Clutch hydraulic damper / delay valve, considering deletion...

[Vne165]

I do have a whistling brake booster though...a little sort of high pitched sigh emanates from under the dash when you push the brake pedal.

[Citroënbender]

I think a persistent whine from near the passenger seat is more common than a whistle in the brake booster. Does the pedal feel "right"?

Per the clutch damper, simply follow the hydraulic lines from firewall to slave cylinder to see if there's something lumpy in the middle.

[johnl]

How do I know if I have one fitted? Would my 2002 916 TS Spider have one?

I haven't examined a Spider / GTV closely enough to know that your car would definitely have one, but I'd say it's a fair bet...

If so then I'd imagine it is not unlikey to be located similarly to the CDV in the 147, which is 'hanging in space' above the gearbox, you can just see it under / behind the battery etc. with a torch.

It's a cylindrical object with a hard line in and a hard line out. Only the hard line from the MC can be unthreaded (at the CDV), the hard line to the slave cylinder is soldered directly into the CDV. Also, the hard line from the CDV to the slave cylinder is permanently attached to the rubber hose that plugs directly into the slave cylinder. To take the CDV out, the threaded hard line needs to be unscrewed, and the rubber hose / hard line detached from the slave cylinder and removed with the CDV as an 'assembly'.

Regards,
John.

[johnl]

For my own amusement as much as anything else, I've constructed some simple diagrams to help illustrate how I think the CDV as fitted to the 147 probably operates, based on the internal components. I've attached these as a Word Doc. with notes.

Regards,
John.

[Vne165]

Interesting, thanks for the diagram. I would agree that the rubber element would add a degree of 'compressibility' to the system, by design it would appear. Deletion would result in a more linear relationship of pedal travel to clutch slave cylinder actuation. Clever little jigger, but you'd have to wonder about it's durability over the years.

[johnl]

Yes I agree, these are impressively elegant, simple and clever widgets. Pity that what they do is so damned counter productive...

Longevity of the rubber may be of some concern, but I have to say that the rubber tube that came out of my cars' CDV appeared to be in truly excellent condition, with no deformation, cracks, excessive softness, or any other obvious issues. It's so clean that it looks like smooth black plastic.

From my experience, removing the rubber tube definitely results in a much more linear relationship between the pedal and clutch function. I still can hardly believe how much better the clutch is to use, how improved its' action is, and consequently how much better the quality of the gear changes are. At first it just seemed a bit better, but the more I use it and the more accustomed I become to the changed characteristics, the more I realise just how much better it actually is (and what I had been putting up with for ages...).

A wierd thing I've noticed since disabling the CDV; since I've had this car its' had what I was interpreting as a chronic significant 'rev hang', whereby the rpm refused to drop sharply when the throttle was closed and the clutch pedal depressed (my old Accord did a similar thing, which I cured by means of disabling the 'Idle Air Control Valve', another story...). 'Rev hang' is a common 'characteristic' associated with emissions control (i.e. a deliberate strategy inflicted primarily to reduce NOx), and degrades gear shift quality.  But, since deleting the rubber CDV tube, the rev hang symptom has largely disappeared...

More thinking needed as to why this has happened...

Regards,
John.

[Vne165]

Is it simply that the crankshaft remains 'coupled' to the transmission for longer when the delay device is in play, thus keeping revs matched to road speed? Remove the CDV and engine is disconnected faster.

[johnl]

Something like that, but not sure exactly how this would lessen the 'rev hang' thing.

Intentionally 'designed' 'rev hang' (for reduction in NOx) is generated by the ECU not allowing the throttle butterfly to close as quickly as the throttle pedal is lifted, or with non FBW throttles, by the ECU briefly opening the air valve that is otherwise used to keep idle speed at X rpm (say when AC is activated, or when other variable parasitic electrical loads exist via the alternator). It's an unfortunate electronic thing, not a symptom of a physical problem.

I just can't visualise how the clutch not properly disengaging could create a symptom that mimicks this. Keep in mind that if my analysis of the way the CDV works is correct, then the clutch disengagement isn't 'lagging' by the time the pedal has reached the floor, though it may well fail to fully disengage if too much pedal motion is 'wasted' by the CDV operation (i.e. deformation of the rubber tube).

All I know is that previous to CDV disablement the rpm would tend to 'hang' and as a result would be too high when the next higher gear was engaged and the clutch pedal released. This caused stiffness in the shift action (synchro resistance due to rotational speed differences inside the gearbox), 'lurching' as the clutch was released, and a sense of general ineptitude as a driver. This also caused issues for 'rev matching' while doing 'heel / toe' downshifts (mostly because response to throttle 'blips' was a bit unpredictable). It could be 'worked around', by adjusting shift technique, adding pauses in the process (like depressing the clutch pedal only after lifting off the throttle pedal, or lifting the throttle almost all the way and then pausing briefly before fully lifting off as the clutch pedal was depressed, and, slowing the speed at which the lever was moved), but this all resulted in unsatisfyingly un-instinctive, slow and clumsy gear shifting.

But now, the 'rev hang' has stopped happening, or at least is so improved as to no longer be an issue (bliss...). Gear shifts are now much faster, smoother, cleaner, lurching has gone, the process is instinctive, and I no longer have to actually think about it while doing it. But I can't figure out why, or the how of the why.

If (when the previous rev hang symptom was occuring) the clutch disengaged enough that it was actually possible to change gears (even with some clutch drag), then the clutch should have been disengaged enough that the engine wouldn't be being 'powered' by road speed acting through the still engaged gears. It doesn't make sense to me, a mystery...

In the end it doesn't really matter, I'm just enjoying how the car is now after ditching the Devils' damnable restricter...

Regards,
John.