Of the variuos intake manifolds available for a twin cams using a progressive two barrel carb,the 1756cc intake from 1974 is regarded as the best one to use.The 75 to 78 intakes are the same as the 74 unit internally (intake port size) except they are more cluttered with tappings and sensor /water port openings.
The 1438cc intake has no provision for a water jacket,but has the same design as the 1756cc intake.The 1608cc intake is very restrictive,and a poor design because of the seperate holes for each throttle plate.
You can also use later model carbed intakes up to 78' if you plug the various ports to suite your needs.Check to see if the tappings match the linkage your using on your Weber carb.Fiat mounted three different linkages styles on twin cams in the 74 to 78 production run.
The 1438cc manifold is a good manifold but the lack of a water port is important to cold starting and warm-up characteristics in cold climates, as well as overall proper fuel atomization under different atmospheric conditions,especially during hot summer conditions(vapor lock,boiling fuel)etc..
Tech chat w/ Marc Matzer
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