What Size Carburetor Do I Need?
We’ve all heard the term - bigger is better. We know that more air and fuel equals more power. So why not buy the biggest carburetor out there if this is so? Unfortunately too much carb is a common mistake made that can actually choke the overall performance of your engine. The right idea would be to proportion the carburetor to the engine's volumetric potential (breathing ability). The engine's volumetric efficiency is a measurable value and with a correctly matched carburetor you will receive the best performance possible out of your engine.
The formula for calculating how much CFM (cubic feet per minute) an engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.
Any ordinary stock engine will have a volumetric efficiency of about 80%. Most rebuilt street engines with average bolt-ons have a volumetric efficiency of about 85%, while race engines can range from 95% up to 110%.
Example: Using a 355 CID engine x 5,500 max rpm = 1,952,500
Take 1,952,500 x .85 = 1,659,625
Then 1,659,625 ÷ 3456 = 480 CFM
Even with about a 10% cushion, a 500 CFM carburetor will handle this engine great. If you try this formula yourself, be honest with how much rpm the engine will see. You’d be better off with a carb smaller than needed rather than something oversized that can lead to poor drivablility and performance.
A vacuum secondary carburetor is usually most fuel efficient when its used on street driven cars that have automatic transmissions. Vacuum secondary carburetors have one accelerator pump and work off sensing the engine load, which progressively open the secondary butterflies as rpm increases. A mechanical secondary carb uses mechanical linkage to open the secondary butterflies and most also have dual accelerator pumps (a.k.a. double pumper). Mechanical secondary carbs are best used on applications with more radical camshafts, high rpm racing situations, and manual transmissions.