Hi Dave,
My experience has been nothing but the best investment in my old beast. Except, they had to make special wires with 90 degree boots to clear each other. Simple to put on. Seems to can open gap on plug a little, they supply the plugs and new set of wires. You can advance timming a little by about 3-5 degrees, on any vehicle I have ever put them on in the last ten years, I have seen no less than a 10% mpgs increase and as high as 25%. I have used to past emissions on used autos I purchased for regular day to day driving. A Cressida, Maxima, and Dodge Caravan. When the vehicles were purchased none of them past emissions until I put these on.
Good Luck,
Tim