Carburetors are rated by CFM (cubic feet per minute) capacity.
The traditional calculation for sizing a carb to your engine goes like this:
CFM= Engine CID x Peak RPM x Volumetric Efficiency / 3456
So for a built-up/modified SBC, or strret engine, we're probably looking at numbers something like these:
CID = 350
Peak RPM = 6000
VE = 75 to 85% (depends on your heads, cam and a billion other things) so let's split the diff and go with 0.80.
Note -- If it's a really efficient, great build, it could be 90%, a full blown - pro built race engine could be 100 or 110%!
For a result of: 350x6000x0.80 / 3456 = 486.11 CFM.
Heck, even if we were 90% efficient we'd only be at 546.88 CFM!
This would tell us that going with a 600 or 650 CFM 4BBL would be WAY more than enough, but suprize, we'd
be wrong! There's one other thing that the "standard rule of thumb" equation doesn't take into
account, and that's the manifold pressure drop.
4V carburetors are rated at 1.5 inches (Hg) of pressure drop (manifold vacuum) and 2V carburetors at 3 inches.
But, 1.5 In Hg is a bit starving for a HiPo engine. At Max RPM, you probably want this to be more like 1.00 or 0.80 In Hg.
Again, let's split the difference and say 0.9. Now, in order to get the proper CFM use the following math:
Real/Final CFM= Calculated CFM / Square Root of (Desired In Hg / Rated In Hg) --- Or from our example:
CFM = 486.11 / sqrt(0.90/1.50) = 627.24
OK, that's a little more like it!
OR, because of this you can just apply a fudge factor as follows:
Use 110% to 130% higher on single-plane manifolds. Example: If the engine needs 590 CFM, select a carburetor
rated in the range of 650 to 770 CFM for a single-plane manifold. A 750 would be right. An 850 probably
would cause driveability problems at lower RPM. A 1050 probably would cause actual loss of HP below 4500 RPM.
For dual-plane manifolds use 120% to 150% higher.
