Dear Car Talk:
I bought a 1969 Chevrolet Chevelle SS 396 recently and want to know if unleaded gas will harm my engine. Also, will using regular unleaded instead of high-octane unleaded hurt? I'd rather use the less-expensive regular, but not sure what to do here. -- Jim
You have no choice, Jim. You have to use unleaded gas. Will it harm your engine? Mostly no. When lead -- a known carcinogen -- was finally phased out of gasoline in the 1970s, older cars like yours did fine, except for one thing. It turns out the lead in the gasoline provided a cushion between the valves and the valve seats. And without lead, the valve seats suffered from something called "valve seat recession." Basically, they got pounded.
When those valve seats got pounded to the point that they wore out, they had to be replaced with hardened valve seats. That's the only kind you can get now. And they've stood up well over time with unleaded gasoline. So if your Chevelle has already had its valve seats replaced (which is pretty likely, unless it's got very low mileage), you already have hardened seats and there's nothing to worry about. If your valve seats haven't been replaced yet, when they do wear out, you'll replace them with hardened seats, and then there'll be nothing to worry about. Either way, there's nothing you can do, so just drive the car and enjoy it.
In terms of the octane rating, your only goal is to prevent pinging. So experiment. First, make sure the valve timing is set correctly. Then try the lowest octane fuel, and when you accelerate up a hill, see if you can hear the engine pinging. If you can, try the next octane level up from there. Do that until the pinging goes away.
Pinging, also known as pre-ignition, is when some of the fuel combusts in the cylinder when it's not supposed to. That'll eventually burn holes in your pistons, which will cost much more to replace than those valve seats. So use unleaded fuel with an octane rating that's high enough to prevent the car from pinging, pick up a pair of striped bell bottoms, and you'll be good to go in the Chevelle, Jim. Enjoy.