When the Firebird was introduced...
When the Firebird was introduced in 1967, its engines were limited to a maximum of 325 hp in order to satisfy GM's corporate power-to-weight standards. Pontiac restricted the Firebird 400 carburetor to limit airflow in all '67-'68 applications, including examples like Rick Mahoney's '68 Firebird 400, equipped with the powerful Ram Air II.
When dealing with popular historical subjects, some of the well-known facts surrounding a particular topic or event seemingly become common knowledge. Most individuals can accurately recite a factual overview of popular examples. As with any historical subject, however, small details are lost over time, and loose factual lore is what's left behind. A portion of First-Gen Firebird history has suffered this fate.
When discussing its introduction in 1967, most enthusiasts are quick to note that the new First-Gen Firebird featured a special carburetor linkage that prevented full-throttle operation. The intent was to restrict airflow, which reduced engine output, making the Firebird package compliant with General Motors' corporate horsepower-to-weight ratio standard. Many will also state that simply "bending the tab" defeats the limiting device, maximizing performance.
While said information is absolutely accurate and the topic is quite popular with early Firebird enthusiasts, the specifics of the throttle linkage and its effect on carburetor operation are somewhat mythical. Though frequently mentioned in Pontiac references or on popular web forums, few hobbyists can describe the Firebird-specific piece. Follow along as we take a closer look at these special Rochester carburetors and discuss their uniqueness with a Quadrajet expert.
Rochester developed the Model...
Rochester developed the Model 4M Quadrajet four-barrel carburetor during the mid-'60s. Pontiac began using it extensively on its '67 performance applications when Tri-Power was discontinued. The standard 4M casting used through 1974 was capable of flowing as much as 750 cfm, while some specialized applications were capable of 800 or more. Maximum airflow of the '67-'68 Firebird-specific casting was significantly reduced to limit engine output. Some experts estimate as little as 550 to 600 cfm in stock form!
General Motors banned the use of multiple carburetion on all its Divisions' vehicles except for the Corvette in '67, and Pontiac adopted the newly developed Rochester Quadrajet for its four-barrel performance applications. To ensure that the '67 GTO's single-carbureted mill performed every bit as well as the 360hp Tri-Power 389 it replaced, Pontiac increased displacement to 400, revised the cylinder heads, and introduced a new four-barrel intake manifold.
The new 360hp 400 H.O. maintained the '67 GTO's position among that year's performance leaders. When the Firebird debuted in midyear '67, the large displacement engine and small-body concept that propelled the GTO to supercar status was quite apparent in the Firebird, and it immediately joined the GTO at the top of the performance heap. The completely new high-performance vehicle weighed 350 pounds less than the GTO, and it was a recipe for immediate success.
The Quadrajet features a progressive...
The Quadrajet features a progressive linkage design that allows the primary throttle blades to open a specific amount before engaging the secondary barrels. The variable tab (shown in circle) floats freely on the primary throttle shaft, while the U-shaped tab at its right is fixed. As the primary throttle shaft rotates, the fixed tab presses on the variable tab, actuating the secondary circuit.
The Firebird 400 was Pontiac's top option for the model line. For it to comply with GM's then-corporate-imposed limit of 10 pounds of vehicle weight for every horsepower, 325 hp was the maximum rating any 400 could have for the 3,250-pound Firebird. Instead of creating a specific 325hp 400 package for the Firebird, along with its base 400 four-barrel, Pontiac took the 360hp H.O. mill used in the 3,600-pound GTO and de-rated it to just 325 hp for use in the Firebird. And the result wasn't accomplished simply by juggling numbers.
Airflow Adjustment
Adjusting the angle of the secondary air-valve in the Quadrajet's air horn to fine-tune a particular carburetor for a specific four-barrel application was standard practice at Rochester. While modern airflow and dyno testing reveals that this practice didn't grossly affect total capacity of the standard Model 4M casting, it proved an effective method of controlling full-throttle air/fuel ratio of the 750-cfm casting to meet the requirements of its intended specific production engine.
Flow bench and dragstrip testing...
Flow bench and dragstrip testing proves that when using a Quadrajet, peak airflow occurs when its primary and secondary throttle blades are at 90-degree angles when compared to baseplate. Quadrajet expert Cliff Ruggles says the baseplate of every carburetor that leaves his shop, (Cliff's High Performance in Mount Vernon, Ohio) has been checked for full opening, promoting maximum possible performance.
To reduce maximum output for the Firebird 400, Pontiac and Rochester developed a unique secondary-actuation linkage that significantly limited the full-throttle angle of the secondary throttle blades found in the baseplate. When the modified Quadrajet was installed on the 360hp H.O. engine, for instance, airflow was reduced so much that it limited the 400's maximum output to approximately 325 hp, which made the Firebird 400 fully compliant with GM's standard.
Rochester assigned specific casting numbers to those Quadrajets used in Firebird applications. It didn't take savvy performance enthusiasts long to learn that simply adjusting the secondary actuation linkage allowed full throttle operation, giving the Firebird 400 engines a solid 20-25 more horsepower. Tech tips were published in a number of performance-oriented publications, and it was seemingly among the first modifications that many enthusiastic Firebird owners performed after taking delivery.
The unique Firebird Quadrajets were used through the '68 model year. The horsepower rating of the GTO's otherwise 360hp Ram Air engine was 335 for the Firebird, while the midyear introduction of the 366hp Ram Air II was limited to 340 hp in the Firebird. It seems that GM relaxed enforcement of its imposed standard for 1969. When the '69 models debuted, the Firebird and GTO engines shared a common carburetor number for specific applications. The Firebird mill was still de-rated, but it was now on paper only.