Delay on two gear DCT downshift

[Benfmd]

I just got my M3 back from the 1200 mile service so have started to open it up. I notice that if I quickly downshift from say 6th to 4th while accelerating with either the paddles or the knob, there is a delay before the revs increase to match the newly selected 4th gear. I do not see any delay on downshifting by only one gear. All settings are in most aggressive settings so that is not the issue here.
Has anyone noticed this as well? Is this turbo lag?

[karussell]

Originally Posted by Benfmd

I just got my M3 back from the 1200 mile service so have started to open it up. I notice that if I quickly downshift from say 6th to 4th while accelerating with either the paddles or the knob, there is a delay before the revs increase to match the newly selected 4th gear. I do not see any delay on downshifting by only one gear. All settings are in most aggressive settings so that is not the issue here.
Has anyone noticed this as well? Is this turbo lag?

its a double clutch so a 2-gear down shift is re-engaging the same clutch. takes a little longer than single gear up/down.

[Benfmd]

So do you notice the same thing?

[karussell]

Originally Posted by Benfmd

So do you notice the same thing?

no delays for me. i have a manual. helpful for going 6 to 3rd in 10a on road atlanta.

in DCT equipped cars though this is normal. been in plenty.

[Benfmd]

You have the delay inherent in your foot and arm movements to make the shift. Likely still slower than the DCT

[Andrew115]

Originally Posted by karussell

its a double clutch so a 2-gear down shift is re-engaging the same clutch. takes a little longer than single gear up/down.

I thought it would use the other clutch for the next downshift, but it would take a moment to get it in place. How would that use the same clutch? Its not as simple as one clutch down shift other clutch up shift. Ill try to find and post the demo I saw on this.

[Benfmd]

Anyone else notice the delay on downshifting 2 gears?

[brianeck]

Originally Posted by Benfmd

You have the delay inherent in your foot and arm movements to make the shift. Likely still slower than the DCT

Actually on the contrary I believe manual should definitely be faster with a 3 gear simultaneous shift, and a 2 should be quite close. 6-3 gear shift is quicker in a manual with just about any driver as there both clutches have to be engaged/disengaged in the DCT. 6-4 (or any 2 gear shift) is gonna be DAMN close if the driver is shifting "fast" (roughly 1/4 of a second/.25 seconds). DCT single gear shifts (from 1 clutch to the other) are going to be faster then manual drivers are capable of 100% of the time at what I think is roughly .08 seconds, nobody can argue that. However, even if the car shifted IMMEDIATELY as you flicked your 3 fingers on the paddle shifter, doing that 3 times puts you at .24 seconds (just looking at simple math), and that doesn't account for the added time from using the same clutch twice which physically has to be more than .01 seconds. . . When you add in the fact that actually "pulling" the DCT paddle 3 times has some inherent user lag in between each pull and you are definitely slower then that .25 second mark.

Someone should do a gopro sideby side looking at the tach/gear indicator doing this in manual vs DCT to confirm

[karussell]

Originally Posted by Benfmd

You have the delay inherent in your foot and arm movements to make the shift. Likely still slower than the DCT

shift times particularly downshifting don't really matter all that much. and still there is a delay while a computer is figuring out what gear to preselect. when you ask for 2 gears down and there is a dedicated clutch for 1-3-5 and one for 2-4-6 it has to disengage and re-engage the clutch. the speed in DCT is from having the next gear pre-selected.

i understand its a little frustrating waiting but thats what it is. waiting. i prefer to do that myself. less wear on the transmission going down 2 gears instead of stepping through each one as well.

DCT comes in handy when there is a turn and you want to upshift in the middle of it without upsetting the car on the limit. It has definite merit in that situation where a compromise must be made in an old fashioned manual.

Although with this cars particular power band which is broad a short shift would probably net the same result. I don't have a faster lap time with a DCT car.

finally one thing the DCT does quite well a manual can't is keep from mis shifting.

anyways i still prefer manual. your delay is from it picking the next cog on the same clutch. its normal. just gotta wait.

[Benfmd]

Thanks. Good information

[Doctor///M]

Originally Posted by karussell

finally one thing the DCT does quite well a manual can't is keep from mis shifting.

^ correct! at least there's no room for money shifting involved

[BhamDavid]

All of the even number gears are on one clutch train and the odd numbered gears are on the other. The DCT preselects the next gear you will use based or whether you are accelererating or decelerating. So for instance when you are shifting from first to second, the trans has preselected second and the shift only consists of releasing one clutch and engaging the second clutch. For a shift from fifth to third, the clutch on the odd gear numbers train has to disengage, select third and re-engage. The second clutch does not come into play. So a two gear change will always be slower than a one gear change.

[karussell]

Originally Posted by notan///m

^ correct! at least there's no room for money shifting involved

amen to that. thats never a good day.

caught a student mid money shift. jabbed it back to neutral. almost spun off the course. wheel lockup at 80mph not fun!

[karussell]

Originally Posted by BhamDavid

All of the even number gears are on one clutch train and the odd numbered gears are on the other. The DCT preselects the next gear you will use based or whether you are accelererating or decelerating. So for instance when you are shifting from first to second, the trans has preselected second and the shift only consists of releasing one clutch and engaging the second clutch. For a shift from fifth to third, the clutch on the odd gear numbers train has to disengage, select third and re-engage. The second clutch does not come into play. So a two gear change will always be slower than a one gear change.

bingo. how about this scenario. 3rd gear early apex into a corner understeer under acceleration on exit correct and down shift to 2nd to be in power band. this isn't a manual vs dct debate as manual would take just as long to shift. just dct has likely preselected 4th based on acceleration however driver chooses 2nd so it has to disengage re-engage all while you are waiting waiting waiting. seen this over and over with a student in DCT M3. no bueno. answer is to apex perfect every time

[gogoevo]

This behavior is present in my GT-R as well. Just the nature of the beast.

[MaynardZed]

Ah, the dreaded money shift, part of the dangerous allure of the manual transmission. Been there, done that. Sloppy mounts on E36, back and forth with Viper at Chuckwalla, gaining on me on straights, red mist, 3rd to 4th (whoops that was 2nd ), mechanical overev, S52 no like 9000 rpm, run very bad, put car back on trailer and go home early, pull head, discover all 12 exhaust valves bent, order many expensive parts from VAC, rebuild head with stiffer springs, retainers, and oversized valves, pay race mechanic $3500, don't do dat again, overhand 3rd to 4th shifts from now on, rebuild shifter linkages, install solid engine and trans mounts, pay more money to race mechanic. Again, don't do dat again.

I love manuals!

[solstice]

Another benefit of the excellent rev matching of the 6 MT. If you initiate an accidental Money shift ( the down instead of up shift type) you will be met with a bomb of a throttle blip where there should be none ( no blips at up shifts ).

[SunnyD]

That's the downside with a DCT transmission compared to say, SMG. I could double or triple click down in my old M5 and it would jump straight to that gear.

[CanAutM3]

I think folks are not understanding DCT. DCT is not a sequential gearbox.

It can skip gears without problem. Pull the (-) paddle quickly twice and it wil skip a gear. Pull it quickly three times, and it will skip two gears. Press the kickdown button while holding the downshift paddle and it will skip as many gears as needed to be in the optimal gear for acceleration.

That being said, when downshifting, DCT needs to open both clutches to blip the throttle and match the revs and then re-engage the proper clutch. There is a time delay to complete this; however it is at least as fast a process than it is on manual or SMG transmission. It may seem slower because all you can do is wait while the servos do their stuff; on a 6MT, your mind is focused on completing the task at hand.

[crypticc]

Drop three gears instead. They're attacked closely enough. Downsides? Bigger blip. Lol

[curehead]

Originally Posted by CanAutM3

I think folks are not understanding DCT. DCT is not a sequential gearbox.

It can skip gears without problem. Pull the (-) paddle quickly twice and it wil skip a gear. Pull it quickly three times, and it will skip two gears. Press the kickdown button while holding the downshift paddle and it will skip as many gears as needed to be in the optimal gear for acceleration.

That being said, when downshifting, DCT needs to open both clutches to blip the throttle and match the revs and then re-engage the proper clutch. There is a time delay to complete this; however it is at least as fast a process than it is on manual or SMG transmission. It may seem slower because all you can do is wait while the servos do their stuff; on a 6MT, your mind is focused on completing the task at hand.

Interesting, I haven't tried the "manual kickdown" with DCT yet, i holding the downshift paddle and pressing kickdown. I guess it will have the same effect as in D mode and kickdown, just that you stay in S mode and have to press the uppshift at the redline. Previously I never paid attention to whether the DCT can skip gears, but it certainly does. As even in Efficient engine mode, it shifts from 7th to 3rd very quickly at kickdown in D ,and doesn't go through all the individual gears.

[Vectors2final]

Originally Posted by CanAutM3

I think folks are not understanding DCT. DCT is not a sequential gearbox.

It can skip gears without problem. Pull the (-) paddle quickly twice and it wil skip a gear. Pull it quickly three times, and it will skip two gears. Press the kickdown button while holding the downshift paddle and it will skip as many gears as needed to be in the optimal gear for acceleration.

That being said, when downshifting, DCT needs to open both clutches to blip the throttle and match the revs and then re-engage the proper clutch. There is a time delay to complete this; however it is at least as fast a process than it is on manual or SMG transmission. It may seem slower because all you can do is wait while the servos do their stuff; on a 6MT, your mind is focused on completing the task at hand.

That's the one...