> Greg, the recommended (by Ford, Holley and any other reputable carb maker)
4
> barrel carb for the 351W is 600-650 CFM.
> Sizing the carburetor is based on air flow in CFM at wide open throttle.
It
> is a simple calculation of engine size in cubic inches divided by 1728 and
> multiplied by max RPM divided by 2. If you use max RPM at 6000 (for a
mildly
> tuned and balanced engine), for the 451W the number comes out to 783
CFM.
====
Hmm, it's a 351in^3, ergo ~609cfm. As for using 6krpm, 5k is more reasonable
for this stocker, or ~508cfm.
====
> This is at 100% volumetric efficiency ( unachievable), an engine with a
> sporty cam, headers, intake manifold and reasonable polishing should
produce
> about 80% efficiency which results in a CFM of 626, right in the range of
> the Ford recommendation.
====
~487cfm, or more realistically for this stocker, 406cfm if this were the
only criteria.
In answer to the original Q, don't remember the cfm of the OEM Motorcraft
model boat anchor (I'm pretty sure it was 500cfm though), but back right
after I bought my B new (like that weekend IIRC), I installed a Holly model
2300 500cfm (0-4412) 2bbl with separate electric choke kit 85R-5175 (you
have to assemble). It's ~a direct replacement (no need to rejet with the one
I got) with much better drivability and slightly better fuel economy and
power. I guess they're still available through speed shops/mail order, but I
haven't been current for about a decade now. Some folks opted for the 600cfm
version, but it didn't perform as well overall. With street carbs, less is
usually more, unless you wind it out on a regular basis.
If I were to convert to 4V on a stocker, then based on the above, 406/1.414
= 287cfm! In this case though there's another consideration in that low CR
mills need a bit more cfm than normal due to the extra dilution caused by
the valve timing used, and why the 500cfm 2bbl works so well overall. So if
it's still available, the 600cfm Holly 4150 or 4160 spreadbore/vacuum sec.,
which IIRC has slightly smaller primary bores than the 2bbl for even better
off idle response, and the vacuum sec should add that little extra squirt
needed with sufficient extra cfm to keep any topend leanout at bay, assuming
the stock pump doesn't do it for you.
Regardless of what design/cfm you choose, spreadbores rule on the street for
overall drivability/performance.
GM
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