With a 4 barrel carb (specifically 750 or 850) if you only use the primary (front 2 bbl) how many cfms are they using? Would it be half?
With a 4bbl carb that has 4 equally sized Venturi and throttle bores, whatever the rated capacity is with all 4 at wot……..using only two will cut the rated flow capacity in 1/2.
This isn’t at all the same scenario that happens on a running engine where you go from using all 4 to only 2.
The flow into the engine will be reduced, but not by anywhere near 1/2.
The engine is not limited to the CFM rating of the carb. That number is just a method of coming up with a way to compare carb sizing against a given standard.
For 4bbl carbs, that standard is a test pressure differential of 1.5” of Hg.
If the carb is a “750”, then in theory when the engine pulls on the carb hard enough to create a 1.5” pressure differential……..750cfm is flowing thru the carb.
You could flow more air thru the carb by creating a higher pressure differential.
This is what happens when you close half the carb.
The engine becomes less efficient because in order to get the air it needs, it has to create a greater pressure differential.
The reduction in flow capacity is a restriction, but the engine will still use way more than 1/2 of the air it was using with all barrels open.
As an example, one of the combinations of circle track engines we were building 20-25 years ago mandated a 2bbl carb on an adapter, on a performer rpm.
The mandated carb was a Holley 4412, which is physically the same size as the primary side of a 750 4bbl.
With the 2bbl, the engines would make about 380hp.
With a 750 4bbl(2x the capacity of the 4412) they’d make about 430hp.
So, in those engines, doubling the carb capacity from the mandated 2bbl resulted in a 50hp gain.
Holley used to sell an adjustable vacuum pod with a knob on it to limit how far the secondaries would open.
I tried one of these on a customers 350SBC street strip build, that made in the 450hp range.
It took a lot of limiting of the secondaries to have any significant reduction in power, and I don’t think it lost as much as 50hp with the secondaries closed completely.
That test was done a looong time ago, and we were pretty surprised at how little the power dropped.
The reason for the test was, this was back when the roll bar rule was still 12.00, and his car ran solidly in the 11’s, and he didn’t want to put a bar in it.
So, he figured the adjustable secondaries would let him tweak it and keep it above 12.00.
It wasn’t quite enough.
Had to numb up the tune as well(timing).
