• 90° Square Bore Adapter

90º Square Bore Adapter Plate to mount Holley/AFB carb on Offenhauser 4bbl Intake manifold.  Machined from 1/2" 6061-T6 aluminum.  Throttle shafts are parallel to engine for symmetrical fuel distribution.

  • Bare adapter plate only. 
  • Throttle linkage is NOT included.
  • Fasteners are NOT included.

The conventional installation of 4bbl carburetors on slant six engines is with the primaries facing the front of the car.  The secondaries would then be facing the rear of the car.  While cruising on the highway, this arrangement should not pose any problem with fuel distribution.  However, when accelerating, the front 3 cylinders would receive proportionately more fuel from the accelerator pump and the rear 3 cylinders would starve for fuel.  Unless the rear barrels of the carburetor are completely identical to the front barrels in every way, the fuel mixture received by the front 3 cylinders will be different than the rear 3 cylinders whenever the secondaries were open.

This adapter corrects this problem by aligning the throttle shafts with the engine so that the all cylinders receive a much more even distribution of fuel.  We recommend mounting the primary barrels farthest away from the engine for three reasons:

  1. The idle mixture and speed adjustments are easily accessible from the left side of the car.
  2. The primary barrels are roughly the same distance from each of the cylinders.
  3. The accelerator pump shot is evenly distributed among all cylinders.  Orienting the carburetor with the primary barrels closest to the engine ensures that cylinders 3 and 4 receive most of the pump's shot.

Under normal driving conditions, all cylinders should receive the same fuel mixture.  When accelerating with secondaries open, careful selection of secondary jets should ensure that all cylinders still receive a symmetrical fuel mixture distribution.

Ensuring that each cylinder receives the same fuel mixture as all of the others maximizes both power and fuel economy.  Otherwise the fuel mixture for the entire engine must be made richer so that the leanest cylinder still runs acceptably well.

The easiest way to fabricate a throttle linkage is to replace the existing throttle cable with one that is sufficiently long.  For the older A-Bodies with shaft linkages, this might involve switching to a later model gas pedal as well.  Vehicles with automatic transmissions will also require that the kickdown linkage is operational.  Lokar makes throttle cables and kickdown cables for applications like this.

90° Square Bore Adapter

  • $65.00