Size Matters

This month, we're testing carburetor engine theory against actual dyno results. You know the deal about bigger carburetion being better and more fuel equaling more power. Well, too often people can't seem to get beyond the air/fuel metering myths. How big is too big, and what happens if it's too small? Many enthusiasts can tune their carbs with their eyes closed, but when it comes to choosing the proper initial carburetor size, there seems to be a lot of confusion.

The idea is to choose a main body with a large enough bore and venturi area to allow maximum airflow through the carburetor and into the engine. At the same time, however, these areas must also be properly sized to promote a high-speed airflow signal into carburetor, a measurement of air speed that the carburetor reads in order to determine how much fuel to deliver to the incoming air stream. In theory, if the bore and venturi are too large, the signal will be weak at low engine speeds and the fuel will not be pulled from the carburetor's discharge ports accurately. The opposite of this effect is when the carburetor's bore and venturi are too small for the engine's airflow demands. While throttle response will be outstanding, the engine's maximum airflow (power) potential will be diminished. The ideal carburetor sizing combination will be able to move the maximum amount of airflow through the carburetor while maintaining the strongest possible signal.

For those readers who like to crunch numbers, there is a commonly used formula for determining carburetor size, but it provides only a rough starting point as all the engine's variables are not taken into consideration. This equation assumes 100 percent volumetric efficiency (VE) and can be worked out using the following equation: VE = (displacement x rpm)/3,456. For example, if we were to run a 350ci engine at 6,000 rpm through this equation, the formula would tell us that a 350ci engine requires a 608-cfm carburetor. However, most 350ci engines do not produce 100 percent volumetric efficiency, even at peak torque. They more likely produce around 85 percent as a strong street/strip engine. The same 350ci engine with 85 percent VE would require a much smaller carburetor (516 cfm) using this equation. As you can see, compression ratio, bore and stroke ratio, combustion chamber design, and much, much more are not even considered. In this case, a good street/strip engine would probably work best with at least a 600-cfm carburetor, and the same engine dedicated to the dragstrip would use every bit of a 650-cfm unit and sometimes more.

Depending on the engine's total displacement, operating range, and intended horsepower output, the most efficient carburetor size varies. While theories make great bench racing conversation, we wanted to see what would happen if one engine were tested using carburetors of various size.

After pondering how to perform this test, it became obvious that a high-horsepower engine would show the broadest results between carburetors, but a 700-plus-horsepower small-block is hard on parts and isn't realistic. For every one of these engines roaming the race tracks, there are a thousand or more making 450 hp or less on the street. We chose a Smeding Performance 383ci engine rated at 440 lb-ft of torque and 440 hp at less than 6,000 rpm. Durability is a key issue when performing a test of so many variables, and Smeding has a good reputation for keeping its engines together.

When our motor arrived, it was complete from intake manifold to oil pan but without the accessories (not needed for the dyno flog). All vacuum, oil, and water holes were plugged prior to shipping to ensure that the motor was clean and ready for action. Since the engine was already assembled, we added the needed parts (see Parts List sidebar) and got ready for the dyno. Since the testing procedure called for every carburetor imaginable, we contacted one of the industry leaders in carburetor performance. Holley couldn't wait to test its different-sized carburetors under the editorial eye. After speaking with a knowledgeable representative, we decided on a standard 4150 HP mechanical secondary four-barrel carburetor in 390-, 600-, 650-, 750-, 830-, 950-, and 1,000-cfm airflow ratings. Because a test of this caliber can be overwhelming, Holley also offered us some West Coast technical support from The Carburetor Shop in Ontario, California. With the dyno mule ready to rumble and the carburetors in hand, we headed for The Carburetor Shop, where we would perform our shootout on the next door Vrbancic Brothers' engine dynamometer.

George and Bob V. also run The Carburetor Shop, so they are intimate with carburetor tuning; they recommended their 13/4-inch Hooker test headers outfitted with exhaust gas temperature (EGT) sensors and oxygen (O2) sensors. These sensors allowed the dyno operator to monitor the amount of unburned fuel passing through the headers, which helped us decide whether to jet the carburetors up, down, or not at all. As long as the EGTs stayed between 1,200 and 1,300 degrees F and the air/fuel ratio hovered around 12.5:1, everything would be considered optimum. We plopped the 390-cfm carburetor on the Edelbrock RPM Air Gap intake manifold, set total timing at 36 degrees, and fired the engine up. Follow along. The results may surprise you.