Originally posted by techinspector1
Matt, there are two problems with pre-71 motors. First, the compression ratio is too high to run on today's cat-piss unleaded gas and secondly, the seats in the heads are too soft to run unleaded without spending the loot to install hard seats.

Bob Parmenter had an excellent explantion of the soft seat syndrome, saying that the lead in the fuel acted as a contaminant to the welding process that goes on between the valve seat and the valve head, preventing seat recession.
I see, thanks for correcting me on the pre '71 350 thing.