CDV = Clutch delay valve. It is essentially an orifice placed inline threaded into the slave cylinder. It allows them to regulate how fast the clutch can go in and out (this is set by the size of the orifice which bmw uses, it can be more or less noticeable depending on how small the orifice is). The effect is a sort of delayed reaction on the clutch that will prevent a noob from dropping it and allows for smoother shifts if you don't know what your doing. If you do know what your doing it just makes it feel like your drunk or something and most people find it to be a hindrance.

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2004 e53 m54 with 6spd manual