From the link below which has additional detail:
" A tapered spacer is different from a restrictor plate. A restrictor plate is thin, while a tapered spacer (shown below) is about an inch thick. Tapered spacers have four tubes running through the spacer. Each tube has a hole at the top and a smaller-sized hole at the bottom (which is why they are called ‘tapered’).
" Tapered spacers have been used in all Nationwide cars since the beginning of this season. The large end of the spacer mates with the carburetor and the narrow end points toward the intake manifold. The tapered spacer gradually narrows the path through which the air must travel to reach the engine. Try breathing through a scuba tube and then pinching the tube down. A smaller hole decreases the flow, which decreases how much air/fuel mixture is getting into the engine and thus how much horsepower the engine can generate. "
http://www.stockcarscience.com/blog/...tapered_spacer