1974 Hood Scoop Vacuum Hook Up - Pontiac GTO Forum
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post #1 of 3 (permalink) Old 09-26-2010, 09:32 AM Thread Starter
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1974 Hood Scoop Vacuum Hook Up

Can anyone help on how to hook up vacuum on 1074 hood scoop?
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post #2 of 3 (permalink) Old 10-03-2010, 02:21 AM
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Originally Posted by Chickenman View Post
Can anyone help on how to hook up vacuum on 1074 hood scoop?
Bump to the top...hope someone can help this guy out. It's tough having one of the less common years/models.

'68 GTO, 400/TH400, AC/PS/PB/PW/tilt, hideaways, Hurst dual gate
'72 442 'Vert, 350/TH350, AC/PS/PB/PW/tilt
'71 Javelin, 401/4-speed restomod
'74 Corvette, 383/4-speed, 450 hp, restomod *SOLD*
'70 Cougar XR7 'Vert, 351W/FMX auto *SOLD*
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post #3 of 3 (permalink) Old 10-04-2010, 04:35 PM
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I owned a '74 back in the day...I think a lot of people thought they would open all the time when ya put your foot into it, but it doesnt...there are a lot of things that work together to make it happen. Heres some info for ya....I hope it helps!!!

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.

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.

Last edited by Bluesbrother; 10-04-2010 at 05:40 PM.
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