
A typical square-bore carburetor...

A typical square-bore carburetor contains four equal or near-equal-size bores that flow similarly. Two are designated primary units, while a mechanically- or vacuum-operated linkage opens the secondary units when additional airflow is required.

A spread-bore carburetor typically...

A spread-bore carburetor typically contains two small primary bores and two large secondary bores to improve throttle response and fuel economy without compromising total performance.

Most Pontiacs through '70,...

Most Pontiacs through '70, including the high-revving R/A II and R/A IV engines, used this type of Model 4M. In '71, its air horn received an adjustable air bleed and large vent tube for improved float-bowl ventilation. Functionality and airflow capacity of the casting remained the same through '74.

Pontiac engineering used two...

Pontiac engineering used two unique Model 4Ms to meet the airflow demands of its Super Duty 455 and 455 H.O. engines. The SD-455 casting had a larger-diameter primary (left) while the high-flow '71 casting (right) was also used on D-port 400 and 455ci engines with manual trans and cast without outer velocity boosters.
Rochester took another approach towards increasing the airflow capacity of its Model 4M during the same period. Originally developed for Buick's 455 and later used by Pontiac for its Super Duty 455, the revised main body featured a primary diameter increase of 1/4 inch to a total of 17/32 inches. Retaining the full velocity booster pack and significantly increasing primary airflow without affecting emissions, these units are regarded as the best for high-performance use.
A modified version of the 4M, designated Model M4M, was introduced for the '75 model year. Not only did the redesigned unit contain several internal enhancements to improve engine efficiency, the main body of every Pontiac V-8 (from 301 to 455ci depending upon the model year) featured the 17/32-inch diameter primary like the Super Duty's Model 4M casting. Scorned by many as smog-era carburetors with very lean fuel metering, the casting changed very little for the remainder of the '70s.
The Quadrajet saw some significant design changes during the '80 model year as computer control command was introduced. Designated E4M, many of the mechanical internals were replaced by electronic components, offering more precise control over the fuel curve, reducing emissions, and improving long-term consistency. The E4M, however, requires an electronic module for normal operation.
Airflow Basics
Carburetor airflow is measured in cubic feet per minute (cfm), and the pressure standard the industry recognizes for rating four-barrel airflow is 1.5 inches of Mercury or its water pressure equivalent of 20.4 inches. It appears, however, that Rochester may have never publicly released airflow-capacity ratings for the Quadrajet. This is most likely due to the fact that the company produced factory-installed carburetors that were uniquely calibrated for each application.
Hobbyist testing tends to indicate that a typical Pontiac Model 4M contains a maximum airflow capacity of 750 cfm. The high-flow '71 and Super Duty 455 4Ms and all M4Ms from '75 forward are regarded as 800-cfm units. But to fit the myriad Pontiac applications, the secondary air valve was used to restrict airflow when the casting's maximum capacity wasn't needed, by limiting the opening angle to a specific degree.
Many hobbyists have often wondered which Pontiac Quadrajets offer the most airflow capacity. Having a number of different Pontiac castings available to us, including a high-flow '71 and SD-455, we decided to compare airflow characteristics using professional equipment. Since it would be unpractical to obtain and flow-test an example of every Pontiac application over the years, the Quadrajets selected for testing represent the major style changes that occurred during the production run and are likely to be used by hobbyists today. The casting numbers and original applications of the test carburetors are shown below.
| Casting number | Application | Accepted Airflow | Represents |
| 7028263 | '68 428ci | 750 cfm | '70-earlier 4M |
| 7044262 | '74 350/400ci | 750 cfm | '71-'74 4M |
| 7041270 | '71 455 H.O. | 800 cfm | '71 high-flow 4M |
| 7044270 | '74 SD-455 | 800 cfm | '73-'74 SD 455 4M |
| 17057262 | '77 350ci | 800 cfm | '75-later M4M |