Carb sizing

[Darrel <darrelK@domain.elided>]

Bill Thebert wrote:

> > From: Tom Mandera <tsm1@domain.elided>
> add a new 4bbl carb to that.
> >
> > Also recall that a 304 doesn't need even 350cfm at the factory redline
> > at 100% efficiency... so the stock 350cfm is overkill as it is.. ;-)
>
> A recent addition to the Binder Bulletin Tech Library is a little "wizard"
> that lets you input: 1) engine displacement, 2) redline RPMs, and 3)
> volumetric efficiency -- and then calculates required CFM.  Thought it was
> a useful tool, given the tendency among quasi- performance enthusiasts to
> select overly large carbs.
>
> http://www.binderbulletin.org/techlib/wizards/cfmwiz.asp
>
> There's a table on the page as well (based on presumed 100-percent VE) --
> just in case my experiment with "Active Server Pages" doesn't perform as
> anticipated.
>
> Bill Thebert
> The Binder Bulletin

Hi Bill!
Before I dive into the "controversial" carburetor size topic, I want to thank
you for a great web site. It's been a great help to me this past year in
taking my Scout II from stock to a somewhat capable off-road machine.

IMHO, These calculations are a good place to start, but like most things it's
not quite so simple.

As you know, CFM is the air flow through the carburetor (at standard temp and
pressure) for a certain pressure drop across the carburetor. But did you
realize that Holley 4 barrels are rated at 1.5, and 2 barrels at 3 In/Hg?

Depending on what the pressure drop across the carburetor is at
wide-open-throttle you may or may not get the rated CFM. Of course, the larger
the carburetor the lower the pressure drop.

The major problem with over sizing is that you don't get enough air flow for
the carburetor to meter fuel correctly. OTOH, if the carburetor is too small
the pressure drop will be too large and you'll lose power because the air/fuel
mixture is less dense.

If you plug in the numbers for muscle cars of the 60's and 70's many are
over-carbed according to the calculations, even at 100% volumetric efficiency.
Why did these major players go oversize? Power! The bigger carburetors simply
made more power.

This is why I'm not so quick to accept the Holley list 7448 as necessarily
being the best carburetor for the 304/345. It may be, but I did to talk to one
304 owner who is very happy with the idle and throttle response of the Holley
500 CFM 2 barrel. And in theory he should be making more mid and top-end power
then the 350 CFM guys....

Darrel Kline
Colorado Springs