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.
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.
The Quadrajet carburetor was developed by the Rochester Product Division of General Motors
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.
A typical square-bore carburetor contains four equal or near-equal-size bores that flow si
A spread-bore carburetor typically contains two small primary bores and two large secondar
Most Pontiacs through '70, including the high-revving R/A II and R/A IV engines, used this
Pontiac engineering used two unique Model 4Ms to meet the airflow demands of its Super Dut
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.
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 |
The Model 4M was redesignated Model M4M in '75, and it contained the same 17/32-inch prima
A Superflow 110 flow bench was used to measure the airflow capacity of the carburetors. We
The Model 4M castings used on most '74 Pontiacs contain a primary diameter of 13/32 inches
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 |
Pontiac used a unique Quadrajet Model 4M for specific applications during the '71 model ye
The primary diameter of the Super Duty 455 Model 4M and all Pontiac Model M4Ms from '75 fo
This tab located on the secondary air-valve shaft is used to restrict airflow in applicati
Though 20.4 inches is the accepted pressure standard for rating four-barrel carburetor air
Other than a few early Quadrajets that utilized a stamped-steel disc, the application cast
Pontiac engines used a divorced-style choke with a thermostatic coil mounted on the intake
From '73 forward, the thermostatic coil was mounted onto the carburetor main body and func
|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 |