Choosing the right fuel will protect your classic

Is it better to buy high-octane fuel with some ethanol or lower-octane fuel with none? What percentage of ethanol is too much for my old cars? — George Malone, Azalia, Michigan

Ethanol and octane are separate issues. Let’s deal with ethanol first. From an automotive standpoint, there is nothing good about E10, the blend of gasoline with 10-percent ethanol sold at most pumps around the country. Ethanol is hygroscopic—it absorbs water. One could argue that the ability of gas to absorb a small amount of water is a good thing. If water is present and isn’t absorbed, it sinks to the bottom of the tank, where the pickup tube sucks it into the engine. But there’s a limit to the amount of water that can be absorbed before separation occurs, so when E10 sits in a humid environment, a corrosive mixture of water and ethanol can accumulate at the bottom of the tank.

These problems are exacerbated in lightly driven vintage cars that often have metal fuel tanks that might be poorly sealed due to degraded or missing emissions systems. This allows moist air into the tank, which gets absorbed by the ethanol in the gas, separates out as water, accumulates at the bottom of the tank, and causes running problems and rust in the tank. Ethanol also ruins rubber fuel lines, gaskets, and plastic and rubber fuel-system parts.

Fuel stabilizer used in vintage cars that sit for months, as I understand it, only prevents gas from going “sour.” It doesn’t alter ethanol’s hygroscopic properties or cause water that has already formed to magically dissolve.

If you can buy ethanol-free gas at a reasonable price, do it. Availability varies state to state. Websites like Pure-Gas.org and BuyRealGas.com make it easy to find ethanol-free fuel.

Now let’s move to octane. I always use the recommended octane for the car I’m driving—no more, no less. A modern car has a digital engine-management system with a calibrated spark-advance profile for a certain amount of octane in the fuel. In addition, most modern cars have a knock sensor, so if the engine is under load (low rpm, open throttle) and it begins knocking, the system should retard the ignition timing until the knocking stops.

In a vintage car without a knock sensor, especially one with a mechanically advanced distributor, regardless of what fuel is in the tank, if the car knocks under load, back off the throttle! Knocking (or pinging, as it’s sometimes called) is terrible for your engine and can blow holes in the pistons in short order. Especially in a vintage performance car with high-compression pistons, running on fuel with the recommended octane, with a distributor timed perfectly to spec, it’s still possible for the engine to knock under load. Try rotating the distributor to retard the timing. If it still knocks under reasonable loads, try switching to higher-octane gas, if you’re not already at the max. I set up my vintage cars for a tiny amount of pinging in a load situation (for example, uphill, wide-open throttle, one gear higher than I’d normally be in) but zero pinging otherwise.

In short, buy ethanol-free gas if you can, and buy gas with sufficient octane that your car doesn’t knock. If you’re forced to choose, take the higher octane to prevent knocking. But if the car doesn’t knock, go with the ethanol-free gas.