Choosing The Correct Carburetor
Choosing the correct carburetor can seem like a daunting task for the those that may be new to the industry or lack automotive experience. Today’s automotive industry is replete with sophisticated computer controlled systems, multiplexing serial data streams, and complex algorithms. Fuel systems are more efficient today than at any other time in history. Throttle body injection paved the way for multi-port fuel injection and now we are seeing more direct injection systems than ever before, and yet, the old antiquated carburetor continues to live on. One of the biggest reasons for choosing a carburetor for your classic build is it’s easy, it’s inexpensive, and it’s typically reliable.
Here on our radio show Horsepower for an Hour, we frequently hear from listeners who are having difficulty choosing the correct carburetor for their particular build. Many companies offer several different carburetors, but with all of the different styles and sizes, selecting one can seem complicated.
The Basic Formula
There is a basic formula for choosing the correct carburetor for your engine and it’s pretty simple. Any first year automotive technology student will readily tell you it’s engine size, (CID) multiplied by maximum RPM, divided by 3456.
This equation gives us the required CFM (cubic feet per minute) for a multi-cylinder four-stroke engine with at least four cylinders and a common plenum area for the intake; with a volumetric efficiency of 100 percent.
Understanding Carburetor Formulas
Over the years, we’ve learned that many folks don’t quite understand the equation or what it means. So, here’s a quick explanation:
* CFM is airflow of the engine measured in cubic feet per minute.
* RPM is the speed of the engine measured in revolutions per minute.
* CID is the displacement of the engine measured in cubic inches.
The number 3456 is more easily understood, and much more easily remembered if it is re-written (12 x 12 x 12 x 2). As there are 12 inches in a foot, we multiply 12 by 12 by 12 to keep the dimensions the same. (Remember it’s a cubic foot) The “2” is required because of the 4-stroke engine only drawing air on ever other revolution of the engine. The equation assumes 100 percent volumetric efficiency of the engine
A carburetor’s airflow capacity, measured in cubic feet per minute (cfm), is critical in choosing the right carburetor for your build. Too large will cause low-rpm response issues and a rich condition. Too small will restrict the engine performance at higher rpm ranges. A dyno will help tremendously when it comes to dialing the carburetor in.
Where To Go For Help
If you are unsure of your choice, or need more information, the carburetor manufacturer’s tech lines are very helpful in pointing you in the right direction. Holley has a very useful website for choosing the correct carburetor that most people find very easy to use, and there are others as well. One thing is certain, even with all of today’s sophisticated fuel injection systems, the carburetor remains a reliable alternative that performs well and is a perfect fit for the budget minded build. Remember, choosing the correct carburetor does not have to be difficult and there are several resources available for you to help ease the struggle.
For answers to these questions or any other automotive type question, feel free to message us anytime here at HorsepowerOnline.com.