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I'm not 100% sure, but I think they are pretty much the same except for the valves.
(Althought LS6 intake is better than LS2)
My understanding is that the heads are basically the same...the difference is the type of valves installed. GENUINE GM LS6 heads have lightweight hollow-stem intake and sodium-filled exhaust valves. Replacement cost of the valves alone is about $650 for 16. The benefit of those valves is the ability to rev to 6600+ rpm, with the 90# LS6 springs without valve float. Those springs rob less HP than ~130# springs needed with standard weight valves to run at that rpm or higher.
Mike
Yes, that's right...same head casting #, same combustion chamber cc, same intake and exhaust ports, same valve sizes (2.00 & 1.55)...just different valves. LS2 valves have the solid stems.
Mike