Aluminum Intake Question - Team Camaro Tech
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post #1 of 8 (permalink) Old Aug 21st, 08, 03:42 PM Thread Starter
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Aluminum Intake Question

I have a question about the aluminum intakes that came on the L78s in 1967. I am finding 3885069 manifolds (that is the right number according to the By the Numbers book) but some have open plenums and some have the slot on one side and two holes on the other side under the carb. Does anyone know about why there would be the difference and what they mean?

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post #2 of 8 (permalink) Old Aug 21st, 08, 04:53 PM
 
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Re: Aluminum Intake Question

May not help at all, but fwiw, the old HPBooks "How to Hotrod BB Chevys" book lists the old 3947083 as being machined to open and the old 3947084 as being completely divided and they come from intake manifold casting number 3933163. (Page 30)..

I think these are aluminum rect. port 427 intakes w/ the open plenum one being used on the L-88 engine..

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post #3 of 8 (permalink) Old Aug 21st, 08, 07:00 PM
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Re: Aluminum Intake Question

Could it be possible that the open plenum design is a square bore pattern, for a Holley carb and the divided plenum design is a spread bore pattern, for a QuadraJet?
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post #4 of 8 (permalink) Old Aug 21st, 08, 08:04 PM
 
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Re: Aluminum Intake Question

No!

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post #5 of 8 (permalink) Old Aug 21st, 08, 08:28 PM
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Re: Aluminum Intake Question

Open-plenum vs divided plenum relate to horsepower ratings?
Holes and slots to equalize vacuum for power brakes?

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post #6 of 8 (permalink) Old Aug 22nd, 08, 07:42 AM
 
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Re: Aluminum Intake Question

Separate but smaller plenums create longer, smaller runners that suck better at lower speed, thus creating more bias towards torque production.

Remove the divider and the one now BIGGER plenum allows for all runners to be like effectively shorter, but they all have more plenum volume to share/suck from, so thus can suck better at high rpm, which effectively bias's the engine to high rpm hp production.

GM didn't make a single plane for the L-88 b/c they just removed the divider to allow it to suck better way up top.

No telling what hp the old L-88 would have made if GM would have been able to put a, "state of the art" (SOTA), modern single plane on it??

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post #7 of 8 (permalink) Old Aug 23rd, 08, 05:51 PM
 
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Re: Aluminum Intake Question

I believe this is correct.In 69 all L78s 396/375 came with Iron rect. port heads, aluminum intake with divided plenum and square bore 780 holleys. I'm not sure about earlier years. I've found several origional intakes that have been modified. Most of the time the divider is removed or drilled, but not by GM.
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post #8 of 8 (permalink) Old Aug 23rd, 08, 11:09 PM
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Re: Aluminum Intake Question

Agree with PDQ67. Open bore means bias to HP and not torque. Depending upon the engine, however, the difference might be minimal and not fussing over if you find a good deal

In general, torque will be more fun on the street if you are not pursuing high speeds.

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