Quote:
Originally Posted by Tom Vaught
A 4 BBL Carburetor is tested to a Standard Test Pressure (established before World War II) of 1.5" of Mercury or 20.4" of Water. (The real number is 20.39265029505" of water).
This means that the lower section of the carburetor (after the throttle blades) has a higher suction pressure vs the pressure at the air horn of the carburetor. They would typically use the same type of Water Manometer as a Flow bench used for testing cylinder heads. Cylinder heads are tested at a variety of test pressures depending on the size of the flow bench. 10" of water, 20.4" of water, 25" of water, 28" of water, 42" of water and even 67" of water test pressure.
A 2BBL carburetor has less overall flow area therefore the carburetors were able to be tested on the same machines at a higher suction pressure. That Standard Test Pressure (established before World War II) was 3" of Mercury or 40.8" of Water. (The real number is 40.7853005901" inches of water)
So naturally if you have a higher test pressure used you get a higher flow number.
A "500 cfm 2 BBL" carburetor if tested at 20.4" of water instead of the 40.8" 2 BBL standard would flow about 350 "4 BBL" cfm.
This means that the "6-pack Carburetor Set-up" that has a "350 cfm" center carb and two "500 cfm" outer carbs (when tested at the 4 BBL standard) would flow about 950 "4bbl" cfm, instead of the assumed 1350 cfm posted for the carbs when tested at the "2 bbl" rating.
Yes, the rating is VERY ACCURATE IF YOU USE THE PROPER RATING SYSTEM.
People can play games with "Suction Pressures" when related to cylinder heads.
Super Flow always used 25" of water "suction pressure". Smokey Yunick had some data from dyno testing that said that if he tested his heads at 28" of water a 5% improvement on the flow bench gave him a 5% improvement on the dyno. So a LOT of people now use that number to quote flow numbers. Barry Grant used to use 28" of water "suction pressure" to make his 850 cfm carbs read a higher flow number (1000 cfm) vs the Holley competition's numbers. Holley had to follow suit to sell carbs so the HP carbs are rated 1000 cfm but if you tested them at the pre World War II Holley standard the carb would still be the old stand-by 850 cfm carb.
Hope this helps.
Tom Vaught
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holy hell, that was an answer, man i'n really glad i asked what i thought was going to be a stupid question, so would it be safe to say that 3 of the small-bore 2Brrl's would flow around 750cfm ? i would think that would be a better match for many stock type Pontiac engines dont you ? Or no ? thanks for the detailed response by the way