It's a little misleading. If i put together everything I've found to be true: heads on some big blocks in 1971 began to come with "flame hardened" exh seats. (casting number's 346) They were first tries, and they were moderately sucessful. I have seen them crack across the seats. I dont see 346s show up much after 1971. Small blocks start having flame hardened seats in '72 on the J heads with smaller exh valves. Those engines also got mild emmissions stuff, like temp controlled spark advance, and evaporative charcoal cans. Big blocks got 452 heads primarilly, but 906s and 902s were also used a lot. '73 is the first years with EGR valves from what I can tell. Not on every vehicle, but they were working on it. It was also as leaded fuels were being cursed. The only change after that is in 1976, when catalytic convertors start apperaing on lighter duty cars. Some heavy W/D350s didnt have cats until 1980. There is a huge difference between "induction" of "flame hardened" seats, and unleaded seat inserts. Hardened factory type seats will last much better, but new seats will permanently solve the problems. If performance on unleaded fuel is you goal, new seats are teh only way to go. A valve job can remove much of the iron that was flame hardened. And as soon as the seat begins to erode, you begin to lose perfomance. Whether you feel it or not, it does happen. But, if a car is not oprating steady state over 2K rpm, or towing, and the spring pressures aren't too bad, the std seats will last for tens of thousands of miles with no really noticable loss.
