Originally Posted by sedanbob
My turn-key ZZ4 crate motor came with a Holley 750cfm dual-feed, with vac secondaries. Some guys on another forum think that is too big for a ZZ4 on the street, and recommend a 600cfm instead, claiming it will be more responsive and still deliver power. I believe they are thinking about a double pumper. When I suggest that I am mostly running on the primaries, and the vac secondaries will open as required at higher rpms, I don't get any answers.
Does anybody here know what the cfm flow of just the primaries might be? I have searched all sorts of Holley specs, but can't find this particular spec. The engine seems very responsive to me, with no lags when I put my foot in it. The newer ZZ4s now come with a 770cfm Holley.
How this averages out between the primary and secondary depends on which 750 body you've got. One uses a square design with 1-3/8ths venturies in each location. Another uses a 1-1/4 primary venturi with a 1-9/16ths secondary.
The math would say for the 4 individual 1-3/8ths venturies the primary and secondary both flow half or 187.5 cfm per bore or 375 on the primary with another 375 on the secondary. The other model with 1-1/4 primary and 1-9/16th secondary venturies would flow 333 primary and 417 on the secondary.
The 350 seems to respond very well to the Holley 750 where 600 CFM carbs always leave the engine hunting for more when you start getting into larger cams, not that the ZZ4 is a monster but it's a lot more than what comes in the truck engines. In some ways the ZZ4 cam times in the neighborhood of the old 300 horse 327 cam.
If this engine is running good I certainly wouldn't mess with it and I'd certainly not go to a smaller carb.
There's a lot more to tuning a carb than meets the eye. If you know how to get beyond just changing the mainjets there's a lot that can be done to make a really big carb function extremely well on a smaller engine than you'd expect.