I know there's been a lot of discussion about how the DCT behaves when sitting at a stop, and also how the "double paddle declutch" behaves as well. The general understanding of this is that pulling both paddles is like pressing the clutch pedal in on a manual transmission, and when sitting at a stop (like at a traffic light), fully stopped, the TCM is also declutching the trans.

I've also read some of the comments from GM engineering on how the TCM manages the engagement of the clutches to provide a smooth feel to the driver when creeping, or coming off a stop. There was a comment which suggested the TCM is controlling clutch engagement at a stop based on brake pedal pressure. So as you release the brake pedal, the clutches engage more, and make it feel like a traditional torque converter configuration, to provide a smooth engagement.

Based on reading this information, I paid close attention to the vehicle behavior yesterday when I was out and about, just running some errands, and here's what I observed:

The clutch engagement is absolutely tied to brake pedal pressure, and you can be at a full stop and still have the clutches slightly engaged even though the vehicle is not moving.

While driving around, I used the double-paddle declutch feature quite a bit when coming to a stop, to get a sense of what was going on. What I was doing was pulling both paddles when starting to slow down for a red light, then I released the right paddle and continued to hold the left. Then after I was fully stopped, I'd release the left. I did the single paddle release to leverage the soft re-engagement instead of the hard re-engagement from releasing both simultaneously.

What I noticed is when I was fully stopped, and then released the left paddle to re-engage the trans, I did experience a slight change in RPM. I thought this was odd, so while still stopped, I re-engaged declutch, and again, small RPM blip. Released the paddles, and a small RPM blip.

Then it dawned on me ... my driving behavior (for decades now) is such where I apply the minimal amount of brake pedal pressure to keep the vehicle from rolling when at a stop. So while still stopped, I pressed the brake pedal harder than needed (ie: put it to the floor), and then tested the declutch again. This time, there was no RPM blip during the engagement or disengagement of declutch.

So ... if I continue the behavior of only applying enough brake pedal pressure to keep the vehicle from rolling, it seems the clutches are still partially engaged and I'll bet will wear faster over time. Certainly that behavior is causing clutch material to shed into the fluid at the very least, even if just a small amount. It's going to take me some time, but I need to retrain myself to apply enough brake pedal pressure to completely disengage the clutches when at a stop; or use the declutch process and continue to hold the left paddle while stopped, until it's time to start moving again.

I wonder if this behavior can explain why some DCT transmission have faster wear than others, or have more material in the fluid than others, etc ... ?

 

In a wet clutch you have different regimes of engagement. When the plates are fully separated there is effectively no torque transmittal. As they get closer they get into a regime where torque starts getting transmitted by hydrodynamic fluid forces but there is no actual friction plate contact. As they get closer the high spots start making contact and transmit more torque. Eventually when enough contact pressure is applied you get full engagement. The difference in RPMs you detect with your foot lightly on the brake pedal is likely due to the hydrodynamic forces.

 

one of these days i will pull both paddles in and bring the revs up to 3 k and see if the car will paddle launch with t/c turned off /on

 

Could be, but there is definitely a threshold where the clutches will make contact. I've been meaning to hook up my MPVI3 and HP Tuners setup to see what is available for logging. Maybe I can see the actual engagement % and TQ transfer through a VCM data channel ....

It will allow you to do a burnout, donuts, etc ... Fast Forward to 6:18 time marker



This one is actually a better demonstration ...

 

but mine is an e ray

 

About 5% of the torque is transmitted before the plates even make contact. Below is a paper exploring the engagement characteristics of wet clutches.

https://onlinelibrary.wiley.com/doi/...20as%20follows.

 

I am going along with RKCRLR and the importance of knowing the DCT is a wet clutch system.
Even a torque-converter transmission has wet clutches which are somewhat engaged when stopped. No one puts the car in neutral and the clutches last.
So I will enjoy the DCT and let it take care of engagement and disengagement. Changing the fluid every 3 years should keep the DCT in good shape.
By now there are over 200,000 C8's out in the wild. The DCT transmission is here to stay for most sports cars.
Best wishes to all.

 

Sure, I'm not discounting that at all, and I'm certainly not concerned with whatever wear may be happening as it is within the engineer's design specs. And this thread is in no way an attempt to convince anyone else to do anything different than they already do.

That said,for me (and no one else), pressing the brake pedal a bit harder than what I'm used to doing, to fully disengage the clutches (including any hydraulic torque transfer) isn't going to hurt anything.

I'm a huge DIY person, although I've no plans to do any major mods to the C8 at this time .... I do know myself, and after the newness wears off, and assuming 2 or so years of trouble-free operation (fingers crossed), I'm sure I'll start to investigate some mods to it. But due to my personality, I don't do any mods until I have a really strong grasp of how things are operating.

So the observations I'm making, and my desire to understand the nitty-gritty on the specifics, is just how I learn and eventually become comfortable with modifying things.

It took me about a year to learn what I needed to become comfortable in tuning my wife's 5.7 Hemi with the Whipple supercharger, and now I'm very confident in how everything in that platform functions ... thus I've no problem experimenting with changes on it.

For the C8, it's very different than my LS2 powered GTO with the M6 trans, so I want to start learning details to become familiar. I've been on the HP Tuners forum for a while as well, and am chatting with some guys there to get an idea of what is available for logging. Today s a rainy Valentine's day, so I won't be testing it out ... but I think I may be able to hook up the laptop tomorrow and see what is available.

https://forum.hptuners.com/showthrea...Without-Unlock

I think it's safe to say the actual clutch engagement does occur before your foot is 100% off the brake pedal, the question I have is ... does that happen at a point where the vehicle is not moving yet, or does it only happen after there is some momentum... hopefully I can answer this for myself, definitively with VCM Scanner.

 

If you are stopped in Drive on level ground and let off the brake pedal but not give it throttle the computer will command some throttle and you will start moving forward until there is no/minimal slippage between the input and output shafts so you know that it doesn't 100% engage the clutch initially. Whether or not the clutch is 100% engaged once there is on slippage I don't know.

 

Yeah, electronic controlled throttle setups allow the PCM to do things like this. The days of the engine throttle being directly controlled by the pedal are long gone.

What I want to know is when physical contact happens though. Not when 100% engagement occurs, but when does the initial physical contact happen. I believe it can happen even when there's no accelerator pedal input and the vehicle is still standing still, but it's allowing enough slippage to not cause stalling or lugging.

VCM Scanner should have some data channels I can log to determine this. If my hunch is right, the most minimal brake pedal input could allow physical contact to be made. Again, I'm not saying I'm overly concerned about it, but I'd like to know.

 

It would depend on how GM set up the system. If it is based on torque then you could have torque being transmitted without contact. Similar for position since you wouldn't know at what position first contact is being made.

 

Exactly, and that's what I want to test and answer with objective data from logging the system.

I'd wager it is a torque based system, and if so, there are a ton of variables at play. More than just throttle is used by the PCM to hit the request as well (spark timing for one). Driver demand is usually tied to the pedal input, but the resulting TQ request is usually much more than just driver demand. Being on an incline or decline has an impact, as does if the AC compressor is on or off, etc ...

 

Yes, as described by Josh Holder, as pressure on the brake is reduced, the clutch pressure increases. What we all have experienced (as @RKCRLR stated), is that as some point when the brake is released enough that the car starts to move, the ECM will open the throttle a crack and RPM will increase.

And I have conducted the same test with the brakes fully depressed and using the double paddle declutch, I feel absolutely no change. So whatever difference there is in clutch engagement declutched and not declutched, with the brake pedal fully pressed is totally negilgible. Not saying there is or is not slippage, only that with the brake pedal fully pressed, there is no noticeable difference. I never explored that middle-world "twilight zone" case where the brake pressure is light with and without declutching, so I don't know about the rpm blip in that circumstance.

But one other thing I did, and reported it in other threads - with the pedal fully applied, switching the gear selector between P,N,D, and R, I do feel something. I think that its the shift fork motion, and there may, or may not be some rotation of input shaft to facilitate the positioning of the shift forks-maybe some torque transmission via the "hydrodynamic torque" of fluid between the clutch surfaces. Its engaging 1rst gear on the odd shaft, and engaging 2nd on the even shaft. Whether there is any remaining driveline tension after the forks are positioned, with the brake pedal fully depressed is inconclusive - all I can say is that with the brake pedal fully pressed, I feel no difference with or without the double paddle declutch.

Folks have discussed pulling the paddles as they slow down for various reasons-some don't like to feel that downshift to 2nd or 1rst as the car slows. I don't do it, but for sure it does avoid clutch actions as the car downshift.

 

The throttle opens because the torque request goes up. More than just throttle opening is happening though. I'm sure spark timing is also changing... Part of the increase in the TQ request is due to more torque needed to overcome the increased friction from the clutch starting to engage. It's a very complicated and spider-web of calculations happening.

No idea what you mean by the "middle-world twilight zone" comment ... but as I clearly stated above, my driving style has me applying the minimal amount of brake pedal pressure needed to keep the vehicle stationary, and no more. This is a subconscious habit over decades of driving, which I am sure is not unique to only me.

That's the premise of my entire observation and resulting discussion.

If your driving style has you applying maximum brake pedal pressure at every stop, then you would not experience this .... but you should explore the light brake pedal situation I am describing and then you can understand what I am describing.

In any case, I'll still connect to the vehicle's computer and collect real data to gain an understanding of what is really going on.

 

I meant exactly what I wrote - "middle-world "twilight zone" case where the brake pressure is light with and without declutching", which I thought was the case you described - light brake pressure, with and without declutching. That is where you said you can feel a difference when you double paddle declutch. I just never explored that situation so can't comment on the rpm blip you observed. I only checked it with brake pedal fully depressed.

 

I guess the "Twilight Zone" comment made it sound like the situation I'm describing (ie: light brake pedal when at a stop) is something which only exists in an alternate/fictional reality, and thus I must be the only person who experiences it.

I don't think that's the case at all, and I'd bet a lot of drivers have this habit ... So knowing if the clutch can make any physical contact at this time (not 100% engagement, not hydraulic TQ transfer, but any physical contact) would be helpful info.

I am going to assume it is, and will work to change my habit (easier said than done) and start applying full brake pressure when stopped (or I'll engage declutch), at least until I can log some data and obtain objective proof as to what is really happening inside the DCT under this scenario.

 

The question is how would use the data to know when first contact is being made? You know there would be no contact at 0 psi and 100% contact at full pressure but how would you know when contact is being made? It will initially take some pressure to overcome the spring force and then some more pressure to overcome the hydrodynamic forces until contact is made.

 

not sure, as I haven’t yet seen what data is available. There will be much more than just clutch pressure PSI though. As a point of reference, I log over 100 data channels on the Durango (both engine and trans) when I collect data on it.

After I’ve had a chance to connect to the C8, see what data is available, log it, and analyze … I’ll then know if it’s possible to answer the question.

 

It would be nice if someone was able to get the clutch engagement % reading from the ECM and then have it display through the HUD with a dash logic. Loved the dash logic in my C6, unfortunately I don’t think they make anything like this for the C8 (not that I’ve seen anyway). Slipping clutches at idle can definitely contribute to increased fluid contamination, engineers have most definitely factored in a certain level of acceptable debris that is allowed to circulate through the system. Different driving habits would explain why some fluid comes out darker than others at similar fluid change intervals.

Imagine someone that constantly drives through an area that, when at rest, they are at an angle allowing the clutches to slip for long periods of time on a daily basis. That slippage is creating dust at a greater rate than someone who has short stop intervals on level ground. Imagine sitting at an incline and feathering between brake and accelerator in a matter that forces the slippage at a much harder rate than someone that doesn’t sit in stop and go traffic. Both vehicles have 7,500 miles, however one may have significantly more dust floating in the fluid than the other. Now all that excess clutch debris is floating through your system, through the solenoids….

Having a better understanding of the clutch engagement % would absolutely benefit us.

 

Nope. Not meant that way. I don't think people make stuff up. When I wrote it, I was thinking of what you reported as the "mystery space" between the "binary" positions - the brake pedal is neither fully pressed, nor fully released - rather somewhere in between, just enough to hold the car still, where the clutches are engaging a little more, and where you noted a difference when you double paddle declutch. I never tested that case, but I certainly don't doubt your result. Its exactly how Josh described it. As the pedal pressure is released, the clutch starts to engage the odd shaft. Naturally, if you double paddle declutch in that situation its going to reduce the clutch pressure.

"Q: “When you are stopped in either drive or manual mode with your foot on the brake, are the clutch discs dragging to allow the car to creep or do they engage when you take your foot off the brake pedal.”

Josh Holder: “Yes, and yes. So the clutch engagement is controlled by hydraulic pressure. We apply pressure to the clutches which is how much friction they are applying and how much torque is being used to creep the car forward and that is a function of brake pedal pressure. Its not just on – off, because that would make it very clunky and awkward driving experience. So the more firm you are on the brake pedal, the less clutch pressure we’re applying. As you back off the brake pedal we’ll start applying more clutch pressure so the car’s ready to move like a torque converter automatic transmission even though this is completely different. So it operates the way people expect in automatic drive mode.”

Tadge: “Yeah, it’s a modern miracle that we can use friction clutches and have a drive away as smooth as it is. You think about you trying to do a manual, a traditional manual, dry clutch and engage it so smooth that it feels like a torque converter – whoo! “

Josh: “And we were paranoid and skeptical about being able to do that.”