well this timing set is a middle of the road set with a nice heat treated machined lower sprocket and a nice japanese chain but the topcog is "normal". i can go -4 0 or +4deg. i think its "advance to make up for the slack". a well respected pontiac engine builder told me on his UD custom flat tappet poncho cams he like to advance them 3-4 so when it does slack out the intake centerline ends up where he wants it. im going to call him tomorrow thanks