Why are 318's low HP and torque?

The LD4B is an older manifold designed for 273/318 there version LD340 intake, performer has an inbetween port size compromised to work on 340/360 and 273/318 heads. Performer Rpm comes in air gap and non air gap but is designed for 340/360 ports only but under 400HP not needed. So if you can find an used LD4B grab it or get a standard performer.