultaneously as one engages and the other disengages. When it performs a rev match, the two clutches are shortly disengaged while the throttle is blipped.

The wear on downshifts is nothing compared to the wear on full throttle upshifts.
What actually gets worn on full throttle upshifts? The gears are engaged quickly so doesn't seem that the clutch discs would get worn much.
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      01-12-2013, 11:54 PM   #19
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Quote:
Originally Posted by sensi09 View Post
What actually gets worn on full throttle upshifts? The gears are engaged quickly so doesn't seem that the clutch discs would get worn much.
Up-shifting or down-shifting, there is similar amount of clutch slip (assuming similar shift times). However, on a full throttle upshift, there is much more force (torque) transmitted through the clutch, hence more wear.
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      01-13-2013, 04:07 AM   #20
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Quote:
Originally Posted by kevin///mlee View Post
It always rev matches in S4-S6. In S1-S3 it depends on driving conditions AFAIK
Yep, noticed this too in the last couple days. Still surprises me sometimes when I'm in M mode (S5 for me) and I downshift mindlessly when I'm slowing to a stop. I think it's my favorite feature right now. So aggressive
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      01-13-2013, 06:57 AM   #21
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Quote:
Originally Posted by CanAutM3 View Post
Up-shifting or down-shifting, there is similar amount of clutch slip (assuming similar shift times). However, on a full throttle upshift, there is much more force (torque) transmitted through the clutch, hence more wear.
It appears to me that a high rpm shift in say in S5 or S6 would have minimal clutch slip (less wear on the clutch disc), but would be harsher on the drivetrain in general

Being stuck in traffic and slow shifts in lower modes would have more clutch slip and wear on the discs. Other than launch control, high rpm shifts in S1-S2 (maybe auto mode too) should have the most slip and wear on the clutch discs.
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      01-13-2013, 08:20 AM   #22
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Quote:
Originally Posted by sensi09 View Post
It appears to me that a high rpm shift in say in S5 or S6 would have minimal clutch slip (less wear on the clutch disc), but would be harsher on the drivetrain in general

Being stuck in traffic and slow shifts in lower modes would have more clutch slip and wear on the discs. Other than launch control, high rpm shifts in S1-S2 (maybe auto mode too) should have the most slip and wear on the clutch discs.
Wear is function of both the slip time and the force applied.

As an example, think of sanding a surface. If you don't apply much force, you are not removing much material with each pass and you need to sand longer. However if you increase the force, more material gets removed much faster, i.e. more wear.

The worst case would be a long clutch engagement time with high forces being trasmitted. As you stated, launch control is an example where the clutch slips while the engine is at max power.
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