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Thread: CDV Hydraulics Explained

  1. #1
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    CDV Hydraulics Explained

    *** Edited 1-27 after moved to new thread ***

    To understand why I'm writing all of this I'll start with an analogy and my experience removing my CDV.

    __________________________________________


    Let's say that "Steve" on another forum states that he has figured out how to remove the top speed governor on our cars and writes up a DIY so that others can do the same. Then "Bob" posts that he did the mod, and while he hasn't been able to test the top speed, he posts that the engine has more power and the car runs much smoother at all driving speeds.

    Since I haven't driven Bob's car either before or after the mod, I can't say that his observation is wrong, but there's really only two possibilities:

    A) His observations are wrong and it's all in his head.

    OR

    B) Something else is responsible for the observed changes because whichever method BMW uses to limit the top speed, it's not going to have any effect on the car at normal driving speeds.


    __________________________________________


    That's where I'm at with the CDV. I had read about removing it on countless forums and the consensus, from those that posted anyway, was that it was a simple and virtually free mod that resulted in a much better "feel" to the clutch. All sorts of adjectives were used to describe the difference, and since the CDV is a restictor valve, they certainly all sounded plausible so why not remove it?

    It sat on my mod list until about a year ago when I needed new brakes. I figured since I was going to be completely fushing out the fluid it would be the perfect time to replace the brake and clutch lines with stainless steel ones and remove that pesky CDV. It was really easy, I just took the old line out with the CDV and replaced it with the new stainless steel line. It was when I got to the flushing and bleeding part that I realized that the whole clutch system wasn't at all how I had imagined it in my head. I could see that it was just two rigid mechanical lever systems, joined together by a simple hydraulic line. That meant that virtually NONE of the claimed benefits of removing the CDV were even possible.

    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.


    So now I'm not sure what caused what, but I am sure of what is and isn't possible and is the basis for my opinion:

    IF YOUR NOT SIDE STEPPING THE CLUTCH OR RELEASING IT SO FAST THAT YOUR FOOT IS COMING OFF THE PEDAL, REMOVING THE CDV WILL NOT HAVE ANY EFFECT


    To show why I don't believe the CDV can explain the changes that people are observing, I'll tackle four things:

    1 - Hydraulic systems and their characteristics
    2 - The layout of the clutch system in our E 46's
    3 - The CDV valve itself and what it can and cannot do
    4 - Pneumnatic systems and how their characteristics can explain a lot of the observations



    ***End edit ***



    Part 1 - Hydraulic Systems and their characteristics

    A hydraulic system is nothing more than a tube filled with an incompressible fluid and a piston at both ends. The idea is to use the incompressible property of the fluid to transmit movement of one piston into movement of the other piston. In the case of a clutch or brake line, where the forces are pushing liquid into the hydraulic line, ANY movement on one side, MUST have a corresponding movement on the other side. More precisely, as Bernoulli figured out a few hundred years ago, a volume of movement on one side will result in the exact same volume of movement on the other side.

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    Diagram(A) is the simplest hydraulic system and is the type used for the clutch in our cars. Because the fluid is incompressible you can see that if you moved Piston(1) one inch, Piston(2) would also have to move one inch. If the clutch line was perfectly straight as in the diagram, you could just replace it with a metal rod and it would behave exactly the same. What's cool about hydraulics though is that it doesn't matter how long the line is or how many bends there are in it, all that matters is that the fluid is incompressible.

    Diagram(B) is a little more complicated and is a simple diagram of what is used in the braking system. Let's assume:

    - Piston(3) has an area of 10 square inches and represents the COMBINED surface area of all of the caliper pistons.
    - Piston(4) has an area of 1 square inch
    - you push the brake pedal attached to Piston(4) with 100 lbs of force and it travels one inch


    When you exert 100lbs of force on the pedal, because I made Piston(4) one square inch in area, the pressure in the fluid is 100PSI - Pounds Per Square Inch.

    On the other side of the system, the 100PSI is exerted over 10 square inches so the force on Piston(3) = 1000lbs

    The idea of turning 100lbs into 1000lbs might seem like black magic but it's really not and there is another ways to show it.

    Using Bernoulli's Principle, the one inch square Piston(4) moved one inch which means one cubic inch of fluid was moved. On the other side, Piston(3) has an area of 10 square inches, so one cubic inch of fluid would move it one tenth of an inch:

    Movement(4) = 1 inch --> We'll call this Distance(4) or just D4
    Movement(3) = .1 inch --> We'll call this Distance(3) or just D3

    Newton figured out a few hundred years ago that all energy within a system is conserved:

    Work is defined as Force x Distance --> W = F x D

    To conserve the energy in our system, the work done on one side of our system must equal work done on the other side.

    Above I set the force on Piston(4) = 100lbs -- We'll call this F3

    Now we have the equation:

    F3 x D3 = F4 x D4

    And solving for the unknown F3 gives us:

    1000lbs x .1in = 100lbs x 1in

    Thats the principle behind the hydraulic brakes in our cars, and the hydraulic jack you use to lift it; you're converting a small force over a large distance, into a large force over a small distance.
    Last edited by wsmeyer; 01-27-2013 at 05:05 PM.
    2006 CiC 6MT
    ZHP, Cold Weather, Xenon
    Sapphire Black / Black Leather / Black Cube

  2. #2
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    Part 2: The clutch system in our cars.

    (reserved)
    2006 CiC 6MT
    ZHP, Cold Weather, Xenon
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  3. #3
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    Part 3: The CDV. What it is, and what it can and cannot do.

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    2006 CiC 6MT
    ZHP, Cold Weather, Xenon
    Sapphire Black / Black Leather / Black Cube

  4. #4
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    Part 4: Pneumatic systems and how it can explain many of the observations.

    (reserved)
    2006 CiC 6MT
    ZHP, Cold Weather, Xenon
    Sapphire Black / Black Leather / Black Cube

  5. #5
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    Moved to its own thread.

    PM me what you'd like the title to be and we can change it.

    Looking forward to reading the rest.
    -Be yourself; everyone else is already taken.
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  6. #6
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    Subscribed for the knowledge drop. So far so good.
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  7. #7
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    Cool stuff, combine that with a compressible gas in a closed system and you have the Citroen 1960's hydraulic suspension.
    325i, 2005, 5MT, Silvergrey, leatherette, Sport package; CDV delete, KONI STR-T & KYB Excel-G, Weisslichts, Stewart H2O pump

    328i, 2010, 6MT, Spacegrey, dakota leather, M-sport packages; has a 330 intake but no tune - yet
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  8. #8
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    I'm all in.

  9. #9
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    Ooh, science!! I'm all ears!

    Travis | '04 ZHP sedan 6MT | WeĭssLĭcht AEs | BMW clears | Plasti-Dipped grilles | BMW Perf. intake | TMS Power Pulleys | DIY aux-in | !CDV
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  10. #10
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    I am very curious about the physics behind it

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