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Thread: CDV Hydraulics Explained

  1. #11
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    I haven't forgotten about this but I've been too busy the last few days to add much to it. I did add something of an intro after it was moved to it's own thread, an in preparation for explaining the CDV I carefully dismantle mine the other day and took pictures of it. Just like the whole clutch system, it wasn't at all how I had imagined it in my head and I spent a lot of time wondering in my head why they made it the way they did. Then, all of a sudden, it was like a light bulb going off in my head and I am now completely convinced that I know why they put it there and what it is supposed to accomplish. It's not a safety feature at all, it's there to reduce driver fatigue.
    2006 CiC 6MT
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  2. #12
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    Quote Originally Posted by wsmeyer View Post
    I haven't forgotten about this but I've been too busy the last few days to add much to it. I did add something of an intro after it was moved to it's own thread, an in preparation for explaining the CDV I carefully dismantle mine the other day and took pictures of it. Just like the whole clutch system, it wasn't at all how I had imagined it in my head and I spent a lot of time wondering in my head why they made it the way they did. Then, all of a sudden, it was like a light bulb going off in my head and I am now completely convinced that I know why they put it there and what it is supposed to accomplish. It's not a safety feature at all, it's there to reduce driver fatigue.
    I appreciate you taking the time to even write it to this point! Driver fatigue eh? That is definitely an interesting point and I'd love to hear the reasons you came to that conclusion.


    After all of that, I DID feel a difference in my clutch, and it definitely felt better, but I had also changed three, possibly four things at once:

    1) Rremoved the CDV.
    2) Swapped out the rubber line for a stainless steel one.
    3) Completely flushed the fluid.
    4 - possibly) If there was air in the system, it was bled out.

    From my own experience:

    I replaced my clutch in June 2012. During clutch job, I also bled my clutch with a full brake flush. My latest brake/clutch flush was in Aug 2012. In Sept 2012, I removed my CDV and rebled the clutch and brakes again. I am still using the rubber clutch line with no "dummy" CDV in place. I certainly feel a difference in my clutch pedal even with use the CDV deleted.

    While this doesn't exactly eliminate all the variables, it does help gauge which variables are more of a factor to the feel of the clutch. My "feel" of the clutch may and very well be completely different from the "feel" your car has. I just think the CDV alone has a significant contribution to the change in the feel compared to the rest of the variables.

  3. #13
    I'm going to copy a couple of my posts from another thread here - I, too, tried to dispel some rumors and misinformation surrounding what the CDV delete does, and I think they'll serve well enough to add to the discussion:

    Quote Originally Posted by RITmusic2k View Post
    The biggest benefit you get from the CDV delete is consistency and feedback.

    The thing to remember about the CDV is that it is a limiting factor, not a scaling factor. In other words, the CDV doesn't make the clutch close at some set fraction of the speed that your foot moves on the pedal - it lets the clutch close at the same speed your foot moves on the pedal *up to a certain point*, after which any faster motion from your foot doesn't increase the speed at which the clutch closes.

    The effect from the driver's standpoint is that while at slow pedal-release speeds the clutch engagement point correlates nice and directly with clutch pedal position, but when you try to release quickly, the engagement point moves artificially (and inconsistently) higher up the pedal's travel. This throws your muscle memory out the window, and most often results in you applying throttle too soon, which translates into excessive and unnecessary clutch slippage.

    For that reason,

    Quote Originally Posted by bimmeryota View Post
    I never did this mod but heard that it made the biggest primary difference in paralell parking where precise modulation is key. Is this true for those that have done it?

    the thing you heard is exactly backwards - at low engagement speeds you will not notice a difference, but when driving aggressively and shifting quickly, there's a significant difference between cars with and without the CDV installed.

    At the end of the day, rev-matching your upshifts and downshifts does much more to prolong the life of the entire driveline, and it's only possible to learn how to do this properly on a car without a CDV installed.
    Quote Originally Posted by RITmusic2k View Post

    Quote Originally Posted by aurelius View Post
    for me the biggest improvement was in low speed parking situations where you're on & off the clutch pedal in 1st gear & reverse. Should be mod #1 on any model that has it.
    Hmm, that brings up a bit of detail that I may have left off my reply - the CDV's effects aren't based on *vehicle* speed, only on *clutch pedal travel* speed. So during parallel parking or other low vehicle speed scenarios, I can see how it could still come into play - especially if you quickly 'pulse' the clutch pedal, or really do anything at all that would exceed its restricted flow rate.

    Zebra Horse Power - PSA: The Hypermiling Thread
    Kevin Savino-Riker
    Cogito Automation, LLC.
    I'm new at BMWs.

  4. #14
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    Well written guys

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  5. #15
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    Hmmm. Maybe a blind test is in order? Have someone remove, install, or do nothing to another person's car (or many cars/drivers to increase sample size) and not inform the driver(s) for a month... The CDV may or may not be deleted every weekend... Record the driver's observations on the feel of the clutch on a per drive basis. If it's fairly evident that the CDV does have a significant impact on the clutch, the driver's observations will improve with the removal of the clutch. Seems simple enough. Of course you can choose to trick the driver by stating a mix of true and misinformation by saying sometimes, "yeah, I removed the CDV" (when in fact you didn't and when in fact you did, etc....).

    Fuelly

  6. #16
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    I think the biggest difference is felt during downshifts. The clutch pedal can be released faster during a downshift without a CDV. With a CDV I feel like I have to leave my foot near the engagement point until it finally engages before fully removing my foot off the pedal.



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  7. #17
    I'm absolutely in agreement with you, Derek. It's a scenario that generates repeatable results highlighting the difference between CDV and no CDV.

    When I first got my car, I tried dropping two gears and stomping on it to see how quickly the car could get out of the way. Coming from a car where pedal position corresponded exactly to clutch disc position, I got hard on the gas as soon as my foot was off the clutch pedal, and the engine revs surged and settled like a failing clutch would, because, well... the clutch was slipping exactly the same way due to that restricted flow.


    Actually, the above is a little simplified... the truth is that the clutch disc makes contact with the flywheel much sooner than the perceived "engagement point"... what we typically think of as clutch disc travel is actually more like "clutch disc travel" over the first 30% of clutch pedal travel followed by "progressively increasing clamping force" over the remaining 70% of clutch pedal travel; the engagement point we feel is somewhere in that 70% range where you hit a threshold of sufficient clamping force to hold the clutch disc and flywheel together without slippage.

    That threshold amount of force rises slightly with increasing disparity between the rotational speeds of the clutch disc and flywheel (e.g., when revs are poorly matched), but for the most part it's in the same place. More accurately, the relatively-fixed amount of clamping force required to engage the clutch corresponds with a relatively-fixed position in the clutch pedal's travel. Or it should, if there's no flow restriction.

    As quickly as you can get to that position, and thus that amount of clamping force, is how quickly you can get on the throttle after a shift. But the rate of application of that clamping force is what the CDV limits. So if your foot makes it to the point where that amount of clamping force is normally applied sooner than that force is actually applied, you'll get slippage until the CDV allows enough fluid through. In my experience, my foot could get there an entire second before the CDV caught up. That's enough time for some very annoying over-revving and slippage.

    Zebra Horse Power - PSA: The Hypermiling Thread
    Kevin Savino-Riker
    Cogito Automation, LLC.
    I'm new at BMWs.

  8. #18
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    Great thread!



  9. #19
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    When I did the CDV on my ZHP, I kept the stock line and did not change the fluid. If anything I introduced air into the system (but I doubt I did since I didn't lose a single drop). I did not feel a night and day difference as some report. However, I did find 1-2 shifts to become considerably smoother and found myself with an improved ability to feel the engagement point.

    I want to point out that this explanation is inadequate. In real life, most systems do not perfectly follow the ideal cases we're taught in high school physics. The biggest thing to note is that the restriction in the CDV is not a smooth reduction; it suddenly becomes a very small diameter. This does make a difference in hydraulics and probably accounts for the difference most people feel. Look in any introductory physics textbook -- every hydraulics problem with diameter changes will specifically note that the change is achieved smoothly. A sudden decrease in diameter will cause resistance and energy loss.

    More info: http://books.google.com/books?id=rbc...action&f=false
    Last edited by terraphantm; 02-06-2013 at 07:30 PM.

  10. #20
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    I'm looking forward to the next installments!

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