FEATURE ARTICLES

Carburetor Math

By John Gunnell

Jon Hardgrove says selecting the best size carburetor for your '50s, '60s or '70s car is "a simple question with a complicated answer." Jon believes it is not so much a question of cubic feet per minute (cfm) ratings as the relationship between your car's "ideal" cfm value and several other factors.

Jon's Carburetor Shop in Eldon, Mo., (www.thecarburetorshop.com) specializes in restoring — as opposed to just rebuilding — classic-car (mostly GM) carburetors. He told us that an engine's air intake volume requirement is measured in cfms. Factory engines represent a compromise between economy, efficiency, reliability and performance. They achieve about 80 percent volumetric efficiency at red line. Engines tuned for performance or using a hot camshaft and valve combination — like many muscle cars — can achieve up to 92 percent efficiency. Racing engines with roller cams and blowers go 100-115 percent.

The more cfms a carburetor delivers, the higher the volumetric efficiency will be — to a point. Hardgrove's formula for calculating an engine's cfm requirement is cfm=CID x RPMs divided by 3456. "The formula gives the cfm requirement for 100 percent volumetric efficiency for any 4-stroke engine with four cylinders or more," he says. "One problem is most people use Maximum RPM in the formula instead of looking at Cruise RPMs."

To avoid this, use the calculator at www.csgnetwork.com/cfmcalc.html. When you enter your engine's displacement, maximum safe engine rpm and volumetric efficiency, the calculator will give you cfm values. These will show both current and optimum volumetric efficiencies based on your entries. The calculator's default values are set for a typical mid-1960s to 1970s muscle car.

Hardgrove warns there's more to consider than just cfm values. "Where the problem comes in, if you are trying to come up with the best carb for an older car, is in the intake manifold design. You have to look at the bolt and flange patterns, since the same ones are not used on all carburetors."

Design features like bolt patterns and throttle bore spacing vary even on older engines of one brand. "The carburetor from a 1961 Chevy V-8 may not necessarily fit the 1956 Chevy V-8," Hardgrove explains. "You also get into other things like differences between GM flanges and Holley flanges. The bottom line is, you can't go strictly by cfm when selecting a carburetor."

For street-driven cars, Jon suggests spread-bore carburetors like the Rochester Quadrajet and Carter Thermoquad as "the way to fly." He says that some Holley four-barrels fall into this category, but he is not a big fan of Holleys. "Unless you have a racing application, spread bores work best," Hardgrove believes. "Assuming that you don't drive wide-open all the time, you're going to be running mostly on primaries and the spread-bore carb has smaller primaries."

Jon also warns that if you are changing from a square-bore carburetor to a spread-bore model, it's important to use the appropriate manifold. "Never use an adaptor," he advises. "For maximum efficiency and performance, you want a manifold that's correct for the type of carburetor you're using.