When I sized my carb to the F4B, for my 5.0L, I took a different
approach. I called Holley, told them my engine size, manifold,
transmission, rear end ratio, tire size, and they told me a 600 CFM
double pumper would be correct.
It is a simpler way, and after jetting changes when the car was dyno
tuned, the car runs great.
Larry
Theo Smit wrote:
> Hi all,
> In a reply to Joey I stated that carb sizes are typically quoted as
> the amount of air they flow at 28 inches water pressure differential.
> That turns out to be wrong, on two counts. It was pointed out to me
> that in the carburetor sizing game, two barrels and four barrels are
> actually rated at different pressure differentials, and according to
> this article:
>
> http://www.carcraft.com/techarticles/56919/
>
> two barrels are rated at 3.0 inches mercury (corresponding to some 40
> inches of water), while four-barrels are rated at 1.5 inches of
> mercury (which is 20.4 inches of water pressure differential). Further
> still, the CFM ratings of carburetors are mostly bunk anyway, because
> if you're dropping more than about six inches of water across your
> carb (0.5 inch mercury, give or take), then your carb is too damn small.
>
> What's all that mean when comparing two barrels and four barrel
> carburetors? Since the pressure drop goes up with the square of the
> flow velocity, according to Mr. Bernoulli (and who can argue with him?
> He's dead...) it means that a 500 CFM two-barrel, tested at 3 inches
> of mercury, would flow about 0.7 of that (or 350 CFM) at 1.5 inches.
> Therefore, a 500 CFM two-barrel would have about the same outright
> performance potential as a 390 CFM four-barrel, but because the
> four-barrel has four smaller throats and only two of them are used for
> just getting around town, the four-barrel will give better throttle
> response (and probably, better mileage because of that).
>
> Best regards,
> Theo
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