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How to determine correct Carburetor CFM?

The carburetor itself has perhaps the most pronounced effect on fuel consumption. Since each carb comes pre-calibrated for a certain cubic inch and RPM range, the best place to start is with the right size carburetor. You can calculate the required CFM using following formula:
Theoretical Carburetor CFM = (Engine CID * Maximum RPM) / 3456
This formula assumes 100% volumetric efficiency at redline, but street motors are more likely to fall in the range of 80 – 95%, depending on head/cam/intake/exhaust selection. Therefore, we should factor that into the equation by using an additional formula:
Required CFM = Theoretical CFM * Estimated Engine VE%

Now that we know the required amount of airflow, the next decision is whether to use a vacuum secondary carburetor or double-pumper design. For drag-race-only applications, a double-pumper will help the car launch harder at the track, but if the vehicle is primarily used on the street, the vacuum secondary is often a much better choice. Lack of secondary accelerator pump shot and a delayed secondary opening will increase fuel economy. The vacuum secondary carb’s fuel calibration is usually more efficient as well. In either case, make sure the idle and main air bleeds located on the top of the carburetor remain clean and unrestricted.
Quick Carburetor Sizing

This is a real quick and dirty formula for figuring carburetor CFM requirements for a street engine. This Ballpark method is very useful at the swap meet, parts store, or when impressing friends and family.
Take the engine displacement and multiply by 2. Then subtract 15% from the total. For a little healthier engine, subtract only 5%.
This a real ball park formula. For a more accurate calculation, see the Carburetor Sizing Calculator. But, this is still a good formula for general sizing purposes.
d = Displacement in Cubic Inches. (d x 2) – 15% = CFM
EXAMPLE: d = 350 (350 X 2) = 700 (700 – 15%) = 595 CFM

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