square or spread bore

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63dartman

63dartman

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I have been looking at some used carbs on ebay. I am not very familiar with holley carbs and need some help. I want to get a 650 cfm for my 360. I am currently running a eddie 600 and feels a little small. I am also running a eddie rpm intake.

I have noticed the spread or square bore carbs. I see that the square bore have pretty much matched ports and spread have larger 4 barrel ports, what are the opertional differences of the two and which one would be better for performance? Would the eddie intake make me have to use one or the other?
 
rpm intake accepts a square bore carb. You'd need some funky adapter to put a spread bore on there, which do hinder the flow/pluses of the spread bores.
 
The spread bore has smaller primaries, which equates to slightly better fuel economy, plus they are a little more "torquey" (for lack of a better word) due to the speed of the airflow through a smaller diameter venturi, and hence a higher vacuum signal. It is a great theory, although a well-tuned Holley square-bore of the correct cfm is tough to top. What size engine are you running? A 600-650 cfm carb ought to flow enough cfm for most smallblocks.
FYI- most spreadbore Holleys were designed to take the place of an existing spread-bore carb (Carter, Rochester, etc) as a direct replacement bolt on. If I was gonna start with a square-bore manifold, I think I'd go with what was designed for it......a square-bore Holley. Adapter plates can wreak havoc with airflow in the plenum, with no way for the average joe to test it without a flow bench. Just my .02
 
63dartman;

Would the eddie intake make me have to use one or the other?

The RPM's squarebore mounting pad doesn't require you to have the like carb. But it will make it easier. The adapters for a spreadbore carb to the squarebore intake DO NOT wreek havoc. But they do add 1 inch or so of height to the overall induction system. Your hood clearance is at stake. Think before you do.

what are the opertional differences of the two

In a overall view, no real difference. There both ready to confound you.
The Carter T-Q's (And all Carters) Have mechanical operated secondarys with an airdoor that opens on engines demand.

The Holleys have a vacuum secondary door. Operation by spring.

which one would be better for performance?

The squarebore carbs have an edge. Even flow of the air and fuel through (about) even bores.
The spreadbores could offer more economy.
 
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