Square-bore vs. spread-bore?

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Bill Crowell

Bill Crowell

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Hey guys, what do you think works better: an Edelbrock Performer or Performer RPM spread-bore intake manifold with a spread-bore carb like a Thermoquad, or an Edelbrock Air Gap square-bore manifold with a Quadrajet or the equivalent carb?

Why do you think the Air Gap manifold is manufactured only in the square-bore configuration?

I've always wondered about this, so figured I'd ask. As always, thanks a lot.

(Corrected after RammerJammer75 pointed out my typo.)
 
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Bill Crowell said:
Hey guys, what do you think works better: an Edelbrock Performer or Performer RPM spread-bore intake manifold with a spread-bore carb like a Thermoquad, or an Edelbrock Air Gap square-bore manifold with a Quadrajet or the equivalent carb?

Why do you think the Air Gap manifold is manufactured only in the spread-bore configuration?

I've always wondered about this, so figured I'd ask. As always, thanks a lot.
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Do you mean square bore? I've only seen the Air Gaps with square bore. If that's what you mean, my guess is they figure most people have a square bore aftermarket carb for this intake instead of trying to use a Thermoquad with it.
 
RammerJammer75 said:
Do you mean square bore? I've only seen the Air Gaps with square bore. If that's what you mean, my guess is they figure most people have a square bore aftermarket carb for this intake instead of trying to use a Thermoquad with it.
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Sorry, my mistake, RammerJammer75, and thanks for proofreading!
 
It depends on what you want to do with your car. Race car it does not matter, on a street car I prefer spread bore carbs. But the Street Demon carb seems like a good street, square bore carb, similar to the TQ.
 
As mentioned I think you'll find that spread bore manifolds are not nearly as prevalent as square bore. And square bore carburetors are definitely the norm. Unless you have your eyes set on a thermoquad carbureted motor which in my opinion is one of the very best street carburetors ever made then you will probably go square bore
 
Square bore with air gap is the best setup.

As mentioned earlier, more sq bore carbs are produced, and with good reason.
 
Aren't Eddy Carbs all square bore bore ? It would make sense to make the intake fit their carbs .
 
The square bore carburetors are more performance and race oriented. Spread bores are better for the street. This is a very general point of course and there are exceptions, but for the most part this is the case.
 
roccodart440 said:
Sorry, misread and I deleted it.
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No worries .
I'm no carb expert. but the fastest Stock a-body 340 I believe had the Thermo quad . Maybe 800 stock cfm? I liked it on my old Demon got decent miliage until you kicked in those big *** secondaries.Great street Carb. It was tough (for me) jetting on another motor I had, Holleys easier to tune and Eddy easier than that. JMO But like I said I'm no expert. Racing ? A whole different ballgame.
 
RustyRatRod said:
The square bore carburetors are more performance and race oriented. Spread bores are better for the street. This is a very general point of course and there are exceptions, but for the most part this is the case.
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For real??? Are you that little of a man? Maybe I'll show a screenshot of what you just did. Pathetic
 
Bill Crowell said:
Hey guys, what do you think works better: an Edelbrock Performer or Performer RPM spread-bore intake manifold with a spread-bore carb like a Thermoquad, or an Edelbrock Air Gap square-bore manifold with a Quadrajet or the equivalent carb?

Why do you think the Air Gap manifold is manufactured only in the square-bore configuration?

I've always wondered about this, so figured I'd ask. As always, thanks a lot.

(Corrected after RammerJammer75 pointed out my typo.)
Click to expand...


Until you start racing, any carb is a good carb. So long as it is well tuned.
The popularity of a spreadbore carb is low. Adapters work OK. They just have to be worked on for best return in power.
The aftermarket offerings with a spreadbore design are factory replacement intakes. With a few exceptions like the MP-M1-single plane and a Holley strip dominator or the Offy line, where the same manifold was offered in ether type. These were mostly designed for a class of racing, Allowing the stock carb. It works well for street guys in terms of a savings of a new carb purchase if you already had a TQ or QJ.

In a post by Bewy where he has done some really cool work and has shown what the dyno says it does, here are the results of a Holley Umtra HP 950, a QJ and a TQ.
I say it to damn shabby for this factory stock TQ carb. The same carb that people throw away for the expensive H brand. The factory stock version can be modified to this level.
Dyno results - Tquad vs Holley vs Qjet - tuning issue, too rich
 
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I always wondered why Holley-when they made a full line of intake manifolds, that most of them were the spreadbore design. While they have the Holley spreadbore carbs-you don't see too many people using them.
 
lemondana said:
I always wondered why Holley-when they made a full line of intake manifolds, that most of them were the spreadbore design. While they have the Holley spreadbore carbs-you don't see too many people using them.
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Not really spread bore specific. Holley tried to design and make a intake that would work for both style carbs. They just look spread bore specific.
 
I'd love to get a Air Gap manifold but I dont want to ditch the 440 TQ on my 340. Don't they make a Adapter/spacer for this?
 
I’ve used this adapter many times over the years without issue. Thou most of the time it was going from a TQ or QJ to a square bore.
 
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