Wondering how to choose a carburetor?
This is a question faced by anyone in the market for a carburetor Either for a street or racing vehicle. Many companies offer several different carburettors, but with all the different styles and sizes, selecting one can seem complicated.
However, there is a logical way to select the best carburetor for your vehicle.
There are three basic types of carburettors: street legal, high performance street / strip, and racing only.
The first thing to consider is the cfm you will need for your engine. There is a simple formula available that will put you in the cfm stage. The formula for naturally aspirated gasoline engines is:
Engine Size (cid) X Maximum RPM / 3,456 = cfm at 100 Percent Volumetric Efficiency (VE)
Example: 350 cid X 6,000 rpm = 2,100,000 / 3,456 = 608 cfm
Approximately 608 cfm would be needed for this engine. However, most urban locomotives are capable of only 80 percent VE; a modified street engine with ported headers, headers, good intake, and carburetor can achieve 85 percent VE; a fully modified racing engine can achieve 95 percent or more VE. The number of cfm obtained with this formula must be factored by this percentage.
Next, you need to decide whether a secondary vacuum carburetor or a mechanical secondary will better fill your bill. As a general rule of thumb, secondary vacuum carburetors perform best at the following:
- Relatively heavy vehicles
- Street gear
- Automatic transmission
- Engines built more for a low-end pair
In contrast, mechanical secondary carburettors seem to work best on:
- Relatively light vehicles
- Band Gear (4.11 or higher numerically)
- Manual transmission
- Engines built more for high-end power
The next decision you need to make is the type of throttle what do you need. Most universal performance carburetors come equipped with a manual or electric choke. Manual chokes can be converted to automatic operation with electric choke with the appropriate kit and vice versa.
Most three-speed automatic transmission kickdown linkage connections will bolt directly to the carburetor throttle lever. Chrysler applications will require the purchase of a bracket if it is not already included with the carburetor. Some applications MOPAR may require a particular throttle assembly. Vehicles equipped with a GM TH-700R4 automatic overdrive transmission will require the purchase of a set of brackets .
For a racing engine, selecting the correct carburetor poses a unique problem. Since the motor it was probably built for a particular racing class, it may be limited to a particular cfm size. The first logical place to start is with your engine manufacturer. Then take a close look at the racing sanctioning body’s rule book. The engine manufacturer should have enough experience to properly advise you which carburetor will work best with your particular engine.
Second, look at what other riders in your class are wearing, especially the ones who are winning. Use the cfm formula as a starting point and only as a guide.
Carburetor Street Legal
Carburettors in this rating contain all relevant fitting and emissions provisions for an emissions legal installation. These carburettors have been designed for a “bolt-on” installation on factory manifolds to replace Original Equipment (OE) carburettors. Holley , Rochester or Carter existing.
Two styles of flange are available: Square (Holley) flange and Extended bore flange. Most square flange carburettors come equipped with an electric choke and vacuum secondaries, and will require an adapter when mounted on an extended diameter manifold. Extended bore carburettors can be secondary mechanical or vacuum and the choke design will vary, depending on the vehicle’s original equipment.
The emissions identification label indicates products that are considered “emissions legal”. California Air Resources Board (CARB) Executive Order (EO) numbers are not required in cases where a product is considered the functional equivalent of the replaced OE unit. The EO numbers …