Old-auto enthusiasts need to be careful when pulling up to the pump—what goes in the tank may not be friendly to it

Spring has sprung, and that means classic cars are coming out of winter hibernation.

They’ll get their oil changed and their brakes checked, which is good for them. They’ll also get a tank of gas—and that might not be quite as beneficial.

Gasoline has been tweaked over the years for environmental reasons, and that can present a problem for older engines.

Before you fill the tank, find out what’s in your gas, and what it might be doing to your vehicle.

Ethanol is alcohol that, in Canada, is primarily made from corn or wheat. On the plus side, it’s a renewable fuel. It also burns cleaner than gasoline, and it has a high octane rating.

But it ticks a lot of boxes on the minus side. It has less energy than gasoline, so it gets poorer mileage. It’s hygroscopic, meaning it absorbs water, including from condensation and even humid weather.

It’s also corrosive, and can chew up cork and rubber seals. That’s seldom an issue in newer vehicles, since they’re built to handle a small amount of it, but older engines weren’t designed for it.

It’s mandated that gasoline has to contain 10 per cent ethanol, known as E10, spread out over all the fuel each company sells. Most oil companies used to blend it into the lower grades, but leave the premium pump ethanol-free.

Unfortunately, it’s now rare to find any fuel that doesn’t contain at least some ethanol—and even if one station sells pure-premium fuel, you can’t always guarantee the next one will, even if it’s the same brand. There are a few online sites that track stations with ethanol-free pumps, including Pure-Gas.org, to help you scout out locations.

If you have to settle for ethanol-infused fuel, most auto parts stores sell ethanol fuel treatment additives, which can help solve water and corrosion issues. We won’t recommend specific brands, but we do recommend the following:

Dealing with ethanol is especially important in older cars, because they tend not to be driven all that much even in good weather, along with that long winter sleep. When you put it back in storage next fall, fill the tank completely – with ethanol-free, if you can find it – and no matter what grade you’ve bought, add a can of fuel stabilizer.

Rather than a problem with the fuel itself, octane issues are usually related to how the engine behaves. With a newer vehicle, putting premium gasoline into one that’s rated for 87-octane usually does little more than put a hole in your wallet.

However, in a classic car, you may want to stop at the pricier pump, no matter what your engine was originally rated to take.

Octane isn’t an additive. Instead, the amount of octane in fuel depends on how it’s refined. While high-octane fuel sounds like it’s the super-powerful stuff, it’s actually less volatile than 87-octane grade. That’s important in high-compression engines, where high heat can cause various pinpoints of detonation in lower-grade fuel, or worse, spontaneous combustion before the spark plug fires.

Premium fuel resists this pre-ignition, and creates an even flame that spreads gradually through the chamber (relatively, of course, since it happens in a split-second). Older engines can be more susceptible to pre-ignition, and using premium fuel can help to prevent it.

Leaded gasoline has been gone since 1990, and that’s a good thing, but old cars were designed with it in mind.

Lead was originally added to gasoline in the 1920s, as a cheap-and-easy way to raise octane levels to reduce engine knock. It also acted as a lubricant, and helped to prevent excess wear at the valve seats, where the intake and exhaust valves seal to close the combustion chamber.

Removing lead removed this protection. After a while, some engines developed valve recession—the valves wouldn’t seal, resulting in reduced engine compression.

Many (if not most) classic-car owners have no problems with unleaded gas, but it can potentially be an issue with hard use, such as if you’re using your old truck to pull a trailer, or taking a muscle car to the drag strip.

If you suspect it’s going to be a problem, there are fuel additives for engine protection. Ultimately, though, the definitive fix is to replace the valve seats with new hardened units—something many owners did when the fuel was initially banned.

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