Some time ago I got an interesting letter from Robert Blinco of Lake Milton, Ohio. I put off answering it because I really wasn't sure I could. I ran across it while cleaning out some files and figured it would be a good subject for a "Workbench."
His letter read in part: "I am building a 383ci stroker engine with 9.5:1 compression, 6,000-rpm Competition Xtreme 4x4 cam, Weiand Stealth intake, good flowing 215cc intake runners with 2.05/1.60 heads, and 64cc chambers. My question is: how do you figure out carburetor size? Is it cubic inches times rpm divided by tire size minus shoe size, or what?"
Yes, Robert there is a formula, and believe it or not, you're pretty close with yours:
Cfm = [(engine size/2) x (maximum rpm/1,728)] / VE %
Here is how it works: engine size is in cubic inches. We divide that by 2 because a four-cycle engine only pulls in air every other revolution. Maximum rpm is just what it is, how fast you'll ever turn the engine. The 1,728 is the number of cubic inches in a cubic foot (or 12x12x12). VE% is the volumetric efficiency of the engine (a guess factor at best), and in fact it can and will change with engine speed. It usually is the highest at maximum torque rpm. A high-performance race motor is very inefficient at low speeds due to cylinder head and camshaft design, but may come close to 100 percent in the upper rpm range. A motor in, say, a farm tractor will be just the opposite.
Here's my breakdown of VE% versus engines:
75% = most stock street engines
85% = top end street/high performance
95% = maximum all-out racing-only engine
As you can see, compression ratio, camshaft design, intake manifold, and other such factors are covered by VE%.
With that established, we can break the formula down a bit simpler to:
(CI x maximum rpm/3,456) / VE
So let's insert your 383ci motor, pick 6,000 rpm as maximum and say that the VE% is 85%.
(383 x 6000)/3,456 = 665 / 85 % = 565 cfm
So the ideal carb would be in the 550- to 575-cfm area.
This is where the formula isn't perfect, and this is where camshaft, cylinder head design, intake manifold, header type, and size play an important part in the equation, not to forget carburetor type and design.
An example is that I just happen to have a 383c performance motor in my own Jeep. For years, I've run a very modified 600-cfm Holley carb. A while back I switched over to an Edelbrock Quadrajet that was rated at 850 cfm. I can unequivocally say that my engine will never flow anywhere near 850 cfm, but this carb offers equal, and in some instances better, performance than the nearly one-third smaller (in cfm) Holley square-bore that I used to use.
How can this be? Well, it's all in carburetor design. The much smaller primary barrels offer good high-speed airflow and proper atomization at low engine speeds. The large airflow-controlled secondaries don't come into play until the higher engine rpm causes a ram effect and the engine can actually make use of the higher airflow.
To some extent, the Holley with vacuum-operated secondaries will work the same way, and even though the engine is "over-carbed" and may actually make a bit more upper-end horsepower, the driveability for low-speed trail use and actual fuel mileage will suffer. For example, '60s musclecars with 302ci motors used carbs approaching 800 cfm, and in all actuality were not very street-friendly, while the street-driveable engines with even more cubic inches had carbs rated below 600 cfm; that even included the 350hp small-block 327ci Corvette.
So yes, using your tire size and maybe even your shoe size in the formula as a variable isn't too far off. As you can see, the variables do make a difference, and while the formula kind of works on square-bore carbs, it only partially applies to a Quadrajet.
To throw one more confusing factor into the pot is the two-barrel vs. four-barrel conundrum. It seems that two-barrel airflow is measured at a different pressure drop than a four-barrel, so there is no direct comparison between them. But that's going way beyond the scope of this "Workbench."
The bottom line: Rely on the intake manifold's or carburetor's manufacturer for a size guideline, plug in the cfm formula, and include a bit of common sense.
