Square Bore vs. Spread Bore Carb

All of the carbs I have ever ran have been square bore. I know that spreadbore is what came on chevys but I am in search for a 650-750DP Holley and have came across a few cheaper ones that happened to be spread bore.

What exactly is the performance difference with the spreadbores and are there any other notable differences?

[img]/phpBB/images/smiles/icon_cool.gif[/img]you will proably find that with the spreadbore you will have better throttle response through about 3k rpm, as well as better fuel economy. how much depends on your tuning and driving skills. after 3k both are going to be about equal performance wise, but you will lose a bit of fuel economy up high as oyu will have the "trash can" secondaries dumping fuel faster than a squareborecarb does.
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The Q-Jet is one of the best carbs made. It's a giant PITA to work on and tune, but people that know run the heck out of 'em. The Holley spreadbores were made as replacements IIRC. For the chevys and the chryslers. Once tuned, it should run as good as anything.
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There is a mounting difference between the two isnt there? The spreadbore uses the wider bolt pattern doesn't it?


<font size=-1>[ This Message was edited by: gork1rogues on 1/20/05 3:16am ]</font>

I love to call them Quadrajunks, BUT, when they work, they work extremely well.

I also like spreadbores in general for a street carb, and have used some Quads (yes, on a Ford), Holleys, Autolites and Carters with mixed results.

I recently got a old 450 Holley 'Economizer' that's a little spreadbore I'll be playing with as soon as I find the intake and adapter (spacer) I want.

I used Quads for a long time for four wheeling (until I got my current 770 Holley Truck Avenger) and again, they work great for that. Much more forgiving then a Holley in the rough stuff, and pulls steep angles better.

I also like the 'kick' they have when the barndoor in the back opens up. It's a pretty cool sound/feeling, if it's set up right and your engine can use what it's giving you...

I'm not a real big fan of the 650 Holley 6210s. I'd just assume have a properly working Quadrajet. I like Holleys in general though...

Hell, Ford even used some Quads, guess what on?
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I'd go spreadbore anytime over square - I just don't like running adapters, and haven't found a spreadbore intake for a Windsor.

Make sure there is clearance for those massive secondaries to swing open, some adapters will cause things to bind.


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There were some squarebore Holleys that were made to imitate a spreadbore. They just altered the venturis instead of the whole baseblate. The rear of the spreadbore seperates the carb bores and puts the barrels closer to the port entries like the Dominators for less plenum disruption.
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The old style holley street dominator was a low profile single plane manifold with a spreadbore carb plate. It had both bolt patterns so that squarebores would work too. I have one for a windsor if anyones interested.

Flip over an Edelbrock carb - it's a "mini" spreadbore on a square flange.