Spacers come in three basic configurations. We didn't test the open-plenum spacer on our tall, single-plane manifold, but open-plenum spacers can be worth power on shorter single-plane intakes. The four-hole spacer also worked much better on our engine than we anticipated. Do yourself a favor and look closely at how well the four-hole spacer performed. It was almost as effective for less than half the cost of the tapered spacer.