Determining the Carburetor size for your Corvette - Figuring Carb Size

Everyone knows smaller carburetors are the better choice when attempting to generate the most efficient power figures in your Corvette. Or are they? Is this the right plan for a modified Corvette? Poll some experienced, knowledgeable engine builders and you might find it isn't. As an example, a Winston Cup car with a restricted carburetor (for the high banks of Daytona and other Super Speedways) will always be down on torque. To offset the small carburetor, the engines are required to spin higher to generate more torque. In years gone by, it was also common practice to build super-high compression ratios to offset the loss in torque, but this is now frowned upon by the NASCAR rulebook. It doesn't take a rocket scientist to figure out the relationship between higher engine speed and higher engine carnage and link that principle to your Corvette street or race engine. On another front, a number of accomplished race-engine builders have done considerable research on the topic, and the common consensus is: As the carburetor cfm increases, so does the maximum torque output of the engine.

Carburetors come in all sorts... 

   

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Carburetors come in all sorts of different shapes, configurations, and, of course, sizes. Typically, Detroit has used carburetors with cfm ratings much higher than what might be considered correct. For example, Chevrolet used carbs as large as 800 (780) cfm on engines such as the Z/28 302. In other cases, GM regularly used large-cfm Quadrajets on any number of engine combinations, large and small. To be fair, the QJ incorporates small primary bores; but, nonetheless, it was still a "large" carburetor.

There's more here, too: Think back to pre-fuel-injection times. General Motors used Quadrajet carburetors for decades on its engines (Corvettes and otherwise). These stock, production QJs typically had flow ratings well in excess of 700 cfm. How many of these Quadrajets were junked in favor of the latest 600-cfm or smaller, flavor-of-the-week carburetor? Plenty, we'd bet.

Of course, there are formulas to determine the correct carburetor size for a given application. The following is a common formula we've relied on for years.

Note: VE = Volumetric Efficiency

The above formula calls for several different numbers to be plugged in before completion. The displacement of 355 ci is relatively easy to determine in the case of a hypothetical engine we'll soon build, but what about the maximum rpm and volumetric efficiency (VE)? Let's assume the Corvette will redline (maximum engine rpm advised) at 5,800, a relatively safe figure for a mild powerplant. The VE figure is much more difficult. Typical race engines will feature a VE of anywhere between 95 and 110 percent, but since this hypothetical street-driven Corvette is no ultimate racecar combination (and for the purposes of this article, we don't have access to a dynamometer to determine VE), we'll have to make an assumption. Assume this small-block is efficient as a street piece, but also keep in mind this is simply a multipurpose car that can be driven to work during the week. A conservative VE figure of 80 percent would work well in our application. Continuing with the formula, this is how it works with the numbers in place