How to Choose a Carburetor for Your Engine

The size of the engine has a lot to do with how much air can be drawn into the cylinders. Rpm range and volumetric efficiency of the engine also play a big role on carburetor selection.
Carburetor manufacturers rate their carburetors flow capacity in cfm (cubic feet per minute). The cfm rating is the amount of air that can flow through the carburetor at wide open throttle when 1.5 inches of vacuum is applied. When a race engine is under WOT it must have at least 1 inch of vacuum differential between the air inside the intake manifold and the air outside the carburetor (atmospheric pressure). This differential causes air to continuously flow through the carburetor at a minimum velocity.

You would be surprised how many inexperienced racers bolt on the biggest carburetor they can find thinking it will help scoot their car down the track. They think if the carburetor was too big it would be smoking black out the exhaust pipes. In fact, bolting on a carburetor that is too large for an engine will do just the opposite. That's right! It actually leans out the engine because the velocity of air slows down significantly anytime you increase the barrel size of a carburetor. This results in lower vacuum in the venturi which results in less fuel being pulled into the air stream. The end result is an engine that bogs when launching the car off the starting line.

Visit Hotrod Pitstop to find out more about Choosing a Performance Carburetor for your engine.