CDV Hydraulics Explained

[wsmeyer]

I hadn't updated this thread in a long time because I really wasn't sure how to move forward. When I started this thread I had hoped that by examining the characteristics of hydraulic systems other people would also realize that most if not all of the claimed "benefits" of removing the CDV simple couldn't be attributed to the CDV. I didn't set out to prove anyone wrong, there are so many people with similar observations that there must be something going on. I just didn't believe that it was the CDV. Unfortunately the thread didn't progress because I didn't seem to convince anyone to even think of other possibilities, and most were so hung up on the CDV that they were just adamant that I was wrong because what I was proposing didn't jive with their observations.

Well, as it turns out, I was wrong. The CDV is definitely having a negative effect on the way the clutch operates. BUT, it's not because it's poorly designed or a "nanny" device there to help overcome poor technique. It's because the vast majority of us have inadvertently altered the system in a way that prevents the CDV from operating as intended. The overbearing CDV is merely one symptom of the problem, not the problem itself.

I stumbled across the key to all of this while thumbing through the Bentley service manual I purchased a little while ago. In the front of the manual, there is a chart with all the fluids listed, it all looks as expected:



But notice at the bottom where it says: "NOTE - See 340 Brakes for more brake fluid information"

In section 340 under brake bleeding procedures, there's this chart:



Which indicates either the use of DOT4 or DOT4 LV depending on the type of traction control system. That chart looks like this:



Our cars use DSC, the one that requires DOT4 Low Viscosity.

I myself had never heard this before and I had never even heard of "DOT4 Low Viscosity".

With a little googling I found a picture of the BMW brake fluid bottle with the part number on it: http://tinyurl.com/k6nrwkx

Depending on if you buy it in a bottle or jug the part numbers are:

12 ounces - 81220142156
gallon? - 81220142155

If you search for those part numbers you'll come across this over at Turner:

http://www.turnermotorsport.com/p-11...-bmw-part.aspx

Which clearly indicates this is the genuine stuff and is DOT4 LV. It also has the Pentosin label on it. That's not too surprising to me as I had read other places that the OEM fluid is Pentosin.

And this over at bmwpartsweb: http://www.bmwpartsweb.com/Part-Numb..._9BA48850.aspx

That lists those part numbers and Pentosin as the manufacturer, again, DOT4 LV.

Now we go over to Pentosin and things really get interesting. http://pentosin.net/f_brakefluid.asp

They list DOT4 for BMW up to 6/2002 and DOT4 LV for BMW's after 7/2002

So what's the difference between DOT4 and DOT4 LV? There are spec sheets for both on Pentosin's website:

DOT4 - http://pentosin.net/specsheets/Pentosin_SuperDot_4.pdf

DOT4 LV - http://pentosin.net/specsheets/Pentosin_Dot4_LV.pdf

The difference that is important here is the viscosity, they list it at different temperatures but I think they chose 40C as one of the temperatures as that would probably be a normal operating temperature ~ 104F

DOT4 = 7.9
DOT4 LV = 6.4

That means that DOT4 is 24% higher in viscosity than DOT4 LV.

If you look at the equations for fluid flow vs pressure: http://en.wikipedia.org/wiki/Orifice_plate there is a one-to-one relationship between viscosity and flow. That means that if you increase the viscosity by 24% YOU REDUCE FLUID FLOW BY 24%. As far as our cars go, we have reduced the maximum fluid flow through the CDV by 24% and that is causing the negative effects.

This also explains why I didn't feel any difference when removing my CDV. Up until that point, the dealer had done all work on my brakes and presumably had been flushing / refilling the system with the correct OEM DOT4 LV fluid.

So now the big question is WHY DO WE NEED DOT4 LV?

That I have no idea, the fact that it depends on the type of traction control system leads me to conclude that the intricate valving used requires a lower viscosity. That's just a guess and I don't know what negative effects on the ABS / DSC system there will be from using DOT4 but I'm just going to flush mine and put the DOT4 LV back in and call it a day. When in doubt, go OEM for me.

[danewilson77]

Wow.....mind blown. Great research. In effect, you're saying most diyer's don't use the correct fluid, and by removing the CDV gain the desired effect of a lower viscosity fluid.

[Crickett]

Well sheet . . . looks like it's out w/the Super Blue & back in w/the OEM!

[wsmeyer]

Wow.....mind blown. Great research. In effect, you're saying most diyer's don't use the correct fluid, and by removing the CDV gain the desired effect of a lower viscosity fluid.

Kind of, the problem is that DOT4 is a higher viscosity than what the system was designed for. The system being the brake and clutch hydraulic systems.

ONE SYMPTOM of the higher viscosity would be the overbearing CDV. Removing it will alleviate that symptom.

Other symptoms would be in the effectiveness / operation of the ABS / DSC systems. I don't really know what those would be but BMW certainly had their reasons for specifying DOT4 LV and the Bentley manual eludes to those reasons having to do with the ABS / DSC.

For me personally, I was using Motul RB600. Not because I felt that I really needed a "racing" fluid but because for a few extra $$ I would rather just purchase something I know is of the highest quality. Knowing what I know now I'm going to flush all of that out and use Pentosin DOT4 LV which I know is the correct viscosity and is also a well respected brand.

[danewilson77]

Got it. Thanks Will.

[johnrando]

Thanks William. I bought a bunch of DOT 4 LV for my brake job, so I have extra unopened liters for sale if anyone wants them. I'll sell for what I paid.

[terraphantm]

I was under the impression that the cars without the precharge pump didn't need DOT4 LV. Let me see if I could find the corresponding info in TIS.

Edit: Other way around according to TIS. So I guess we all need DOT4 LV. I wonder if Castrol SRF is considered LV. (despite what the Bentley says, 2003+ M3s have the MK60). Ideally I'd like to flush the system completely before putting in the SRF as it's quite expensive

Also I want to mention, I was always on OE fluid with my old ZHP. I did notice a difference with the CDV, but I will admit it wasn't night and day like a lot of folks claim. It was noticeable however -- and that makes sense. Just because it is "low" viscosity doesn't mean the effect will be eliminated. I also don't know if you can blame the issue entirely on brake fluid, since the CDV was installed in cars that don't specify DOT4 LV






I do think you might be right about the effectiveness of the ABS system. I noticed last winter that my ABS didn't seem very effective. It felt like it was getting triggered way too early, and more importantly, it felt ineffective at actually allowing me to stop. I didn't make the connection with SuperBlue until now.

[quikryptonite]

Thanks William. I bought a bunch of DOT 4 LV for my brake job, so I have extra unopened liters for sale if anyone wants them. I'll sell for what I paid.

How many liters did you end up using to flush and fill?

[M0nk3y]

I've never noticed a difference in ABS ability at high speeds with super blue, high speeds being 130.

The only time abs was kicking in early was when I started to overheat pads

Sent from my XT907 using Tapatalk 2

[Crickett]

I've never noticed a difference in ABS ability at high speeds with super blue, high speeds being 130.

The only time abs was kicking in early was when I started to overheat pads

Sent from my XT907 using Tapatalk 2

Is the Z4MC's DSC the MK 60 type (no precharge pump) like us E46s or the MK 20 (w/precharge pump) like the M3s?