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[M30B35] Triple weber 45 DCOE installation

jc971

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Hello
i am rebuilding my euro m30b35
I will mount the original ignition system of the csi, a stainless steel sports exhaust manifold, with a weber 45 dcoe carburetor kit

I adapted the alternator and the power steering pump of the 735i

I would like to know what configuration of the carburetor to use

thanks
 

Stevehose

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Are you referring to the jetting of the carbs? Do you have any modifications to your engine: compression ratio, cam, etc?
 

jc971

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i don't have the carbs yet
i'm waiting for the end of the containment to buy them
so i will buy the good jets in the same time
 

bfeng

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Yes the jettings of the carbs
my b35 is stock, just a headcylinder surfacing
Not just the jetting but
(1) progression hole layout (related to the “type” number of the carb). You can customize this with lead solder and some small diameter “number drill” bits.

(2) air correctors (or not).
 

rsporsche

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45's are a lot of carbs for a stock b35 - IMHO. i had twin 45's on my 2002 with 10.5:1 / schrick 316 cam - all producing about 185 hp when newly built by Korman.
 

Stevehose

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Yes I am thinking 40’s with 34mm chokes. Can go up to 36mm. If they are new carbs they will be 152’s with 5 progression holes I think.
 

Markos

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Yes I am thinking 40’s with 34mm chokes. Can go up to 36mm. If they are new carbs they will be 152’s with 5 progression holes I think.
My 152’s are new redlines tailored to the m30. I can check if it helps. I checked the jets a while back, but I can’t recall what they were. Same with the chokes. Significantly different than my friend’s setup. Anyway they are very handy so I am happy to look.
 

rsporsche

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yes i do, but i have become a fuel injection guy, i do LOVE the sounds that triple sidedrafts make along with the wonderful m30 valve train. @Stevehose knows a lot about this subject and he is one of the best sources around.
 

Belgiumbarry

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Smaller is better for torque and throttle response (as 40 DCOE with 34 mm chokes)
Bigger for top end power ( as 45DCOE with 36 mm chokes)

i used 45 /36 but lost 60 NM torque... till i build this setup and had again the full "oem" 300 NM. ( and 210 HP as oem injection … )

SAM_3188.JPG


SAM_3210.JPG
 
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nosmonkey

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Have you considered going for DCOE throttle bodies with a standalone management system? Best of both worlds of side drafts and FI.

I'd go for 40s on a relatively standard B35
 

Stevehose

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If it were mine I'd start with DCOE40 and 34mm chokes. You could go to 35 or to 36mm chokes if necessary, which is the largest that fit into 40's. Most importantly for setting up jetting is installing an AFR gauge.
 

Gary Knox

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When I put triple Webers DCOE's on my '73 coupe back in the late '70's, I used 40s as well. I'm a big fan of trying to get the most torque from an engine with much less interest in HP (where on US highways are you going to be trying to push through the air at speeds above 120 mph??). I was pleased with the 40s,which I ran for the three final years I owned the car.
 

jmackro

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If it were mine I'd start with DCOE40 and 34mm chokes.
I agree with Steve's numbers. If you want to apply some science to it, the rule of thumb for Weber sizing is that the choke diameter in mm = (individual cylinder displacement in cc's X RPM at maximum power / 2500 ) ^ 1/2. And if you can't find your slide rule, you can see it in graphical form at http://www.dvandrews.co.uk/ by clicking on "Selection and tuning of DCOE carbs". Andrews' chart is for 4-cyl engines, but taking the 3,453 cc displacement of a "3.5" BMW engine and multiplying it by 2/3 gives you the appropriate point on the X axis.

For a BMW 3.5, plugging 575.5 cc / cylinder and 5,200 rpm into that formula gives a choke size of 34.6 mm. But of course, Weber chokes are only available in integral mm sizes, so 34 mm is your closest, readily available choice (do they make 35's?). Andrews says that the ideal carb size is 1.25 X the diameter of the choke, so 1.25 X 34 mm = 42.5 mm: halfway between a DCOE 40 and a DCOE 45. Since 40's are typically lower-cost than 45's, and since you probably aren't going to regularly go as high as 5,200 rpm in a street car, a DCOE 40 with 34 mm chokes is probably a good starting point.
 
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Ohmess

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I too would start with the 40 DCOEs and a 34mm choke. As to your comment on jetting, you probably are going to need to plan to acquire more than one set of jets. There is no standard weber jet configuration for a BMW m30b35, and you are likely going to want to tune to your particular engine. In my experience, this takes a few interations.
 

jmackro

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As to your comment on jetting, you probably are going to need to plan to acquire more than one set of jets. There is no standard weber jet configuration for a BMW m30b35, and you are likely going to want to tune to your particular engine. In my experience, this takes a few interations.
Dave Andrews' website that I cited in post #17 also gives some formulas (formulae?) for sizing the main jets, air correctors, emulsion tubes, and idle jets. But that will just get you close - as Ohmess says, dialing in a jetting configuration is an iterative process.
 

jc971

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Hi guys
thank you for the informations
so if I understand the carbs in 40 with 34 chokes for my engine to have the maximum torque
and maybe 36 chokes to get the horses higher rpm
the m30b35 making its maximum power at 5700 rpm
i will try to install an afr gauge for the set of the jetting
but i will start with the formula for it
 
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