Originally Posted by JesseT
I could have sworn that only the S1 had reworked exhaust ports and sodium filled valves. Some first S2s might have had the sodium filled valves too, but generally these are not considered as of very good quality and often cause more trouble than what they are worth. I'd take that head any given day if the price is ok.
EFi heads are all the same. The MFi S2 rst head is the same as the MFi XR3i head. The S1 had the sodium valves, but that same port size and combustion camber shape as the 3i heads. HTH