Teach me CDV

[AndyK5]

I have the basic knowledge but feel free to explain like ELI5. How does CDV work? What does it do? Does it kick in while pressing the clutch or releasing it or both. Why is it good, why is it not good. What can CDV screw up, what does CDV save you from?

This of course if you truly know. Not looking for:

I think....
It sounds like....
My friend said....
I saw a youtube video....
A member on this forum claims... etc..

If you have the mechanical knowledge, please do share.

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[lucky phil]

I have the basic knowledge but feel free to explain like ELI5. How does CDV work? What does it do? Does it kick in while pressing the clutch or releasing it or both. Why is it good, why is it not good. What can CDV screw up, what does CDV save you from?

This of course if you truly know. Not looking for:

I think....
It sounds like....
My friend said....
I saw a youtube video....
A member on this forum claims... etc..

If you have the mechanical knowledge, please do share.

It's a simple restrictor port in the clutch line. Easily removed. Plenty believe removal is the path to shifting nirvana. I have my doubts but I've never seen a blind test to verify the claim and a blind test is the only test worth a damn in my view.

Phil

 

[Oodis]

I have the basic knowledge but feel free to explain like ELI5. How does CDV work? What does it do? Does it kick in while pressing the clutch or releasing it or both. Why is it good, why is it not good. What can CDV screw up, what does CDV save you from?

This of course if you truly know. Not looking for:

I think....
It sounds like....
My friend said....
I saw a youtube video....
A member on this forum claims... etc..

If you have the mechanical knowledge, please do share.

Bloke

Basically it slows down the fluid pushing the clutch plate therefore increasing the time it takes for the clutch to gain full clamping pressure.

Pros: easier on drive train, easier to drive ie hard to stall.

Cons: not good for spirited driving, launching etc and in my opinion increases the chance of excessive clutch wear due to exposing the clutch to a small amount more of slip in how it operates.

I have read that it can effect clamping pressure as well but i have no experience or data on that.

my 2 cents

cheers

 

[AndyK5]

It's a simple restrictor port in the clutch line. Easily removed. Plenty believe removal is the path to shifting nirvana. I have my doubts but I've never seen a blind test to verify the claim and a blind test is the only test worth a damn in my view.

Phil

Yes, same reason why I am asking . There are people who claim supra transmission is awful due to cdv. I never felt like it was awful but am also wondering if it would be better without cdv. Precisely why i made this thread because everything out there is he said she said and assumptions.

Bloke

Basically it slows down the fluid pushing the clutch plate therefore increasing the time it takes for the clutch to gain full clamping pressure.

Pros: easier on drive train, easier to drive ie hard to stall.

Cons: not good for spirited driving, launching etc and in my opinion increases the chance of excessive clutch wear due to exposing the clutch to a small amount more of slip in how it operates.

I have read that it can effect clamping pressure as well but i have no experience or data on that.

my 2 cents

cheers

So i am thinking if the fluid is slowed down, its slowed down in both ways…meaning both engage and disengage is slow. I can see how this in theory should make taking off and shifting less abrupt but conversely people are saying cdv makes shifting harder…which makes no sense.


In general if I take a 10-15 year old car with a manual transmission, even with a brand new clutch, the slower i am in engaging and disengaging the clutch the more i am letting it slip against flywheel and the more material I wear off. If done very very slowly you get that dreaded smell of burning clutch.

The nirvana is releasing clutch as fast as possible while still not jerking the drive train around and I am not sure if CDV tolerance is above or below that (where above would cause excessive wear)

 

[Oodis]

Yes, same reason why I am asking . There are people who claim supra transmission is awful due to cdv. I never felt like it was awful but am also wondering if it would be better without cdv. Precisely why i made this thread because everything out there is he said she said and assumptions.



So i am thinking if the fluid is slowed down, its slowed down in both ways…meaning both engage and disengage is slow. I can see how this in theory should make taking off and shifting less abrupt but conversely people are saying cdv makes shifting harder…which makes no sense.


In general if I take a 10-15 year old car with a manual transmission, even with a brand new clutch, the slower i am in engaging and disengaging the clutch the more i am letting it slip against flywheel and the more material I wear off. If done very very slowly you get that dreaded smell of burning clutch.

The nirvana is releasing clutch as fast as possible while still not jerking the drive train around and I am not sure if CDV tolerance is above or below that (where above would cause excessive wear)

Bloke

i track my car with the CDV still in, does it cause an issue well sort of but nothing to bad.

But i am sure that it does cause excessive wear how much i dont know?

I feel it more street driving with first to second shifts.

Dont feel any issues down shifting due to rev matching on down shifts but im sure it is still using more clutch material then necessary

cheers

 

[Stvee]

I have the basic knowledge but feel free to explain like ELI5. How does CDV work? What does it do? Does it kick in while pressing the clutch or releasing it or both. Why is it good, why is it not good. What can CDV screw up, what does CDV save you from?

This of course if you truly know. Not looking for:

I think....
It sounds like....
My friend said....
I saw a youtube video....
A member on this forum claims... etc..

If you have the mechanical knowledge, please do share.

There is a thread that covers all of this. The short version is that it makes starting from a stop and shifting from 1st to 2nd feel much more natural.

https://www.supramkv.com/threads/diy-clutch-delay-valve-cdv-removal.18268/

 

[ApexTheory]

I removed mine after about 1000km. It's not a blind test, but I feel it made the 1-2 shift slightly more direct in relation to the clutch pedal.

It's technically a reversible mod if you don't mangle the restrictor on removal. Here is mine...definitely looks to be a 75% restriction or so. Someone with fluid dynamics knowledge would have to chime in on how this affects pressure/flow in the line and on the slave cylinder.

My thoughts are as above...it's purpose is to bias wear away from the drivetrain and towards the clutch in cases of imperfect shifting.

 

[AndyK5]

I removed mine after about 1000km. It's not a blind test, but I feel it made the 1-2 shift slightly more direct in relation to the clutch pedal.

It's technically a reversible mod if you don't mangle the restrictor on removal. Here is mine...definitely looks to be a 75% restriction or so. Someone with fluid dynamics knowledge would have to chime in on how this affects pressure/flow in the line and on the slave cylinder.

My thoughts are as above...it's purpose is to skew wear away from the drivetrain and towards the clutch in cases of imperfect shifting.

I think so too, some people are saying it is to protect the clutch but if anything this would increase clutch wear not decrease.

 

[bk5]

Cons: not good for spirited driving, launching etc

I disagree. I think the CDV helps with launching as it provides a nice consistent slip, so all I need to worry about is "dumping" the clutch, not modulating it.

I pulled a 1.83s 60ft time in a stock (to the tires) A91MT. Which, IMHO is a pretty good time for a RWD manual car on PSS.

That being said, if someone on a stock tune and tires with a CDV managed to do better, I'd be willing to give it a go.

i suspect some of the hate attributed to the CDV might have to do with the anti-stall "help" the car seems to have. On all my other cars, I very occasionally lug the engine, especially starting in second gear. I don't recall ever actually lugging the Supra in an entire year of driving, and I start in second almost every time I make a right turn. Sometimes the engagement is just too smooth.

 

[AndyK5]

I disagree. I think the CDV helps with launching as it provides a nice consistent slip, so all I need to worry about is "dumping" the clutch, not modulating it.

I pulled a 1.83s 60ft time in a stock (to the tires) A91MT. Which, IMHO is a pretty good time for a RWD manual car on PSS.

That being said, if someone on a stock tune and tires with a CDV managed to do better, I'd be willing to give it a go.

i suspect some of the hate attributed to the CDV might have to do with the anti-stall "help" the car seems to have. On all my other cars, I very occasionally lug the engine, especially starting in second gear. I don't recall ever actually lugging the Supra in an entire year of driving, and I start in second almost every time I make a right turn. Sometimes the engagement is just too smooth.

My issue is at the race track. I never launch this car anywhere, but at the track I change from 2->3 and 3->4 pretty fast, even if I am purposefully slow with actuating the shifter, I dump the clutch completely after the shifter is fully engaged.

In those situations the clutch pedal is too slow to follow my foot where I can lift faster than the pedal can disengage and the pedal slaps the bottom of my sole at the top of the range as my foot got there half a second earlier than the pedal did.


I think CDV can be blamed for that for sure. No idea if that is a good thing for the clutch material. It can not be good for it, that is for certain.

 

[Pizza]

My issue is at the race track. I never launch this car anywhere, but at the track I change from 2->3 and 3->4 pretty fast, even if I am purposefully slow with actuating the shifter, I dump the clutch completely after the shifter is fully engaged.

In those situations the clutch pedal is too slow to follow my foot where I can lift faster than the pedal can disengage and the pedal slaps the bottom of my sole at the top of the range as my foot got there half a second earlier than the pedal did.


I think CDV can be blamed for that for sure. No idea if that is a good thing for the clutch material. It can not be good for it, that is for certain.

This has happened to me at the track as well. I've gotten used to it now but it's definitely weird and I agree that removing the CDV would probably fix it. I'll probably eventually remove it, but it really doesn't bother me in normal driving.

 

[94boosted]

I'm still on the fence myself. Somewhat tempted to remove it as I'd really like to avoid premature clutch wear/failure. I do autocross the car very often which results in a hard & fast launch and a hard & fast 1->2 shift quite frequently, so longevity of the clutch, and drivetrain as a whole, is far more important to me than any potential gain in smoothness.

Stvee's excellent write-up makes it seem fairly straightfoward, especially if you buy the E90, non CDV containing slave. But like I said the #1 thing for me is longevity of the clutch & driveline.

 

[lucky phil]

My issue is at the race track. I never launch this car anywhere, but at the track I change from 2->3 and 3->4 pretty fast, even if I am purposefully slow with actuating the shifter, I dump the clutch completely after the shifter is fully engaged.

In those situations the clutch pedal is too slow to follow my foot where I can lift faster than the pedal can disengage and the pedal slaps the bottom of my sole at the top of the range as my foot got there half a second earlier than the pedal did.


I think CDV can be blamed for that for sure. No idea if that is a good thing for the clutch material. It can not be good for it, that is for certain.

When was the last time you changed your clutch fluid and bled the system out of interest?

Phil

 

[bk5]

I never launch this car anywhere,

To be fair, you used launching as an example of where the CDV has a negative impact. I just brought some hard data so we might compare notes and test that idea. As my personal observations are that the CDV helps with launching, especially at the track.

In those situations the clutch pedal is too slow to follow my foot where I can lift faster than the pedal can disengage

I have noticed this too. It's noticeable even when just sitting parked, if you "pop" the clutch pedal from the floor, it definitely moves slower than your foot.

But like I said the #1 thing for me is longevity of the clutch & driveline.

I think these are at odds with one another. If you're driving these cars hard, something has to take the beating, either the clutch or the driveline (axles, probably).

 

[94boosted]

I think these are at odds with one another. If you're driving these cars hard, something has to take the beating, either the clutch or the driveline (axles, probably).

Fair point bk5, if I had to choose clutch wear vs. braking stuff in the trans, diff or axles, I'll take the clutch wear. So in that case maybe sticking with the CDV is my best bet.

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