Tech Answers January 2014 Carburetor
David Stribling
January 20, 2014

Carb Numbers

I have seen a formula for choosing the correct carburetor size. It is carb size = (volumetric efficiency x cubic inches x maximum rpm)/3,456. But many of the engines in the Fords featured in Modified Mustangs & Fords have carburetor cfm ratings much greater than what would be obtained by using this formula, yet these engines have great horsepower and torque ratings. Is this formula outdated?

I enjoy reading MM&F. Keep up the great work!

Tom Brown
Carson City, NV

The formula for carburetor selection is good science, but it’s only one tool in our toolbox. It’s frequently misused and misunderstood. Although there are many applications where the engines are over-carbureted, we need to use more than this formula to select the right carb for our engines. First, let’s understand what the formula means and how it gets misused.

The formula for calculating the CFM rating for a carburetor is CFM = (VE x CID x RPM) / 3,456. Let me explain this in reverse order:

• The 3,456 is cubic feet in inches = 12 inches x 12 inches x 12 inches x 2. Since we calculate based on the cubic inch displacement of the engine, we use a cubic foot in inches (12 x 12 x 12) and we multiply by 2 since we have a four-stroke engine and are only drawing air on every other cycle.

• RPM = revolution per minute. Most people use this to calculate the carburetor at wide open throttle, and people are usually not realistic about what the engine is capable of running. They pick a fantasy number like 8,000 rpm (thinking well, that’s what my tach reads), ignore the volumetric efficiency, and pick a carburetor slightly bigger than they calculate. These guys usually flood their engines on takeoff.

• CID = cubic inch displacement of your engine.

• VE = volumetric efficiency. VE usually gets dropped or ignored because most people don’t understand it. VE is your engine’s ability to move the air/fuel charge in and out of the cylinders. Imagine a short-block sitting there rotating without a cylinder head on it—100 percent VE—no restrictions. Now put valves, ports, NASCAR restrictor plates, and other obstacles in its way and the ability to pull the air charge into the cylinder is reduced. Most street engines are in the 70 to 85 percent VE range, whereas better-flowing street engines are between 80 to 90 percent efficient with good headers and good-flowing intake manifolds. It’s possible to achieve better than 100 percent efficiency by forced induction (superchargers and turbochargers) and in naturally aspirated engines with radical cam timing.

With that, let’s put the formula to a tried engine. I had a 351 street engine that made around 456 horsepower at 6,800 rpm. This engine was around 85 percent efficient, so the formula becomes:

• CFM = (0.85 x 351 x 6,800) / 3,456 = 587 cfm

Now solely based off of this formula, you’d think a nice 600-cfm carburetor would be the choice, but we actually went with a 750-cfm-rated with vacuum secondaries.

Why? One of the things the above formula assumes is a dry charge. In the ’60s, someone determined that the “wet” charge (air/fuel mix) could reduce the cfm up to 10 percent to the rating. Also, carburetors are as different as the manufacturers that make them. The size of the venturis (carburetor openings), size of the accelerator pumps, jetting, vacuum secondaries or mechanical, and single/dual plane manifolds (or even ram air) all play a part in determining what carburetor to select.

Back to our 351 engine. We actually tried both a 700-cfm and a 750-cfm carburetor, and the 750 only made 10 more horsepower on the dyno. That tells me we were reaching the limit on the cfm rating on the engine, but we chose the 750 for the bigger accelerator pumps.

And for a bit of trivia, why would Ford have put a huge 780-cfm on the Boss 302 engine when the 428 CJ only got a 735? While you would think the Boss would flood like the Titanic after the iceberg, it works just fine. The reason was to certify the engine for certain NHRA classes. Also note those huge ports on a Boss 302 flow 290 cfm of air at 0.500 lift, where a CJ flows just under 250 cfm! So the science is good when using the carburetor formula, but it isn’t the only tool available.

The formula for carburetor selection is good science, but it is only one tool in our toolbox. It’s frequently misused and misunderstood.

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