Why are 318's low HP and torque?

302 heads came out in '85 and ran until the magnum engines came out, so '85-91. If your 318 has 302 heads, it should also be a roller cam engine unless the heads were added later.

Keep in mind though that not ALL of the LA 318's from '85 to '91 had 302 heads or roller cams, there were still non-roller, non 302 headed versions of the 318 that ran from '85-'89. If the last 3 digits of the casting number on your heads is 345, then you don't have a roller cam. If you have "646" heads you may or may not have a roller engine. In either case, you don't have 302 heads. If your 318 is '89-91 its has a roller cam, but it could still have 646 heads instead of 302's.