Holley cfm selection

[jmarkaudio]

Here is something to think about. Say an engine has the ability to draw in "X" amount of air at 100% efficiency. And there are NA engines that are over 100%, but at the given peak RPM they can draw a certain amount, say 3200 CFM at 8000 RPM, 400 CFM per hole. Since the engine runs with a common plenum in most cases will it only need a 400 CFM carb? No, because of the overlap and pulsing in cylinders it will want more, 400 CFM will be a restriction. Carbs are typically rated at 1.5 " of manifold vacuum, say the engine demands 1000 CFM of air. And then its most efficient point of vacuum is .8". Is that 1000 CFM carb going to flow enough? The engine in the lane next to it is nearly identical, built with the same parts, but surface finish is different in the induction requiring higher vacuum to get sufficient vaporization and distribution, so the carb restriction needs to be smaller to make that happen. Take an engine with a dual plane and change to a single plane and the carb size needed can change significantly. So there is no simple answer...

[Right hand drive]

[BradH]

Yep.

[Right hand drive]

[hysteric]

[rgalajda]

This is my first post so I will give it a try.

I restore a number of quadrajet carburetors. The secondary air valve opening preset at the factory varies a a lot from one carb to another on a quadrajet and is not adjustable without some grinding. This is how the factory was able to use them on such a wide variation of engine combinations. So a 750 a quadrajet could have been setup from the factory anywhere from 500 cfm to its full potential of 750 cfm . I measure and record these secondary air valve openings. If I am doing a matching numbers restoration of the carb I will leave it as the factory preset. If I am doing a custom carb (quadrajet) I will set the air valve opening to suit the engine and car combination. This can be aceheived by grinding the stop mechanism on the secondary air valve and or installing an adjustment mechanism.

The secondary throttle plates were also not always set to full opening either.

Pontiac GTO would be one of the quadrajets that you you would find full opening of the secondary air valve. ( larger cubic inch engine )
I have recorded huge differences measuring secondary air valve openings from Quadrajets used on 305 cu in motors to 455 cu in factory setups.

The 780 holley used on the 302 and 350 Z28 was a vacuum secondary carb on a dual plane heated intake manifold which was a true dual plane with full plenum divider. So the carb cfm seen by any one cylinder is almost halved. I believe the secondaries had a slow opening also.