Spaces and Plenum Dividers
Spacers and plenum dividers provide an easy way to change the configuration of the intake tract and the relationship it has with the carburetor.
Adding a plenum divider to an open plenum manifold will help keep the left to right fuel distribution balanced for oval track applications when the car is in the corners. This is especially helpful on alcohol engines. These dividers usually do not have a measured affect on torque or horsepower.
Using spacers between the carburetor and intake manifold can produce dramatic results. The use of a four hole spacer can improve low end to mid-range by helping the carburetor draw and atomize fuel. An open center spacer increases the plenum area and can benefit the mid-range and upper rpm power. It is not uncommon to see combinations of spacer types or stacking of similar type spacers being used. It is also not uncommon to see results that are totally opposite from the general tendencies.
The actual results from any spacer or combination of spacers can only be measured during a test and tune session on the specific engine combination being run. This information can be a very useful tool when tuning to find the best horsepower or to change the power characteristics to suit a specific track condition.
Excerpt from -Barry Grant - Basic Carburetor And Fuel System Maintenance And Tuning-