Of course, everyone reading the headline of this article has the answer to the first part. “85-octane is bad because it’s low-octane, duh!” Simple enough, but why is the lower octane actually bad — and potentially dangerous — for modern engines? And if that is the case, why is it sold at all? Well, to answer those questions, we’re lucky to have Jason Fenske from Engineering Explained to put his usual pizazz on the topic and really explain the factors in-depth.

The first thing to address is that, yes, 85-octane gasoline does exist. In certain areas, it is sold as “regular unleaded” gasoline. But, we all know “regular unleaded” as 87-octane at the pump. So, why does this extra-low-octane fuel even exist? The answer is simple: cost and the fact that at higher elevations, you can get away with lower octane gas… Or can you?

Using the R+M/2 method, 85 Anti-Knock Index (AKI) gasoline gets that rating by taking the Research Octane Number (RON, which measures engine knock resistance during low-load operation) and the Motor Octane Number (MON, which measures knock resistance during high-load operation), adding the two together and dividing by two, to find the average octane number. That “85” rating is two points lower than the traditional 87-octane “regular” gasoline, but why?

As Fenske explains in far greater detail, as elevation increases, atmospheric pressure decreases. With less pressure, less oxygen is making it into your cylinders during operating, which means the amount of fuel required — and the associated requirement for knock resistance — is reduced proportionally. So, as you go through high-elevation cities and towns, you’ll start to see 85-octane being sold as regular.

Here, Fenske explains (quite enthusiastically) what knock is, and the visualizations of the differences in oxygen content of atmospheric-pressure air at different altitudes.

So, case closed, right? Wrong. While 85-octane might have been tolerated by older, less efficient engines, in modern engines, it can pose an issue. As Fenske points out in his video, he found studies showing that “modern” engines (in quotes, because the study was in 1987) saw a reduction in tolerance to lower octane as elevation increased, by a factor of 10.

Where the 1942 study saw a change of about 2.0 points of octane requirement per 1,000 feet of elevation, the 1987 study showed that number had dropped to 0.2 point per 1,000 feet. Meaning if your 1987 vehicle required 91-octane gasoline at sea level, it still had a requirement of 89.6 at 10,000 feet, because the first 3,300 feet of elevation showed zero reduction in octane requirement. So, it only stands to reason, that the numbers in today’s modern vehicles are even lower.

Now, again, you might be asking, “With modern electronic controls, shouldn’t my modern engine be more tolerant of lower quality gasoline?” And the answer to that is yes and no. While there are safeguards built into the most modern of engine control systems, it’s a safety net, not really an operating system. All the technology in today’s control systems allows the engines to be run incredibly efficiently, with leaner air-fuel ratios and much more spark advance to ensure complete combustion.

This graph was from a large study in 1942 regarding the difference in octane number requirements as altitude changes. Looking at it, it is pretty clear why 85-octane became an option at higher altitudes. However, those graphs no longer hold true for more modern cars.

Whereas older fuel delivery systems — carburetors and even early fuel injection systems — were much less precise, that generally gave them a larger tolerance window, because it had to operate in a wide variety of conditions with the same settings. Because modern engines are continuously variable on the fly, they are operating in a much more narrow set of parameters, because of the system’s ability to adjust all of the engine’s parameters to the immediate operating conditions. Those narrow windows are far less tolerant of lower-quality fuel.

“At the end of the day, I think it’s very unlikely that if you put 85-octane gasoline in your modern vehicle that requires 87-octane, you’ll do any damage,” Fenske says. “But it’s crazy to me that we sell a fuel that could potentially cause issues in modern vehicles.” Fenske wraps up by explaining that, realistically, if you have an older, carbureted, naturally aspirated engine that consistently operates at high-altitude that doesn’t see much throttle, you’ll be much less likely to encounter any issues with 85-octane gasoline.

And, as to why it is even sold in the first place, instead of 87? According to a US Department of Energy report Fenske found, “85-octane fuel was originally allowed in high-elevation regions, because it was cheaper and because most carbureted engines tolerated it fairly well.” Not exactly glowing praise for the fuel. So, with all that said, the conclusion is probably one that you have already reached, being enthusiasts: Even though it’s available at the pump, just say no to 85-octane gas.

As you can see in this video, it does exist in the wild. 85-octane gasoline is sometimes sold at the pump as “regular” while 87-octane is bumped up to “mid-grade,” and like those of us on the west coast, 91-octane is the “premium” option.