e9 3.0CS Weber Carburetor installing?

[tom]

hello all,
i´m tom from germany and driving 1972 cs 3.0.

now i´m trying to find out what the difference will be if i would change to weber Carburetor? (i´m using 2 standard zenith Carburetors)

- more power? (how much approx.)
-what will happen to the consumption (17ltrs now approx)

- what is the coding on the Carburetors? Weber xx DOCE? xxx?
(double...)
-whats the diffenrence between 40/45...?
http://search.ebay.de/search/search...ion=compare&copagenum=1&coentrypage=search
- is it necessary to use 3 of the same? or can i mix up?

it would be great to get some infos/tips from you what to dio!

thank you!
best regards,
tom

 

[texcarguy]

Webers

Tom,
I'm no expert, but do have a triple Weber DCOE40 setup on my engine with a Korman manifold Pics here: [url]http://auldridge.org/bmwCS/index.htm [/url]. On a recent 2800-mile trip, I averaged about 18 miles per gallon, this at 70-80 mph cruise... However, this was with the 5-speed overdrive transmission. You'll have to convert this to metric terms!

My belief is one of the advantages of the DCOE's is simplicity of operation.. these carbs are dead simple, and just take a little attention to get synchronized, then leave them alone. Many similar installations use DCOE 38's (the numerical designation is the size of the primary bore of the carb.. presumably larger allows more gas, more power, less fuel economy... However, there is a limit.. too large, and the engine suffers from low-rpm torque... just too much air volume to get good flow. I suspect the 40's are about the max size you'd want to use on this engine.

I recently had a '72 Ferrari 365GTC/4 V12 with 6 DCOE38's, but then as a 4.2l engine, individual cylinder displacement was less than with the BMW. Interestingly enough, at wide-open throttle, the Bimmer 6 sounds VERY similar to the Ferrari V12 (that's the engine sounds, NOT the exhaust.. sigh)

About 20 years ago I did have my first 3.0CS with the Zeniths, and I remember always struggling with those carbs... maybe just my inexperience, but I always dreamed of putting on the Webers. I'll certainly vouch for the installation... trouble-free, good pulling power... of course, no way to objectively judge power output over the stock units.

Hope this helps!

 

[velocewest]

The number (38, 40, 45) refers to the size of the venturi (the choke, or minimum diameter of the hole in the carb) in mm. You want to have all the same, mixing is not a good idea because they will flow different amounts of air for a given throttle opening and you'll never get them balanced.

 

[jmackro]

velocewest had written: "The number (38, 40, 45) refers to the size of the venturi (the choke, or minimum diameter of the hole in the carb) in mm"

Well, that's sort of right, but let me take it a step further. Yes, a 40DCOE mounts to a hole that's about 40mm in diameter, while a 45DCOE mounts to one that's 45mm. Those are the two common sizes for Weber DCOE's (I think that 48's and 50's may also be available, but that's getting sort of obscure).

Webers have MANY replacable parts, which is what makes them so easy to tune (as well as to screw up). One of these parts is called the choke. Another is called the aux venturi. Both slide into the bore, and both determine the working diameter of the carburetor. For example, my 2 liter Alfa uses 40DCOE's with a 32mm choke. My 1.6 liter Alfa uses the same carbs, but with 30mm chokes.

As texcarguy wrote, bigger is not necessarily better. You need to match the carb's choke diameter to the displacement of the engine. Too small a choke may strangle an engine, but too large a choke will cause problems at low rpm, with no offsetting benefit at high rpm.

My "bible" for explaining all of this is a brief paper written by Dave Andrews and available at http://members.aol.com/dvandrews/webers.htm

Note that Webers also have model numbers and serial numbers. The model numbers follow the "DCOE" - e.g., a 40DCOE32. The model # defines how the progression holes are sized and located. You also want to maintain consistency between model #'s on a given car (though a real guru would know which model #'s are close enough to be paired).

 

[decoupe]

Tuning for air flow

Jay - Do the same general guidelines for the Aux venturi apply to the DGAS downdrafts?

I'm guessing that it does and getting the best airflow and fuel atomization will improve performance. I've got the jetting just about perfect using a wideband o2 sensor with different fuel and air jetting but haven't pursued the optimal air flow. Guess this would be trial and error testing?

 

[velocewest]

velocewest had written: "The number (38, 40, 45) refers to the size of the venturi (the choke, or minimum diameter of the hole in the carb) in mm"

Well, that's sort of right, but let me take it a step further. Yes, a 40DCOE mounts to a hole that's about 40mm in diameter, while a 45DCOE mounts to one that's 45mm.

I stand corrected, or perhaps more accurately, reminded! That's exactly right -- chokes can be had for both 40's and 45's in a range of sizes from about 26 or 28mm up. Correspondingly, idle jets, main jets, air correction jets also are available for tuning the carbs to the engine capacity.

I've kind of let carbs go -- once you grasp the simplicity and reliability of fuel injection, carbs seem like a lot of work.

 

[jmackro]

decoupe had asked: "Do the same general guidelines for the Aux venturi apply to the DGAS downdrafts?"

Gee, I'm sorry - I know nothing about DGAS or any other Weber model other than the DCOE's.

velocewest had commented: "I've kind of let carbs go -- once you grasp the simplicity and reliability of fuel injection, carbs seem like a lot of work"

Oh, I agree. I view playing with carbs as being analogous to working with tube amps (rather than transistor), or wood drivers (rather than metal) - its an old technology that is kind of elegant in its simplicity, but no way does its performance approach modern-day equipment.

 

[YBNormal]

Carb Terminology

To dot the T's (and cross our eyes?), the "hole" is correctly known as the throttle bore, and is how Weber identifies their carbs, 40mm, 45mm, etc.
The choke is actually the venturi, choke more of an anecdotal term that describes the function to some degree. It is the difference in size of the two that creates the vacuum which draws fuel through the jets. So the venturi controls the amount of flow, but will flow a little more with a larger throttle bore.

The DCOE's and IDA's have removable venturis for ultimate tunability and are designed for IR (individual runner) applications, with a throttle bore or "throat" for each cylinder. This is why a relatively small engine can utilize lots of carburation. If the same setup were run on a typical manifold with a plenum chamber, the engine would be unresponsive except at high RPM. And it is the individual cylinder capacity that determines the size requirements, since each cylinder is literally breathing through it's respective throttle bore. For example, a 2 liter Four would require 44mm carbs to run at 7000 RPM, while a Six of the same 2 liters needs only about 36mm. Makes sense when you consider 4 throats vs. 6.

For our Big Sixes, 45mm with 34mm venturis is sufficient for most applications, and for a 3.7 with the Schrick 292 or more cam, the 45's with 36mm is plenty. Edoardo Weber's formula is stated in an earlier post in a slightly different form, but here it is. The square root of: displacement in liters times peak HP rpm divided by the number of cylinders. Multiply the result times .6 for the venturi size - the throttle bore size should be 1.25 times larger.