Doug,The 18c is a better head than the 18x,especially on a big motor.I have a 412 motor with 10 yr old edelbrock victor 18 heads done by Chapman.I never tried any of the other 18 heads but at 3100 lbs on 10 inch tires my car has run 1.35 60,5.99 115.65 1/8 9.38 114.97 1/4.Comp 14.4 to 1 850 carb,T&D rockers 5.50 offset in 1.70 offset ex.