I need some help on the differences of square/spreadbore carbs and when they are used and for what applications. :confused:

Thanks Rudy

Think of the spreadbore as Holleys answere to the Q-Jet. Small prinaries and big secondaries. To my knowledge the Spread Bore was never installed on any car from the factory it was always aftermarket only. I believe that the Spread bore bolts right onto the Q-jet manifolds. No adapters and none of that nonsense. The base is VERY different between the square base Holley and the S-Bore. You can adapt the S-Bore to a square bore base manifold with an adapter that is intended and sometimes marketed as being for adaptation of Qjet carbs on Holley square bore flanged manifolds.

As you can see Holley wanted to take the perf. portion of the market away from Roch for those cars that did not have to run stock carbs.

I like the Q-jet and though I never had a S-Bore I think the concept is a good one and would try one if I had a project car I was workin on. As far as I know all the internals of the holley such as center hung floats, jet installation and secondary operation are more or less the same as the square bore carb unless a difference is dictated by the Spread bore design itself.

Not a carb expert but I have a 1966 iron SBC Holley manifold mounted on my 64. The new 3310 mounts up to it.

Actually, TONS of cars came from the factory with Spreadbore carbs- it was probably the most prevalent type of factory carb in history. The Qjet is a spreadbore carb for the simple reason that it has massive secondaries and tiny primaries. The tiny primaries and MASSIVE secondaries ARE why it is called a "Spreadbore". Squarebore speaks for itself: 4 bores of equal (relatively) size.

The idea behind it is having both performance AND mileage, since most of the time you are running off of two barrels. Gm more than likely figured that why not make those front two barrels as small as possible to allow for better mileage. Chrysler saw the same thing, and threw the ThermoQuad out there in the mid '70s.

The Holley squarebore's design has basically been around since the 1950's which pre-dates spreadbore carbs like the GM Qjet and Carter Thermoquad for quite a few years.

Holley makes the 4175/4165 series, which is their double pump and vac. sec version of a spreadbore carb. They are NOT Holley "Qjets"- they are Holley Spreadbores. For the most part, they are the same as other Holleys in jets, plates, springs, pvs, etc... BUT- the primary bowl gaskets are different, the shooter is different, and the idle circuit is usually COMPLETELY different (Reverse Idle), as is the needle and seat arrangement.

Most squarebore carbs will bolt to ANY aftermarket manifold...Holley really is the king of the carbs, so all the manifolds seem to be geared towards them. However, specialty Qjet manifolds are out there, like the Performer RPM Qjet, which allow you the choice to run the more economical carb on a hipo manifold. If you want a Holley on one of those manifolds, run a Holley spreadbore.