Help with carb spacers?

Ok I need the final word on the effect on carb spacers and what they do! I searched and looked around but I get conflicting info.

Adding a spacer to the intake will raise or lower the effective RPM range? Does this change when it is on a single or dual plane intake?

In general when are open spacers and 4 hole spacers used? Does this change when it is on a single or dual plane intake?

Thank you for setting me strait!
 
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