Vacuum And tuning Help

1wild&crazyguy said

in the stock world...look at the super six....look at what a 750 on a stock 340 does to et's.....look at the 440 and what an 850 does for et's....
heck look at the thermo quad, the added cfm helped those low comp 340's, let alone had to pass smog=tiny 240cfm primarys with like 600cfm secondarys

what I'm saying is 'reasonably speaking' for example...you take a sb stroker and put smog heads on it, that air speed is flying and makes plenty of signal to run the larger carb to an extent[as in not talking about dominator carbs or the farthest reach of debunkment to this] thats stay in the relm...say a 700cfm was recomended where really you could get away with a 850-900cfm carb.

Click to expand...

You are missing a very import law of physics. As air flows through a restriction the velocity increases and when the restriction is reduced the velocity decreases.

I agree your example of smog heads on a stroker will likely result in a very high velocity air flow in the ports of the head but that has no impact on the velocity of air flowing through the carb. Appling Bernoulli's principle; the air flowing through air cleaner is moving at a slow velocity, as it enters the carb the venturi's in the through cause the air velocity to increase, when it enters the plenum of the intake manifold it slows down again, depending ton the ports of the intake it may increase or decrease and in your example it increases moving through the ports of the head. The CFM does not change in this example.

A big carb on an engine that can not use it results in the air velocity through the boosters and venturi of the carb not creating enough vacuum to draw fuel from the bowls properly that results in driveability issues. With a street car you spend 95% of your time running on the idle and transition circuits so it only makes sense to size the carb to perfrom best in this range. Worrying about .1's of a second when you go to the strip once or twice per year just doesn't make common sense.