Is it true that the 1971’s were built with heads that are acceptable to unleaded gas. Just bought a 71 and am wondering if I need to change my heads or use fuel additive. Anybody got an answer? L.M.
Actually no! The major difference between cylinder heads for engines designed for leaded and no lead fuel is valve seat hardness. To make a proper no-lead fuel engine the valve seats must be hardened to resist recession. This did not universally happen at GM until 1973.
If you think about it valve seats take a real beating in all engines. As the valves are pulled shut by the valve springs they bang against the valve seats. To visualize this, strike the anvil portion of your bench vise with a steel hammer. You will hear a distinct metal on metal ringing sound as the hammer hits the vise and you will see a tiny dent in the anvil where the hammer struck. For comparison take an old lead wheel weight and position it on top of the spot where you hit your vise and hit the lead weight with the same hammer. This time you’ll hear a dull thud rather than a metal on metal clang. This time there will be no dent in the vise either.
That difference in sound illustrates one of the big things lead did in gasoline. The lead in the gas cushioned the blow of the valves as they were pulled back against the valve seats. Without the lead each time the valve closes there is a noticeable strike much like the strike of the hammer against the vise. After hundreds, tens of thousands or perhaps millions of strikes a miniscule fleck of metal would be hammered off the valve seat.
Mile after mile this hammer on anvil scenario continues and much like water wearing rock the valve seat metal would one spec at a time slowly disappear allowing the valves to move farther into the head. Eventually the erosion of the valve seats would allow cylinder pressure to escape and performance would suffer. You then needed a valve job. Also the harder the engine was used the more likely and rapid the valve seat recession. In other words, an engine used on the track or in a dump truck would have much more rapid recession than in the family sedan or sensibly driven, prized Corvette.
The lead provided a soft cushion that eliminated the metal on metal strike between the valve and valve seat making valve seats last much longer. Two other notable benefits to the lead was as a lubricant and heat transfer material. The style of lead used in gasoline acted as a lubricant on the valve guides helping them to live longer in their extremely hostile environment.
Then there’s lead as a heat transfer material. Inside an engine the valves are subjected to extremely high temperatures caused by burning gasoline. Temperatures so high that the metal of the valves can actually melt if not cooled. That cooling is mostly accomplished in the fraction of a second that the valve is closed and touching the valve seat. So we have to transfer lots of heat from the valve to the valve seat and lead helps that transfer happen. Finally lead increased octane which is simply the gasoline’s resistance to auto ignition (pinging/knocking). Octane does not affect temperature or power output by itself it just makes the fuel less volatile.
Now that we know a few of the basics what about using lead-free gas in an engine without those hardened valve seats? Well as noted above how the engine is used has a dramatic effect on whether or not there will be problems.
Bottom line if the car isn’t used for long highway trips at sustained high speed (like a trip from New York to Miami), towing a trailer, participating in track days or being used for serious off-roading it will probably live a long time without additives or machine shop work.
Then there’s the practical financial side of things. If you keep the car for a while and put miles on it or rarely drive it, which is actually worse than driving sensibly, sooner or later other parts of the engine will wear leading to the need to open it up. When that happens it’s time to do the heads with hardened valve seats, better valves and valve guides. It costs a lot more to do it now than it would to do the job in conjunction with other internal repairs. Plus, the money you save on expensive additives over time could almost pay for that part of the job.