kenny at indy told me the biggest differnce between the two are the 13* valve angle.he also said they moved the valves around on the 572-13,s.thats why they use there own jessel rockers and stands.they are differnt from -1,s.he also said on a 4.5" bore or larger the 572-13,s will out perform the -1,s.laying the valves back he said really helps to unshroud them.