[Tom Vaught]

Holley 4 bbl carbs are "sized" and tested at 20.4" of water to flow 850 cfm.

20.39" inches of water is basically 1.5" of mercury.

Holley 4 bbl carbs are "sized" and tested at 20.4" of water to flow 850 cfm.

40.80 inches of water is basically 3.0" of mercury.

so if the cernter carb is rated at 355 cfm at 40.8" of water then the formula goes like this:

355 cfm at 3" divided by 1.414 = cfm at 1.5"
or 251 cfm at the 4 bbl rating.

so if the end carb is rated at 500 cfm at 40.8" of water then the formula goes like this:

500 divided by 1.414 = 353 cfm

353 x 2 for the end carbs = 706 cfm (4 bbl rating)

The center carb we calculated at 251 cfm

251 cfm plus 706 cfm = 951 cfm using real math vs the assumed numbers.

so 951 times 1.414 = 1344 cfm by the 2 bbl rating.

So your number is close. The confusion comes in with how Holley rated them. People say that the ends are 500 in the advertising and 355 for the center carb but the reality is the end carbs are basically the same dimensions as a 850 cfm carb not a 700 cfm carb so the carb rated numbers do not match actual Holley flow numbers which is what my number was based on.

Tom V.