Carb question

[Ken B.]

Sorry if this seems like a dumb question but......Can someone tell me the difference between a spread bore and a square bore carb...Sorry I just don't have alot of carb knowledge'

[Joe L.]

Re: Carb question

Sorry if this seems like a dumb question but......Can someone tell me the difference between a spread bore and a square bore carb...Sorry I just don't have alot of carb knowledge'

Ken-----


The term "spread bore" is used to describe what amounts to the Rochester Q-Jet configuration. This configuration has very large secondary "bores" and relatively small primary bores. This creates a unique intake manifold and carburetor throttle body configuration. While originally developed by Rochester for the 1965 model year, they do not have an exclusive franchise on the spread bore configuration as spread bore carburetors were also later manufactured by Holley, but otherwise of their own design. Also, Carter manufactured some Q-Jets for Rochester during the early to late 70's. Q-Jets are no longer manufactured by anyone. However, the Holley spread bore type carbs are still available new.

A "square bore" carb refers to one that has more-or-less equal size bores. This configuration creates a more-or-less "square" appearing carburetor flange. Conventional Holley carbs as well as Carter AFB's use this configuration.

[Stuart F.]

Re: Carb question

Many of the square bore carbs look as though all four bores are on equal centers, even though their bores may be of different sizes (larger secondaries). Spread bores are just that: the centers of the bores in the secondaries are much wider apart than those of the primaries.

Stu Fox

[Ken B.]

Re: Carb question

Will a square bore carb work on a manifold that was used with a spread bore?

[Joe L.]

Re: Carb question

Will a square bore carb work on a manifold that was used with a spread bore?

Ken-----



Generally, no. However, there used to be available adapters to make the conversion. Of course, the adapters raise the carburetor by the thickness of the adapter and that won't work with many Corvette hoods. I don't like adapters, anyway.

There are also some manifolds that are designed for use with EITHER a square bore or spread bore carb. However, NONE of these were ever used in a PRODUCTION application; they are aftermarket or GMPP only.

[Stuart F.]

Re: Carb question

Hypothetically, you could use most any carb on most any manifold as long as it will bolt up and you can seal it properly, and that can be a big IF! Engines don't like vacuum leaks for too long. Even if it seals ok and there is no throttle butterfly interference, the manifold mismatch can greatly affect the engine's performance characteristics. Best to match them up.

Stu Fox