[chiphead]

If you've got some experience, it's possible to estimate the idle vacuum for a particular combo. It comes down to static compression, engine size, intake tract design and camshaft specs.

A 400 at 10:1 with a stock intake and 068 cam is going to have 15" or better vacuum. The compression creates a good pull during the intake stroke, the 068 cam has a early intake closing and a wide LSA, and the stock intake has a small plenum volume that promotes port velocity.

Take the same 400, lower the compression to 9:1, use a single plane intake, change the cam to RAIV spec. It's going to struggle to make 10" at idle, and want a lot more idle fuel. The vacuum is too low and reversion too high to draw fuel through a idle circuit sized to feed the 10:1 engine that is more efficient at breathing.

That's why these new holley carb calibrations are so fubar. Holley expects most knuckleheads to slap it on a low compression chevy 350 with a single plane, and a "super rattler hog momma" cam that's way too big for the application. That carb better dump fuel like a garden sprinkler, or said knucklehead is gonna say the carb is bad and send it back to summit.