Today this post from a local car dealership, explaining regular versus premium gasoline, showed up as a sponsored post in my Facebook feed.
This particular claim caught my attention:
If you’re putting premium gas into your vehicle thinking that ‘premium’ means better, you could be damaging your engine.
I have never heard that before. I have always heard that putting premium gas in a car that only requires regular gives you no benefit and is a waste of money, but I’ve never heard that it can actually cause damage (other than to your bank account). Can it?
I mean, sure, I know the inverse is true. Using regular in a high-compression engine designed for premium can cause damage if you do it too often (although as I understand it modern engines can detect the lower octane fuel and adjust the timing accordingly, at the expense of performance and/or fuel economy). But using premium in an engine designed for regular?
Higher octane gas requires greater compression to ignite.
Every car engine is engineered for a particular fuel (which means one of a very few commonly available fuels). Other fuels may still work but your engine will be less efficient.
So, higher octane gas is meant for higher compression engines. It will work in your Geo Metro but not as well as you’d like which will reduce the efficiency of your engine and result in worse emissions as well. Put another way, you car runs better on the cheaper gas it was made to run on if it was built to use lower octane gas.
So, check your owner’s manual and use the fuel it recommends. That is what your engine was made to use and will run best on. Buying a higher octane gas than your engine is rated for only helps the oil companies.
If your fuel is completely wrong, it won’t burn fast enough, and will still be burning when going out the exhaust valves – burning the valves and poisoning the catalyst.
But my understanding is that in a modern relatively-low-pressure engine the flame speed for modern high-octane fuels is about the same as the flame speed for modern low-octane fuels. The only thing that burns out is your money.
In most vehicles that I am aware of, using a higher octane fuel than what is recommended makes your mileage go down a bit, so it’s not just wasting money but it’s actually making your engine perform slightly worse. But generally that’s it.
I have run across one issue on older air-cooled cars. If you put higher octane gas into them on a hot day, the engine can overheat, which could potentially cause damage.
Think of it this way. Higher octane and lower octane fuel both contain basically the same amount of energy. If the engine doesn’t run as efficiently on higher octane, then that lost energy has to go somewhere. If it’s not being converted into mechanical movement, then it’s being converted into heat.
This is no big deal for a water cooled engine, as the cooling system can probably handle the extra heat without any issue. It might be an issue for a water cooled engine if you are driving through someplace like the Arizona desert, but otherwise, probably not. But for an older air-cooled engine where the cooling is a bit marginal to start with, overheating might be an issue on a hot day.
I have a couple of older air-cooled cars, and I did have one of them overheat when I accidentally put higher octane fuel in it on a hot day. Kinda surprised me, too. I didn’t expect there to be a real measurable difference between fuels. Since then I am much more careful to only use the octane that they are designed for. Haven’t had a problem since, even on hot days.
(1974 Beetle and a 1960 Beetle kit car converted to look like a 1929 Mercedes, by the way - the kit car is the one that overheated)
In my 45+ years doing auto repair I’ve never heard this claim before. My guess is it was written by a half-informed journalist who misinterpreted something he heard or read.
It will make no difference other than being a waste of money. Octane rating is only a measure of how resistant the fuel is to autoignition and detonation. It has nothing to do with the fuel’s burn rate or energy content.
This is not true. A higher octane means a fuel is “more resistant” to autoignition at high temperatures and pressures. “more resistant” means it will tolerate elevated temperatures for slightly longer before spontaneously igniting at random locations around the combustion chamber (giving you slightly more time to get the normal burn completed), but high-octane gasoline and low-octane gasoline are both reliably ignited by the ultra-high temperatures produced by a spark plug’s electrical discharge.
Octane number has no relation to laminar flame speed. It’s strictly a measure of the fuel’s resistance to autoignition. if you ignite a low-octane gasoline/air mixture and a high-octane gasoline/air mixture at the same time, they will both finish at the same time. This assumes the same air/fuel ratio in each case.
Lean mixtures do burn more slowly. Since they finish their burn later in the expansion stroke, less of their energy is converted to mechanical power, making for hotter exhaust that can indeed roast the exhaust valves, and may also cook the cat (which is not the same as “poisoning” the cat, and has nothing to do with octane rating).
Rich/unburned mixtures can release their heat in the cat, cooking it, but again not the same as “poisoning” it (and nothing to do with octane rating). Misfires that send unburned mixture into the exhaust are typically detected by the ECU. If you see the MIL lamp on your dashboard FLASHING (as opposed to just a steady light), this an indication that misfire is happening and you need to shut the engine down or risk expensive cat damage.
Gasoline octane rating these days is typically boosted by adding alcohol, which lowers the per-gallon energy content. A modern engine that’s been optimized for high-octane gasoline will have a higher compression ratio and/or greater spark advance, providing the desired efficiency. Low-octane gasoline (with less alcohol) may have greater per-gallon energy content, but if you put it in this engine, the knock sensor will detect knock and retard the spark timing, compromising efficiency. OTOH, if you have an engine optimized to tolerate low-octane fuel, and you put high-octane fuel in it, the engine won’t advance the spark timing; the lower per-gallon energy content just means you’ll burn more gas per mile. In either case, running the engine with a fuel other than what’s recommended by the manufacturer is likely to compromise fuel economy.
This. From the article:
So far, so good.
Not true. A/F ratio can effect knock, but even if you’ve got a perfect stoichiometric blend, knock can still happen.
Very light knock is an indication that the burn is mostly happening by normal combustion, with detonation occurring only in the last few morsels of mixture. It’s not severe enough to cause damage, but it’s an indication that you can’t dial in any more spark advance without causing problems. At one time it was called “the sound of economy”.
Latter? yes, if it can’t adjust for it. Former? A hard nope.
Any doubt? Listen to the manufacturer, not the dealer. Read your owner’s manual. Where it talks about fuel octane, “recommended” and “required” matter: “recommended” means you’ll get less-than-best performance if you use lower octane, and “required” means you run the risk of engine damage if you use lower octane.
The manual for a Honda CRV for example (PDF here) says on page 644 “Unleaded gasoline, Pump octane number of 87 or higher”. If you have too much money in your wallet, go ahead and put 93 octane in the tank; you won’t hurt the engine (but you won’t improve your fuel economy, and may actually get worse economy if alcohol was used to boost octane).
The manual for an Infiniti Q50 (PDF here, see page 9-4) is a bit more nuanced:
Use unleaded premium gasoline with an octnae rating of at least 91 AKI (Anti-Knock Index) number (Research octane number 96).
If unleaded premium gasoline is not available, unleaded regular gasoline may be temporarily used, but only under the following precautions:
Have the fuel tank filled only partially with unleaded regular gasoline, and fill up with unleaded premium gasoline as soon as possible.
-Avoid full throttle driving and abrupt acceleration.
If this were true, it would be impossible to idle an engine on high octane fuel (since, when idling an engine, cylinder pressure at TDC before ignition is very low).
Thankfully, it’s not true. I can ignite high-octane gasoline and low-octane gasoline fuel with a spark plug equally well under a given pressure.
Running an engine slowly damages it, that’s why engines don’t last forever. So saying that using gasoline with higher than recommended octane rating can damage your engine is true, and so is saying that using gasoline with the recommended octane rating can damage your engine.
It’s been said to dissuade people from using higher octane gasoline under the mistaken belief that it is better more powerful gasoline. Providing a detailed explanation is usually a waste of time, telling someone it will damage their car is more effective.
Ref @Machine_Elf’s excellent deconstruction of the Folsom article. …
When the article is describing knocking, they’re trotting out 1960s era facts about 1960s or earlier cars. No modern cars make loud knocking noises if fed a too-low octane; the engine controller is smarter than that.
Unless Folsom is specifically targeting the antique market, this sounds like their author is somebody who doesn’t really know what he’s talking about quoting an ancient article as received wisdom. Which it may well have been at the time. But isn’t now.
There’s a rule about limiting jocularity (especially before the OP’s question has been answered), but AFAIK there’s no rule against good-faith provision of incorrect info.
I’ve heard the claim before, although I always understood it to be false. If I recall correctly, it was a very minor plot point in the movie Never Been Kissed, in which Drew Barrymore’s character borrows her brother’s beater to help pull off the guise of a high school student. When it breaks down, she tells him she put premium gas in it, and he tells her she shouldn’t have done that because the car is a “regular girl.” It was kind of played for laughs, like this guy was overly attached to and a little bit irrational about his car, not like he was an expert. But certainly the idea has been around for a while.
More explicitly, it can reach higher compression and will not ignite without a spark. With a spark it will ignite at any compression.
Low-octane gas will ignite from compression alone at lower compression, so when using it in a high-compression engine it will ignite before the spark–preignition, or what we hear as “pinging.” Which is bad.
A somewhat related question that all this talk of octane ratings made me think of:
Are there any cars out there that are designed for “mid-grade” fuel, which is typically 89 octane around here? Just based on my personal experience, it seems like most run of the mill cars are designed to use 87 octane. Some higher-end, more performance oriented cars require 91 octane. But I’ve never heard of a car that required 89 octane. So why do gas stations sell it? Am I correct in my assumption that it’s just a marketing thing, an attempt to get some people to pay a little more by making them think it’s “better” (but less expensive than premium)?
Come to think of it, I’m just barely old enough to remember when they still sold leaded gas, and I don’t remember mid-grade existing back then. Back then the three pumps at the gas station were Leaded, Unleaded, and Premium Unleaded. It kind of seems like Mid-Grade was something oil companies created after leaded gas was phased out so gas stations would have something to do with that third pump.
darned if I can remember what it was, but I remember being told about a couple of vehicles which did recommend at least 89 octane. But it’s exceedingly rare.
I’d wager. It also doesn’t cost them that much to offer; if the dispenser has a single nozzle and you press a button for the grade, the dispenser is blending 87 and 91/93 to give you 89.
I used to own a Toyota Camry that required 89 octane. IIRC it got around 2 to 3 mpg worse mileage on 87 octane and about 1 mpg worse on 91 octane.
I currently own an old Cadillac that recommends 91 octane but requires 89 octane. It just barely starts to ping on 87 octane (and would probably be a lot worse if the knock sensor and engine computer weren’t doing their job), but it runs fine on 89 octane. It gets better mpg on 91 octane but not enough better to justify the higher cost.