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Carburetor Sizing
by Larry D'Argis


If enough is good…more is better…and too much is just right.  Yes the old saying can often get you in trouble whether it's too much Christmas turkey, or too much carburetor on your old Ford.  Many have complained that replacing their worn out Holley four-barrel carburetor with the standard Holley 600 CFM (cubic feet per minute), replacement carburetor, turns their Y-block or FE Ford into a gas guzzler.  Why?

If we look at the factory carburetor you removed and size it up with the 600 CFM replacement, we see that the barrels or venturis are noticeably smaller than the new carburetor.  While going with the larger carburetor on a modified engine will boost power, it does little on the stock engine in your '50s Ford cruiser.  Wide-open throttle may feel a bit better, but slower throttle response, a rich idle and poor gas mileage are a lousy trade-off.  While that 600 CFM. carburetor should be quite happy on top of a 390 V8, it's just going to cause trouble on a smaller engine.  What works is to choose the correct CFM replacement and there's nothing magical about it.


Most naturally aspirated engines operate on an 83 to 85% volumetric efficiency, meaning they can fill about 85% of the cylinders volume under wide-open throttle.  To properly size a carburetor we need to know the cubic inch displacement of the engine and the maximum RPM, (revolutions per minute), it will be running.  An example would be a 427 V8 turning a maximum of 6,700 RPM should only require a 703 CFM carburetor.  In comparison a 312 V8 turning 5,600 calculates out to just 430 CFM.  The actual equation is 100% VE = cubic inch displacement X maximum RPM divided by 3456 X 85%.  Don't knock yourself out with a pencil and paper, you can find this handy CFM calculator at www.4secondsflat.com/Carb_CFM_Calculator.html

Holley makes a very nice 465 CFM four-barrel carburetor (part number 0-1848-1) that is a direct replacement for the '57 312 V8.  That same carburetor is very close to the one used on the late '50s 332 and 352 V8 FE series engines.  The smaller venturis allow for better throttle response for even acceleration and being sized for the engine it will offer a smoother, cleaner idle and better fuel economy.  The key is to think small, it pays off big down the road.


For a complete listing of Holley carburetors made up to mid-1970, including the original application and the OEM part numbers, see here.