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Old 12-30-2011, 04:25 PM
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Spread Bore VS Square Bore Advise

Hey guys,

What's the difference between a spread bore and square bore carb? I'm putting it on an Edelbrock LD340 on my 340 1971 Challenger. Which do I need? I'm looking at the Holley 650 double pumper. Thanks.

Dave

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Old 12-30-2011, 04:55 PM
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The spread bore has smaller primary throttle plates as compared to its larger secondary throttle plates. The eddy LD340 takes a square bore carb in which the primary and secondary throttle plates are the same size.

The holley is a square bore so you can use it, there are many other choices of carbs as well most being square bore. Double pumpers are best on single plane intakes and with a hotter cam and 4 spd trans. The LD340 is a double plane intake, if the rest of the build is not too radical personally I would not use a double pumper, that is not saying you can't.
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Old 12-30-2011, 04:56 PM
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http://www.summitracing.com/parts/EDL-7176/?rtype=10
This is a square-bore RPM intake, the one I would use on a street or street-strip build. In all the DynoSim tests I ran, the RPM made more power idle to 6500 than ANY other manifold. Mount a square-bore carb in the range of 650 CFM and you're good to go.

This is a spread-bore carburetor, the Carter Thermoquad that was used as original equipment on the Mopar 340's....Notice that the secondaries are much larger than the primaries, requiring larger holes in the intake manifold for the rear secondary throttle plates....this carb is the same design as the Rochester Quadrajet spread-bore carburetor, which was original equipment on GM products.
http://www.google.com/imgres?hl=en&b...t:429,r:4,s:11

Use a spread-bore carburetor on a spread-bore intake manifold. Use a square-bore carburetor on a square-bore intake manifold. Don't fool yourself into thinking that an aluminum adapter will make it work, it won't.
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Old 12-30-2011, 04:57 PM
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in a search for that intake, it looks like a performer

and it looks like you can use either one

spread bore carb base

square bore carb base

i wonder if you could put a Quadra-jet on it

Q-jet---that would be the best
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Old 12-30-2011, 05:12 PM
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Quote:
Originally Posted by Custom10
The eddy LD340 takes a square bore carb in which the primary and secondary throttle plates are the same size.

Quote:
Originally Posted by matts37chev
in a search for that intake, it looks like a performer
do you have a pic of the intake?
it looks like it came both ways, depends on the age and applacation
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Old 12-30-2011, 05:39 PM
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I happened across this one matt, square bore in this case, not sure if there is a spread bore version? but I did not think so, hopefully the OP can give us a look at his and help solve this Mopar Mystery
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Old 12-30-2011, 07:23 PM
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Below is what I have. I had to dig it out of storage. I've always used Holley 650 double pumpers on my small blocks with success.

But if you guys have a better idea of a better carb please let me know.

I'm not the best at tuning and I've had good success with the 2 Holley DP's that I've bought new. I really appreciate it. Thanks

Also it needs to be a regular carb height so it fits under my shaker hood. Thats why I'm using the LD340. Thanks

Dave
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Old 12-30-2011, 07:32 PM
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square bore would be best for that baby
and my recommendations for topping it off would be this thingy or this doodad
but that's just me
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Old 12-30-2011, 11:54 PM
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I personally prefer spreadbores and here's why: tiny primaries tend to help throttle response and cruise MPG. On an all-out performance build that is not really a big deal.

For intakes I almost always choose the spreadbore option if I can. A spreadbore intake is almost always drilled and tapped for both bolt patterns, and both types of carb will fit without consequence. Almost all squarebore intakes won't accept a spreadbore carb because the big secondary butterflies hit the mounting flange. Because of that, putting a spreadbore carb on a square intake requires an adapter that often times really hurts performance.
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Old 12-31-2011, 04:36 AM
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I perfer spreadbores myself. The Quadrajet is my carb of choice. The small primaries make for great low end torque due to the higher velocity of the smaller size. With a good street combo, such as a small block with a small RV cam, headers, 9:1 compression and the correct tune up pieces the small primaries make for fuel injection like real sensitive throttle response.
When wide open throttle is needed the massive secondaries flow all the engine needs and provide it with plenty of power.
I also just about always use a intake with a spread bore pattern and why is because both style carbs will bolt on and you dont need a adapter plate. If you have a spread bore intake and your using a square bore carb you can bolt it on and use a thin square bore gasket with a .050 thickness open metal plate then another gasket. Some spread bore intakes already come with the metal plate and others don't include it. The metal plate assures the gaskets have enough surface for the gasket to cling to on the secondary side where the flange is larger. One time I used a 4 hole square bore gasket on a Professional products Vortec intake that a customer had brought by. I bolted on a holley and it was then I noticed on the passengers secondary side there was a hole big enough to stick my pinky finger into due to the gasket not covering enough surface on the spread bore intake. It was then I realized what the .050 thickness metal plate was for that came with the intake.
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Old 12-31-2011, 07:28 AM
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Still Another Carb Question

Hey guys,

It was suggested on a Mopar site that a Proform carb is the way to go. Does anyone know anything about these? About $75.00 less than the Holley. The money isn't the price I just want to make sure it's almost plug and play with no issues. Thanks
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Old 12-31-2011, 07:53 AM
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I think most guys will tell you, stick with the holley
but I dont know anything about carbs, compared to some of the other guys on here
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Old 12-31-2011, 08:08 AM
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proform blues
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Old 12-31-2011, 08:17 AM
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Quote:
Originally Posted by daveh
Hey guys,

It was suggested on a Mopar site that a Proform carb is the way to go. Does anyone know anything about these? About $75.00 less than the Holley. The money isn't the price I just want to make sure it's almost plug and play with no issues. Thanks
Well I am not familiar with the proform? got a model number maybe someone else can chime in with there input on it. Personally I like the edelbrock performer carbs and if it were me I would be mounting a 1405 on there, no hassles and easy to tune with jet/rod changes, square bore, designed for your intake and they are closer in design the old carters than a holley is so you would be more along the lines of an original mopar setup as well.

Spread bores are OK but you would need an adaptor plate, IMO dont go there
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Old 12-31-2011, 09:45 AM
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I wouldn't use the proform body, unless it's got annular boosters.
If it's got downleg boosters those things are completely worthless.
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