Since it was Pontiac who made ram-air household terminology in the '60s, it knew excitement could be created easily with thoughts of cold-air induction. Ask any automotive enthusiast to quickly name any vehicle associated with a factory shaker hood scoop and replies will be severely limited. While you may hear the occasional Mopar or Ford example, many would ultimately provide Trans Am as the most overwhelming response. In many hobbyists' eyes, the shaker hood scoop is as Trans Am symbolic as the large hood bird decal optioned on so many '73-81 models. The shaker hood scoop provided an already intimidating package with an aggressive appearance and in some instances was fully functional for improved performance.
As commonly associated with the Firebird Trans Am as the hood scoop is, many passive enthusiasts do not correlate them with '74 GTO or '77 Can Am models. Our 2-part series will take a close look at differences between hood scoop's design over the years and different applications on which they were found. Our first installment will cover functional scoops with the latter depicting those non-functional.
THE '70 1/2-72 TRANS AM SHAKER
In February 1970, the world was introduced to a freshly redesigned Firebird with a more contoured and aerodynamic body shape, revised suspension geometry for improved ride and handling characteristics, and the Trans Am option was now a Firebird model unto itself. The Trans Am incorporated a lower front air dam, front and rear wheel flares, a rear spoiler for high-speed downward force, and a rearward facing engine mounted scoop peering through an opening in the hood. Since the scoop was mounted directly to the air cleaner, it nervously danced with engine harmonics giving its name "shaker hood scoop" descriptive meaning. The '70 1/2 Firebird Trans Am shaker hood scoop was functional, allowing the engine to ingest cooler outside air via a throttle-actuated, solenoid-controlled "air valve" at the rear of the scoop. Dual solenoids were mounted on an internal bracket within the scoop housing using a small T-shaped rod extending to the air valve, which was basically a hinged door. The air valve was used to keep weather elements and foreign objects from entering the air cleaner assembly under normal conditions but for more spirited driving, specific throttle position caused the solenoids to energize, opening the air valve and allowing outside air into the air cleaner.
This design carried through 1972 on Trans Am models before ever-increasing federal standards on drive-by noise and emissions output rendered the hood scoop inoperable. From 1973 through the end of Second-Gen Firebird production in 1981, no other Trans Am would boast of a functional shaker hood scoop from the factory.
The '70 1/2 and '71 Trans Am models used a carburetor-mounted switch to control solenoid activity. A specifically-shaped linkage fastened to the carburetor throttle shaft would contact and depress a stationary spring-loaded switch mounted on the carburetor body. At approximately the same time the secondary barrels opened, the linkage began depressing the switch thereby energizing the solenoids for an enhanced 4-barrel effect from the carburetor. Adding much wiring and general clutter under the hood and for reasons most-likely to reduce repair costs from broken components, a revised actuation system was incorporated for the '72 model year. While many '72 Trans Am owners and enthusiasts are aware differences exist, changes were not noted in the 1972 Pontiac Service Manual or any 1972 Pontiac Technical Bulletins. The only reference to the updated design was found on page 33 of 1972 Craftsman Service News, issue number 2.
Throttle position still controlled solenoid activity, however, carburetor-mounted contacts were replaced by a pedal-mounted transmission kickdown switch. Since automatic transmission-equipped '72 Trans Am kickdown switches had a dual-purpose, controlling both transmission kickdown and hood scoop operation, a specific switch with higher resistance, PN 1235480 was required. Manual transmission cars also received a transmission kickdown switch, though required for only one role, shaker hood scoop solenoid activation.
THE '74 GTO SHAKER
The '74 GTO was unique in many respects. Most notably, it was the first time since 1964 that the GTO was built on a platform other than the A-body. Based on the X-body chassis, the GTO package was optional on the Ventura model and included color coordinated decals, a 350ci 4-barrel engine, and a functional Trans Am-style shaker hood scoop. With operational characteristics similar to that of earlier Trans Am models, a larger single solenoid replaced dual solenoids to control air valve operation.
However, unlike earlier Trans Am models, which used throttle position to activate the solenoids, the '74 GTO shaker scoop used a series of electronic switches and manifold vacuum to operate a vacuum switch to produce the electrical connection for the solenoid. A temperature-activated switch mounted on the passenger side cylinder head ensured solenoid activation would only occur at or above 140-degree coolant temperatures. A specific vacuum-actuated switch mounted within the scoop housing was connected to both a 12V lead and manifold vacuum through an electric switch mounted on the intake manifold. The vacuum switch was held open by manifold vacuum and anytime manifold vacuum dropped below 1.5 to 1.9 in/hg, internal spring tension would overcome vacuum forces causing the switch to close, current to pass, and solenoid activation to open the air valve.
As with the '72 Trans Am, operational characteristics for the '74 GTO were not mentioned in the 1974 Pontiac Service Manual or any 1974 Pontiac Technical Bulletins. Page 25 of 1974 Craftsman Service News, issue number 2 provides a general description and abbreviated flowchart for technicians to diagnose any malfunction. The small article also provides the procedure for checking solenoid operation, which consists of a coolant temperature above 140 degrees, an ignition position of "ON," and the engine not running to prevent manifold vacuum. Under these conditions, the solenoid will energize, opening the air valve allowing technicians and owners to check for proper component operation. For reasoning most likely due to the smaller displacement 350ci engine not producing as much drive-by commotion as a larger displacement powered Trans Am, Pontiac was able to install the functional hood scoop on the GTO hoping to create attention for the package. Since vacuum levels as low as those required to close the vacuum switch only occur under aggressive acceleration, hood scoop activity was limited to a narrow window of operation thereby reducing any unwanted carburetor noise under moderate acceleration.
THE DIFFERENCES
While Firebird Trans Am and GTO functional shaker hood scoops appear identical, they are in reality only similar. Scoop assembly PN 481689 was used on all '70 1/2-72 Trans Am models and fastened to the lower air cleaner by 2 wing nuts and studs protruding through tabs welded to the lower air cleaner itself. Solenoids were energized by a single 12V black wire with orange striping, which fit into a rubber connector under the scoop housing. An individual braided-wire ground strap fastened to the bottom rear of the scoop assembly with a male-spade connector fitting a female connector mounted on the firewall to provide sufficient ground. Without both connections, proper scoop operation may not be possible.
'70 1/2 R/A-IV and all '71-72 Trans Am models received dual-snorkel lower air cleaner assemblies and while they appear identical, minute changes in Thermac vacuum routings required different part numbers for each application. Lower air cleaner assembly PN 6486669 is specific to '70 R/A-IV Trans Ams while PN 6486686 is specific to '71-72 Trans Am models. The '70 1/2 R/A-III Trans Am used PN 6486668, featuring a single snorkel but is otherwise identical in overall appearance to its dual-snorkel counterparts.
Contrary to what many believe, Trans Am-specific lower air cleaners are unique to Trans Am models due to the required drop producing specific scoop height and clearance. While appearing very similar, standard 4-barrel dual-snorkel air cleaners may not locate the scoop in the proper location nor produce the correct drop without modifications. Because of reduced clearance inside the air cleaner assembly, the shorter and smaller diameter AC A366C filter was specified as the original air filter used on all '70 1/2-72 Trans Am applications.
To accommodate the taller hood height of the Ventura body, a Trans Am shaker hood scoop was modified by riveting a stamped sheetmetal spacer with specific front and rear heights to the bottom to properly position the scoop for the '74 GTO hood. The unique scoop assembly has an internal bracket for solenoid mounting similar to the '70 1/2-72 Trans Am design but features a single solenoid as opposed to the dual-solenoid set up found in the Trans Am. Unlike the Trans Am, which had a separate 12V power lead and ground strap, the '74 GTO had 1 plug with 2 terminals for both power and ground sources. These wires originated from the main wiring harness but were routed through an electric switch, which monitored coolant temperature to limit operation only above 140 degrees. Next to the electric plug was a port to which a vacuum hose supplying manifold vacuum was connected.
The GTO shaker hood scoop assembly, PN 497085, will not interchange with Trans Am applications without modification. With what began on the Trans Am in 1973, the '74 GTO shaker hood scoop was fastened to the single snorkel lower air cleaner base by a large band clamp eliminating the need for welded tabs and wing nuts for scoop retention. Additionally, the '74 GTO lower air cleaner, PN 6488274, was the same base used on '73-74 Trans Am models, which again will not interchange with standard 4-barrel applications due to specific drop.
Like all other Pontiac models beginning in 1973, the lower air cleaner featured a molded plastic duct, PN 497081, which was connected to its single snorkel and extended to flexible tubing that was routed to take air from below the inner fender to reduce intake air temperatures. And as with the Trans Am applications, the GTO also specified AC filter A366C.
CONCLUSION
As similar as many enthusiasts feel the '70-72 Trans Am and '74 GTO scoops were, we might now realize just how different they really are. Made famous by the '70 1/2 Firebird Trans Am and infamous by the '74 GTO, the functional shaker hood scoop was more than a hot-selling gimmick to draw potential customer traffic onto the showroom floor. They allowed the carburetor to draw cooler, denser air, possibly giving owners a slight edge on their competition. Whether the sales literature aerodynamic drawings showing high-pressure air at the base of the windshield are accurate or not, the phenomenon it created with performance minded owners was real. In addition to any added performance, the functional shaker hood scoop also gave a mental edge by providing the low-toned "4-barrel moan" true performance enthusiasts would come to expect from the not only the '70 1/2-72 Trans Am but also the '74 GTO.
Stay tuned for our next installment in which we will explore the non-functional hood scoops from the most common to the most rarely seen.
The term "Ram Air" is as synonymous...
The term "Ram Air" is as synonymous with Pontiac as the shaker hood scoop is with the Trans Am. The new-for-1970 1/2 Trans Am featured a solenoid activated "door" at the rear of the scoop allowing the carburetor to ingest cold air for added performance. While this feature was used through 1972, not many associate it with the '74 GTO. Being held wide open here, the air valve only opens about 1 inch when under heavy acceleration due to the limited travel of the solenoids, which is still enough to allow cool air into the air cleaner.
A typical shaker hood scoop...
A typical shaker hood scoop and air cleaner assembly as found on '70 R/A-IV and '71-72 455 H.O. Firebird Trans Am models. '70 R/A-III Trans Am models used an assembly nearly identical to that shown, though its air cleaner only had a single snorkel. Though barely visible in our photo, a stud on each side of the scoop assembly fits into corresponding L-shaped brackets welded to each side of the air cleaner base. Wing nuts are then used to secure the shaker scoop to the air cleaner base.
The underside of a functional...
The underside of a functional hood scoop assembly is one not many enthusiasts are familiar with. You can see here that the '70 1/2-72 Trans Am scoop used dual solenoids connected to T-shaped rod to open the rear air valve under hard acceleration. With the reproduction of nearly every part required, replacement parts are available through a wide variety of Pontiac restoration houses allowing once-functional shaker scoops to again be operational.
All '70-72 Trans Am shaker...
All '70-72 Trans Am shaker scoop solenoids received their power from a black and orange 12V power lead originating from the main engine wiring. Notice the single male spade inside the rubber grommet.
The braided-wire ground strap...
The braided-wire ground strap is attached to the rear of the scoop and fits into a female spade connector mounted on the firewall providing the solenoids with additional ground. Though ground is transmitted through the carburetor and air cleaner, levels are limited and may be insufficient for proper solenoid operation.
While '72 Trans Am models...
While '72 Trans Am models used the pedal-mounted transmission kickdown switch to actuate the solenoids, '70 1/2 and '71 Trans Ams used a spring-loaded switch mounted on the carburetor body and a corresponding linkage on the throttle shaft to operate the solenoids. We can only speculate that the additional wiring and delicate parts mounted on the carburetor were often damaged requiring the actuation system be redesigned for future models operation.
This series of photos show...
This series of photos show the points at which the linkage contacts and depresses the switch as throttle position increases on all '70 1/2-71 Trans Am models. The first picture shows the linkage position when at idle or under normal driving conditions.
As throttle position increases...
As throttle position increases in the middle photo, the linkage is beginning to depress the switch at the point when the secondary throttle blades begin to open.
Under full throttle conditions...
Under full throttle conditions as shown in the last photo, the switch is fully depressed activating the solenoids, which open the air valve at the rear of the scoop.
The '74 GTO used a functional...
The '74 GTO used a functional scoop very similar in appearance to that used on the '70 1/2-72 Trans Am. Due to different hood-to-carburetor relations between the 2 body styles, a spacer was riveted to the shaker base to properly locate the scoop in relation to the taller X-body hood to achieve proper height. Notice the ring clamp, which is used to fasten the shaker scoop to the air cleaner base on not only the '74 GTO but also all other '73-81 Trans Am models.
As with the Trans Am, the...
As with the Trans Am, the '74 GTO rear air valve was also solenoid controlled. However, unlike the Trans Am's dual solenoids and T-shaped rod, the GTO scoop used a single, larger solenoid with a U-shaped rod. A unique vacuum switch, which can be seen just left of the solenoid, would not allow solenoid activity until manifold vacuum fell below about 2 inches.
The '74 GTO shaker hood scoop...
The '74 GTO shaker hood scoop was connected to both a power source and manifold vacuum. Rather than have separate wires for both, the 12V lead and ground like the Trans Am, the GTO scoop had one plug receiving both as evident by 2 male spade connectors within the grommet. A yellow 12V lead and black ground wire were fit into a single T-shaped female connector, which then plugged into the grommet. Additionally, a vacuum hose running to a manifold vacuum source was connected to the vacuum port on the scoop, which led to the vacuum switch within the scoop housing as shown in the previous photograph.
The '70 1/2-72 Trans Ams and...
The '70 1/2-72 Trans Ams and '74 GTO were among the few GM applications that specified the AC/Delco A366C air filter. With an outside diameter of 13 inches, this filter was shorter in overall height to accommodate the reduced clearances under the shaker scoop. With limited availability, many owners elect to replace the A366C with K&N filter number 1590, which provides the same overall dimensions and is reusable.