Hasn‘t been rebuilt. Old 289 from 66 mustang. I‘m talking about how fuel in the 69s had lead which I think protected the valves but now with unleaded gas I heard that it could ruin the head.
needs hardened valve seats to run on the unleaded crappy corn gas we have now
An engine from 1966 would have been designed for leaded gasoline. Lead was added as a lubricant to prevent wear on parts such as valve guides.
If it has never been rebuilt and hardened valve seats have never been added then yes you will need to add a lead additive or lead substitute to your fuel. You might get away with using regular unleaded if you drive the car very easy but I would not take that chance. Lead substitutes are cheap and you do not have to use much in a tank of fuel. Better safe than sorry later on.
