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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