These are 360° open-plenum, single-plane manifolds designed for high-performance street, drag, oval track, and marine applications. Their effective range is 2000-6500 rpm, and they work best with at least a 9.5:1 compression ratio, 280° or more of cam duration, and headers with 1-3/4" primary tubes. The X-CELerator series intake is the ultimate street/strip performance manifold for single 4bbl applications where increased torque and power is required in the mid and high rpm ranges.