I just bought a Quick Fuel 4150 "Slayer" rated at 450 cfm. I was looking at the specs and noticed that the throttle bore size is the same as the 600 cfm version, 1 9/16". I was surprised, since I thought it was the throttle bore size that affected the flow rate on otherwise identical carbs. Apparently this isn't the case, or only partially the case. What does affect it, what is it that makes one carb a 450 and one a 600? Here are the specs, in case anyone is interested. 450:https://www.holley.com/products/fue...ick_fuel/street/slayer_series/parts/SL-450-VS 600:https://www.holley.com/products/fue...ick_fuel/street/slayer_series/parts/SL-600-VS
Three factors enter into it, throttle bore, venturi diameter and the venturi boosters. As you have noticed the throttle bores can be the same yet one carb can flow more because the venturis are larger but that will be at the expense of low RPM response. The boosters will effect flow with their size, but also the location is a factor. The 3310 Holley, when introduced on the 1965 396 Chevy was a 4150 rated at 780 cfm. Eventually Holley brought out an updated version for the mass market that was rated at 750 cfm. It was still known as the 3310, (with the addition of a - number on the end) but now it was a 4160, and the secondary boosters were straight rather than the “down leg” boosters the 780 carb had. The change in booster configuration resulted in the 30 cfm loss of airflow.
The shape and placement of the venturi as well as the boosters make as much difference in flow for any given carb as the size of the throttle bore.
The Jalopy Journal
