It seems that the Rochester Quadrajet is a popular hobbyist choice for Pontiacs that are regularly street-driven. Not only is the carburetor well known for its excellent balance of low-speed throttle response and full-throttle performance, Pontiac selected the Quadrajet for many of its performance applications in '67 and all four-barrel applications from '68 forward. So there are a number of original castings that, when modified correctly, make excellent performance units while maintaining an unmodified underhood appearance.

Over the years, the basic Quadrajet casting saw a few design changes, including attempts at increasing airflow for high-performance applications. These highly coveted units are the ones most sought after and demand hundreds if not thousands of dollars on the open market. But you may wonder what airflow characteristics separate them from an ordinary Pontiac Quadrajet. We gathered several different Pontiac castings for a detailed comparison. Follow along as we share the results. You might find a low-buck casting that contains the desired airflow qualities for your combination.

History
The carburetor's role is to provide the engine with metered amounts of fuel and air that vary with workload. A throttle valve controls the volume of air passing through the bores and over the fuel discharge nozzles. The resultant pressure depression draws atomized fuel from the float bowl through the nozzles.

As displacement and power output increased over the years, engines consumed greater amounts of fuel and air. Although the simplest solution to satisfying those demands may seem as easy as increasing carburetor size, doing so can lessen air velocity, which reduces nozzle signal, ultimately degrading throttle response and low-speed performance.

To retain maximum performance in all conditions, a dual-stage carburetor with four near-equal-size bores (or barrels) was developed. Airflow was directed through two primary barrels for maximum low-speed performance. As engine workload increased, a progressive throttle linkage opened the remaining two barrels for maximum heavy-throttle performance.

Although the basic four-barrel design proved quite successful, carburetor engineers discovered that throttle response and fuel economy could be improved by decreasing primary size, and by increasing the secondary size, the engine's maximum flow requirement could be fulfilled to maintain strong full-throttle performance. A highly efficient, well-balanced unit that was practical for virtually any driving condition was the end result.

The first spread-bore carburetor in the industry was the Quadrajet. It was developed by the Rochester Product Division of General Motors in the mid-'60s and designated Model 4M. Its throttle body featured two 1-3/8-inch primary and two 2-1/4-inch secondary bores, and its main body had a primary diameter of 1-3/32 inches. It also featured a vacuum-actuated secondary air valve that provided varying amounts of airflow based on engine workload.

Model 4M first appeared on select '65 Chevrolet applications. Its use expanded into other GM applications over the next few years. By '68, the Quadrajet was the only four-barrel used by the GM divisions save for a few specialized Chevrolet applications that used a Holley.

A very unique Model 4M was developed for Pontiac in '71. Limited to 400 and 455ci manual-transmission engines and all 455 H.O. applications, the main body was cast without the outer velocity-booster rings that surround the fuel-discharge nozzles in the center of each primary. Aimed at increasing overall airflow, former Pontiac engineer Skip McCully noted that the missing boosters degraded nozzle signal, which negatively affected off-idle and low-speed emissions. The design lasted only one year as regulations tightened for '72.

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

Airflow Testing
Before our flow test could begin, we had to create an adapter that would allow us to mount a carburetor onto the flow bench. Using a bare Quadrajet throttle body as a template, we drilled four correct-size holes into a piece of 1.25-inch-thick soft pine. By applying tape over a hole, we could effectively seal off any portion of the carburetor we wanted to eliminate during the flow test.

After properly warming and calibrating our Superflow 110 bench and eliminating secondary flow through the adapter, we placed each carburetor on the bench and recorded airflow. The testing procedure included ensuring the choke and throttle valves were wide open, checking port alignment and for air leaks, and installing a modified air-cleaner base to smoothen the incoming air. The results were recorded at 9 inches of water and mathematically converted to 20.4-inch values.

Secondary airflow was a little more difficult to measure. As previously mentioned, Rochester used the secondary air valve to limit total airflow of its 4M and M4M models for applications that required less. Surprisingly, this includes most standard-performance Pontiacs. It seems that the air valve of high-performance units like the high-flow '71 and SD-455 open furthest. So we used them to establish base flow.

After eliminating primary airflow and replicating the testing procedure, we found peak secondary airflow of the high-performance castings to be around 600 cfm. Maximum airflow through the remaining castings varied from 520-580 cfm (depending on the application), which indicates that engineers spent time determining the optimal amount of total airflow for each application.

Simply removing a slight amount of material from the secondary air-valve tab can increase airflow. Using a carbide cutter, we applied this modification to a test carburetor. The goal was to remove enough material that the rear half of the secondary air valve opened nearly 90 degrees from its closed position. Subsequent flow-bench testing revealed that secondary airflow had significantly increased. But keep in mind that the added airflow won't necessarily translate into additional performance. Typically only engines that are airflow-restricted will gain from this.

Approximate Primary Airflow Converted to 20.4 inches
  7028263 7044262 7041270 7044270 17057262
Primary airflow 176 cfm 182 cfm 228 cfm 212 cfm 210 cfm
Approximate Total Airflow at 20.4 inches Assuming Secondary Flow of 600 cfm
  7028263 7044262 7041270 7044270 17057262
Total airflow 776 cfm 782 cfm 828 cfm 812 cfm 810 cfm
HPP Pocket Carb Chart
Number Year Application Type Approx Flow
7026260 '66 A, OHC 6-cyl, auto trans 4M 750 cfm
7026261 '66 A, OHC 6-cyl, manual trans 4M 750 cfm
7027260 '67 A, F, OHC 6-cyl, auto trans, w/o AIR (early) 4M 750 cfm
7027261 '67 A, F, OHC 6-cyl, manual trans, w/o AIR (early) 4M 750 cfm
7027262 '67 400/428 auto w/o AIR 4M 750 cfm
7027263 '67 400/428 manual w/o AIR 4M 750 cfm
7027268 '67 A, F, OHC 6-cyl, auto trans, w/o AIR (late) 4M 750 cfm
7027269 '67 A, F, OHC 6-cyl manual w/o AIR (late) 4M 750 cfm
7027272 '67 Firebird 400 auto w/o AIR 4M 750 cfm
7027273 '67 Firebird 400 manual w/o AIR 4M 750 cfm
7037260 '67 A, F, OHC 6-cyl, auto trans, w/AIR (early) 4M 750 cfm
7037261 '67 A, F, OHC 6-cyl, manual trans, w/AIR (early) 4M 750 cfm
7037262 '67 400/428 auto w/AIR 4M 750 cfm
7037263 '67 400/428 manual w/AIR 4M 750 cfm
7037268 '67 A, F, OHC 6-cyl auto w/AIR (late) 4M 750 cfm
7037269 '67 A, F, OHC 6-cyl manual w/AIR (late) 4M 750 cfm
7037271 '67 GTO w/Ram Air 4M 750 cfm
7037272 '67 Firebird 400 auto w/AIR 4M 750 cfm
7037273 '67 Firebird 400 manual w/AIR 4M 750 cfm
7037276 '67 Firebird w/Ram Air 4M 750 cfm
7028260 '68 A, F, OHC 6-cyl, auto trans 4M 750 cfm
7028261 '68 A, F, OHC 6-cyl, manual trans 4M 750 cfm
7028262 '68 400 auto trans 4M 750 cfm
7028263 '68 400 manual trans 4M 750 cfm
7028264 '68 Firebird 400, auto trans 4M 750 cfm
7028265 '68 Firebird 400, manual trans 4M 750 cfm
7028266 '68 A, F 350 H.O., auto trans 4M 750 cfm
7028267 '68 High-performance 400, manual trans 4M 750 cfm
7028268 '68 High-performance 400, auto trans 4M 750 cfm
7028269 '68 A, F 350 H.O., manual trans 4M 750 cfm
7028270 '68 A, F, 400 auto w/Ram Air 4M 750 cfm
7028271 '68 High-performance Firebird 400, manual trans 4M 750 cfm
7028273 '68 A, F, 400 manual w/Ram Air 4M 750 cfm
7028274 '68 A-body 400, auto trans, w/Ram Air 4M 750 cfm
7028275 '68 A-body 400, manual trans, w/Ram Air 4M 750 cfm
7028276 '68 Firebird 400, auto trans, w/Ram Air 4M 750 cfm
7028277 '68 Firebird 400, manual trans, w/Ram Air 4M 750 cfm
7029260 '69 A, F, OHC 6-cyl auto 4M 750 cfm
7029261 '69 A, F, OHC 6-cyl manual trans 4M 750 cfm
7029262 '69 Bonneville 428 auto trans 4M 750 cfm
7029263 '69 350/400/428 manual trans 4M 750 cfm
7029268 '69 350/400/428 auto trans 4M 750 cfm
7029270 '69 A, F, 400 auto w/Ram Air 4M 750 cfm
7029273 '69 A, F, 400 manual w/Ram Air 4M 750 cfm
7040262 '70 B, 400/455 auto trans 4M 750 cfm
7040263 '70 A, F, G, 400 manual trans 4M 750 cfm
7040264 '70 A, F, G, 400 auto trans 4M 750 cfm
7040267 '70 A, G, 455 manual trans 4M 750 cfm
7040268 '70 A, G, 455 auto trans 4M 750 cfm
7040270 '70 A, F 400 auto trans w/Ram Air 4M 750 cfm
7040273 '70 A, F 400 manual trans w/Ram Air 4M 750 cfm
7040562 '70 B, 400/455 auto trans, Calif 4M 750 cfm
7040563 '70 A, F, G, 400 manual trans, Calif 4M 750 cfm
7040564 '70 A, F, G, 400 auto trans, Calif 4M 750 cfm
7040567 '70 A, G, 455 manual trans, Calif 4M 750 cfm
7040568 '70 A, G, 455 auto trans, Calif 4M 750 cfm
7040570 '70 A, F 400 auto trans w/Ram Air, Calif 4M 750 cfm
7040573 '70 A, F 400 manual trans w/Ram Air, Calif 4M 750 cfm
7041262 '71 A, F, G, 455 auto trans 4M 750 cfm
7041263 '71 A, F, G, 400/455 manual trans (high flow) 4M 828 cfm
7041264 '71 A, F, G, 400 auto trans 4M 750 cfm
7041267 '71 A, F, 455 H.O. manual trans w/o Ram Air (high flow) 4M 828 cfm
7041268 '71 A, F, 455 H.O. auto trans w/o Ram Air (high flow) 4M 828 cfm
  • «
  • |
  • 1
  • |
  • 2
  • |
  • 3
  • |
  • 4
  • |
  • View Full Article