From BG web site,
Tuning With Spacers
Spacers and plenum dividers provide an easy way to change the configuration and the characteristics of the intake tract and the relationship it has with the carburetor. Adding a plenum divider to an open plenum manifold can help keep the left to right fuel distribution balanced for oval track applications This is especially helpful on alcohol engines.
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.
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.
Luv the smell of NITRO in the morning.