Ground Pounder Street 470

I'm kinda interested to hear the expanded version of that answer too, but despite my lack of familiarity with those products, I'd guess Edelbrock makes that distinction as a general advisement not to expect the powdered metal style seats to last as long or take the same abuse as the hardened ductile seats in the RPM heads. The valves are also supposedly a little heavier than their RPM counterparts, according to Edelbrock. FWIW, most domestic new mfr engines these days use powdered metal connecting rods, and as long as they're not abused*, they last just fine.