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Holley rates 2bbls differently. The 2bbl rating for the Six Pack is 1350, 2x500 outboards, 350 center.

To get an idea of the outboard flow rated like a 4bbl, the throttle bore and venturi is the same as a 750DP. So just the 2 outboards total 750cfm. The center 350 is estimated to have a 4bbl rating of 230-240.
Total Six Pack flow is 980-990.

venturi and throttle bore sizes for the end carbs is the same as an 850 4bbl; 1 9/16" venturi and 1 3/4" throttle bores. the original end carbs were rated at 390cfm each. the center was 250cfm.



I stand corrected, I just measured an end carb. The throttle bores are 1 3/4" but the venturi is not 1 9/16", it's 1 7/16".
Holley made a 2300 carb with the same specs and it was rated 650cfm. The #6425 attained a big part of it's 650cfm flow from the lack of boosters.