I hve hc pistons, pr bak, pciii, went 1 colder on plugs, and gapped to 32 or 36.(I dont remember), d&d exhaust. When I put the 4-1 at first I thought I installed wrong (loose wire, broken plug wire, something) but I started thinking logically. If im not getting good burn, why? not enough power to the coils or coils have too much resistance to the plugs.