How Tos / Tech / Tech Articles

How to Choose a Carburetor

 

Wondering how to choose a carburetor?

This is a question that confronts anybody in the market for a carburetor, whether it’s for a street or race vehicle. Many companies offer several different carburetors, but with all of 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 carburetors: street legal, high performance street/strip, and race-only carbs.

The first thing to consider is the cfm you will need for your engine. There’s a simple formula available that will put you right in the cfm ballpark. The formula for naturally aspirated gasoline engines is:

Engine Size (c.i.d.) X Maximum rpm/3,456 = cfm at 100-percent Volumetric Efficiency (VE)

Example: 350 c.i.d. X 6,000 rpm = 2,100,000/3,456 = 608 cfm

Approximately 608 cfm would be required for this engine. However, most street engines are capable of achieving only about 80-percent VE; a modified street engine with ported heads, headers, good intake, and carburetor can achieve about 85-percent VE; a fully modified race engine can achieve 95-percent or greater VE. The cfm number arrived at with this formula must be factored by this percentage.

Next, you need to decide if a vacuum secondary or a mechanical secondary carburetor will best fill your bill. As a rule of thumb, vacuum secondary carburetors work best on the following:

  • Relatively heavy vehicles
  • Street gearing
  • Automatic transmission
  • Engines built more for low-end torque

Conversely, mechanical secondary carburetors seem to work best on:

  • Relatively light vehicles
  • Strip gearing (4.11 or numerically higher)
  • Manual transmission
  • Engines built more for top-end horsepower

The next decision to make is the type of choke you need. Most universal performance carburetors come equipped with either a manual choke or an electric choke. Manual chokes can be converted over to automatic electric choke operation with the proper kit, and vice versa.

Most three-speed automatic transmission kickdown linkage hook ups will bolt directly to the carburetor throttle lever. Chrysler applications will require the purchase of a bracket if it’s not already included with the carburetor. Some MOPAR applications may require a particular throttle assembly. Vehicles equipped with a GM TH-700R4 automatic overdrive transmission will require the purchase of a bracket kit.

For a racing engine, selecting the correct carburetor poses a unique problem. Since the engine was probably built for a particular racing class you may be limited to a particular cfm size. The first logical place to start is with your engine builder. Then take a good close look at the racing sanctioning body’s rule book. The engine builder should have enough experience to correctly advise you as to which carburetor will run best with your particular engine.

Secondly, look to see what other racers in your class are using, especially those who are winning. Use the cfm formula as a starting point and guide only.

Carburetor Descriptions

Street Legal Carburetor

Carburetors in this classification contain all relevant emission provisions and hook-ups for an emission-legal installation. These carburetors have been engineered for “bolt-on” installation on stock factory manifolds to replace existing Holley, Rochester, or Carter original equipment (O.E.) carburetors.

Two flange styles are available: square (Holley) flange and spread bore flange. Most square flange carburetors come equipped with an electric choke and vacuum secondaries, and will require an adapter when mounting to a spread bore manifold. Spread bore carburetors can be either vacuum or mechanical secondaries and the choke design will vary, depending on what was original equipment for the vehicle.

The emission identification label denotes products that are considered “emission legal.” California Air Resources Board (CARB) Executive Order (E.O.) numbers are not required in cases where a product is considered to be the functional equivalent of the replaced O.E. unit. E.O. numbers may be required in other cases where the replacement unit is not considered to be the functional equivalent, for various reasons. In these cases CARB grants E.O. numbers to signify the legality of these products.

High Performance Street/Strip Carburetor

Carburetors in this popular classification are primarily non-emission in nature and, as such, must be used only on off-road vehicles, racing vehicles, and pre-1966 (non-emission) vehicles. These carburetors are usually equipped with a manual choke, electric choke, or a hot air choke and may have some emission provisions, but are not designed to meet California emission standards. There are some carburetors that are application specific, and are specifically designed and calibrated for certain years and applications and are considered to be 50-state legal. They have been assigned an emission identification label.

Race-Only Carburetor

These carburetors have been designed and calibrated for racing only, and are available for strip, circle, oval track, and marine racing.

Approved by both NHRA and IHRA, strip carburetors are used across the country. Many carbs in this category have mechanical secondaries, double accelerator pumps, dual feed fuel bowls, and range in size from 500-1,250 cfm. Many are not equipped with a choke, and have no provisions for one. These are universal, non-emissions legal carburetors that are intended for off-road and racing vehicles only. Gas and alcohol versions are available in various sizes for most any size racing engine or class.

Other race carburetors are used in circle track or oval track racing. Approved by NASCAR and local tracks around the country, these two- or four-barrel carburetors are not intended for street use. They are calibrated for racing and generally have mechanical secondaries, double accelerator pumps and dual feed fuel bowls. Some newer styles have superior air flow and fuel metering capabilities. They range in size from 390-830 cfm, and feature a dedicated high-flow main body and metering block castings that have been designed for all-out circle track performance. The premium features of these carburetors enable them to perform much like the more expensive “modified” units. Gas and alcohol versions are available in various sizes for most any size racing engine or racing class.

 

Editor’s Note: Holley Performance Products contributed to this post.

 

Tags: , , ,

7 Comments

  1. I am glad to read above post here i am providing variety of power carburetors. Troubleshooting manual for Tecumseh carburetors for your own safety that includes Maintenance, Instructions and proper working of carburetor in the engine.

  2. Bruce Guertin says:

    Or you could go here and learn how to tune that puppy up from a winner of the Bo Laws Performance Carburetor Challenge. http://racingfuelsystems.myfunforum.org/index.php

  3. I was wondering what would be the downfalls to putting to big (CFM) a carb on my car. It is a 1965 Galaxie with the 352 interceptor. The hot-rod shop has ordered a 750 cfm performer saying it would be fine even though I did state that I thought nothing bigger than a 650 would be more than enough.

    • OnAllCylinders says:

      Thanks for reading! Agreed–first impression says it will most likely be too large for just 352 cubic inches. But, what, if any modifications do you have? What will be the max RPM of the motor? We’d suggest you call the Summit tech line with this information, and they can help you figure out what’s best–330-630-0240.

  4. Pingback: A Quick Guide to Carburetor Heights (and Why They're Important) - OnAllCylinders

  5. Pingback: Carburetor Basics 101: A Quick Overview of Primary and Secondary Barrels for Street or Race Applications - OnAllCylinders

  6. AnalogDan Wilson says:

    Articles about choosing the correct carburetor for a particular application generally contain the essential basics such as calculating the required CFM, vacuum or mechanical secondary venturi operation on 4-V carbs, choke types and more.

    The features listed by the various carburetor manufacturers always includes several adjustments pertaining to engine idle speed because all engines regardless of the intended usage will have to idle at times. Carburetors rely heavily upon intake manifold vacuum in varying degrees for proper operation, especially during operation at less than wide open throttle.

    Since the camshafts valve opening/closing events and degrees of open duration have a direct effect on intake manifold vacuum, shouldn’t this information be included in technical discussions and especially by carburetor manufacturers when they list their operating parameters for a particular carburetor ?

    Sometimes the descriptions for Demon Carburetors will include their compatibility with camshaft durations up to a certain level as measured in degrees at 0.050″ valve lift. This seems like an important specification and should be used by all carburetor manufacturers to help customers in making the right choices for purchase.

Leave a Reply