Reply to "Phenolic carb spacers"

There are several common types of spacer. Most are alum others Phenolic or Plastic. The idea between the Phenolic or Plastic types is that they transfer less heat to the carb and therefore produce more HP. That is the only difference in the materials.

Spacers also come in Open or Closed version. The Open is where the space is wide open in the center, the closed has four holes to match the butterflies of the carb. The open version is best for large manifolds, cams and big heads. Therefore creating improving high RPM flow resulting in top end HP. The closed version is better for low RPM torque and well suited for dual plane intake manifold.

There is no simple answer other they trying different versions. Their height (1" or 2") reacts differently to each engine, so experimentation is required to find the right match. Often part of a dyno is to play around with spacers to get the right unit.