off list carb question

[Wes Branchflower]

In the case of webbers, the reason theirs not much info on thier CFM's is 
because of their applications. Their usually used in sets of 2,3 or 4. The 
CFM of 3 webbers connected to a common intake plenum, will have a much 
higher CFM than of them connected to non common intake.

An example of a non common intake, is the auzzie valiant charger. It uses 3 
2bbl webbers on a 6 cylinder engine. Effectivly one carb for every 2 
cylinders, or one throat each. If it used all 3 on a common plenum, there 
would be way to much CFM to work problerly

Because the carb is only being sucked through 2 times every 720degree's
it creates a 'different' CFM, than the almost constant suction which would 
be present with a common plenum.

45's are meant to flow 250cfm per barrel,


Webbers don't really use CFM, they use revs vs. cylinder size. This formula 
will calculate choke size of a give RPM -

Choke = 20 x SQRT(((cc/cyl) x rpm) / 1e6

This formula only works for 1 throat per cylinder.

The formula for RPM is -
rpm = 1e6(choke/20)^2 x (cyl / cc)



I learnt most of this stuff of the australian chryslers discussion forum.. 
so i don't take cred for the formula's etc..

hope this helps.


>From: "Bruce Plecan" <nacelp at bright.net>
>Reply-To: diy_efi at diy-efi.org
>To: <diy_efi at diy-efi.org>
>Subject: Re: off list carb question
>Date: Sun, 28 Jan 2001 08:39:55 -0500
>
>
>I used to do alot of work with Webers, but don't recall seeing any CFM
>related numbers for them.   They had been used on so may cars, that the
>applications, had the chokes pretty well sorted out or picking a similair
>engine to what you were working on would get you close to start with.   
>I've
>seen 45s used with just a single tube intake, with the other side blocked
>off.
>    Now being EFI specific,  it seems to be *common* knowledge a GM dual 2"
>TBI is 670 CFM, the 1.8" butterflies about 450.   Needless to say the type
>of manifolding makes a huge difference for butterfly sizing, ie Independent
>Runners allow for very large butterflies, and that is why the 45s seem to
>always use such large chokes in em.
>    I've used a reworked 45 in place of a Holley 600 CFM carb, with pretty
>good results.
>HTH
>Bruce
>
>
>
>
> > any one know how to work out CFM for a weber, eg 45mmDCOE?
> > just would like to know how to work it out
>
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