Can you use unleaded fuel in a 1968 390 ci ford engine?

Answer 1

Yes, and no. There is a term called Lead Memory. Although this term's a bit misleading, it's a reasonable description, and possible good news for you. As leaded fuel's burnt, it leaves a deposit on the combustion chamber areas, most notably the valve seats on both valves and heads, and valve stems. This deposit builds up over increasing mileage to a point where after some ten's of thousands of miles a decent layer has formed - hence the term 'lead memory'. This makes it perfectly feasible to run on unleaded fuel only without any problems for many miles before the severely abrasive nature of unleaded fuel totally removes it. How long the lead memory lasts is dependent on mileage and use. A vehicle that's done over 40,000 miles on leaded fuel will probably do 10,000 miles on unleaded before problems occur. Those vehicles used conservatively on mainly light throttle openings (pootling around town, demonic economists, or speed-abhorrent types) will certainly achieve this. Extended periods under maximum load (the progressive, lead-booted, fast and furious driver, driving in motorsport, 'mountainous' regions or long periods at high speed on motorways) will shorten this by about half to two-thirds. Purely because of the exhaust temperatures achieved and throughput of extremely abrasive unleaded fuel. Unless you are going to do lots of driving at high speed or under heavy load you are good for at least 10-15,000 miles. After that you may need to use a lead additive.