I’ve read Cecil’s column on the subject, but that thing’s 13 years old. Automotive technology has advanced by leaps and bounds. So I’ll ask the question for today: What will happen if you fill your car with premium when it was designed for regular? Other than lighten your wallet. My local Arco station has 3 grades: regular, plus and premium. Today I grabbed the plus handle by mistake. Cost me a whole dollar more for the fill-up! But am I going to see any difference? Like an extra mile per gallon or something? Or did it just cost me the equivalent of a morning donut.
Everything I have read on the subject says that it will do no harm, but no good either. Most just go on about wasting money and “stick to what it says in the manual” which is not the question you asked.
no, nothing.
the octane rating of gas denotes its ability to resist autoignition. Nothing more. if your engine is not pinging/knocking on regular, you don’t need anything higher.
There will be no effect on the mileage or any other performance metric of your vehicle.
In fact, if your engine isn’t designed for it you may get worse gas mileage, not better. Octane is a measure of gasoline’s resistance to ignition, so higher octane means you have more to overcome to get it to ignite. But other than that minor impact you won’t see any difference unless your car in pinging.
The spark plug’s spark is extremely hot and will have absolutely no problem lighting off high-octane gasoline; high octane only gets you some resistance to ignition by random hot spots on the combustion chamber surface while under the high pressures/temperatures developed during the compression stroke.
The problem with putting high-octane fuel in cars made for low-octane isn’t ignitability, it’s energy density. Many high-octane gasolines these days are made by adding ethanol, which reduces the per-gallon energy content. So if you’ve got an engine that’s tuned to tolerate low-octane fuel, and you feed it fuel that doesn’t burn any differently in that engine but delivers less energy per gallon, then you need to burn more of it to get where you’re going: you’re going to get lower MPG.
While engines back in the 1970s and 1980s would definitely ping with too low of an octane, most modern engines won’t noticeably ping. The engines have knock sensors and will fiddle with the engine controls (adjusting timing and such) so that there won’t be any pinging or damage to the engine. Your mileage will usually drop, though.
It depends on the car. Some will get worse mileage on too high of an octane. Others won’t have any noticeable difference. A few will get better mileage, but typically it’s not much of a difference in my experience.
We used to have a Toyota Camry that was designed for 89 octane. The mileage went down a bit if you put 87 in it, but it didn’t noticeably ping. The mileage also went down a bit if you put 91 octane in it, though not as significantly as it did on 87.
On the other hand, I have an old Cadillac DeVille that requires 89 or better octane. It gets worse mileage on 87 octane (but again, no pinging), but gets better mileage on 91 octane. The mileage difference is fairly small though, and I get the best cost per mile on 89 octane as the better mileage from 91 octane isn’t enough to overcome the cost difference between 89 and 91 octane.
Most likely, the OP just cost himself the value of his morning donut.
wherever did you get this idea? it doesn’t take any more “work” to ignite 93 octane gas than it does 87.
if it did, and using higher octane caused a loss of mpg, then you’d very likely have your check engine light flashing at you because of all of the misfires.
I’ve been told to buy higher octane for my Volt because the gas will often sit in the tank 60 to 90 days and “degrade” or some such thing. Any truth to that?
Unlikely that higher octane will help. But yes, gas does degrade.
back to the Op
If you buy the el cheapo gas, there* may* be a advantage of running a tank of high test “top tier” gas thru it, due to added detergents. Especially if you still have a carburetor.
if it’s “Top Tier” licensed then it has sufficient detergents regardless of octane rating.
the Volt has built in strategies to deal with fuel aging. octane rating won’t affect it.
cars have sealed fuel systems; gas will degrade much more slowly than it would in a gas can or your lawnmower, which has a vented fuel tank.
You should use high octane in small engines that only get seasonal use now because it contains less crap added to it that will gunk up the carbs. Very helpful for 2 stroke engines.
For hybrids that don’t run through fuel quickly you may want to avoid ethanol because it absorbs water over time. Don’t know if they’ve worked something out for that with just ethanol additives in gasoline.
Cite?
there seem to be a lot of people posting myths and “folksy wisdom” in this thread.
Small engines can have issues with ethanol in fuel, most notably two strokes with diaphragm-pumper carbs. the ethanol degrades the rubber part of the diaphragm and it’ll eventually stop pumping fuel effectively. On my gas R/C boat, I’d go through two diaphragms a year on pump gas, then after I switched to getting ethanol-free gas at the marina I’ve never changed one.
Ethanol seems to be the major crap that is a problem. I’ve had this recommendation from small engine suppliers. If it’s just the problem of ethanol degrading plastic/rubber parts I don’t care, you don’t want it in your small engines unless you don’t care if they run. ThisHusqvarna site also points out the lubrication issue when using gasoline with ethanol additives. I’ve seen the same recommendation from Briggs&Stratton also. Hi octane gas is often the most available form of ethanol free gasoline and sometimes costs less than other sources.
This area has been investigated many times and the answer is every car is different. Without carefully controlled testing, nobody can authoritatively state that using higher octane fuel than specified produces no horsepower improvement. I doubt it would produce any more fuel economy.
In general a modern engine designed for premium fuel will produce less power on regular.
But will an engine designed for regular produce more hp on premium? And will an engine designed for premium produce even more hp on super-premium?
There’s no simple answer. Some cars tuned for regular will produce a little more power on premium, if the knock sensing and ECU logic allow this. In fact prior to 2005, some manufacturers exploited a loophole in SAE J1349 horsepower specification, testing their cars on high octane fuel, despite being certified for regular octane. This allowed some mfgs to claim higher hp ratings:
C&D April 2006 “Horsepower Confusion and Resolution”:
http://www.caranddriver.com/features/06q2/horsepower_confusion_and_resolution-column
Other cars tuned for regular will not produce more hp on premium.
The November 2001 Car & Driver magazine tested regular vs premium fuel and dyno tested various cars under controlled conditions to confirm impact of varying octane on engine power: http://www.caranddriver.com/features/regular-or-premium. In general they found more highly tuned cars like a BMW M3 responded better to higher octane fuel.
The TV show Fifth Gear tested a Mk5 VW GTI and found it produced a little more power on super-premium than premium: https://www.youtube.com/watch?v=gQghB4asSnI
They later tested how a Mk6 VW GTI responded to higher octane fuel. It also produced a little more hp on super-premium fuel: https://www.youtube.com/watch?v=WTaBngvsPrc
Modern turbocharged engines with sophisticated multi-sensor acoustic or ion flow knock detection will take elaborate measures to modify engine output to ensure thermodynamic margin. For lower octane fuel or higher ambient temps, the ECU will pull ignition timing, lower turbo boost, richen the mixture or modify valve timing. For more optimal conditions or higher octane fuel the ECU will modify engine parameters to deliver more power. This obviously has limits but each car and engine control system is different. As shown in the above tests, some cars specified for regular octane will produce a little more horsepower on premium and some specified for premium will produce a little more on super-premium.
With other cars (maybe most) there is no difference, and it’s questionable whether you could detect the difference solely from octane anyway. Max engine output is constantly changing based on many other factors and octane is only one. E.g, if you filled up with premium vs regular, during that period the car cooled down a bit so will briefly deliver more power – regardless of octane.
A note about octane numbers:
Canada and United States use the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI) or (R+M)/2. Thus the numerical octane rating in the U.S. is about 4 to 5 points lower than the the same actual octane fuel elsewhere. E.g, 87 octane fuel, the “regular” gasoline in Canada and the US, would be 91-92 in Europe.
However most European pumps deliver 95 (RON) as “regular”, equivalent to 90-91 US AKI=(R+M)/2, and deliver 98, 99 or 100 (RON) (93-94 AKI) labeled as Super Unleaded. Thus regular petrol sold in much of Europe corresponds to about 91 octane premium sold in the United States. Octane rating - Wikipedia
Around my parts the only ethanol-free gas you can buy is high octane, so I can understand how it might get confusing.
A lot of the confusion that people have about the 3 grades of gas comes from the words used to describe them. As if there is a stair step up in quality for each grade, and that is wrong.
You like your car and want to take car of it, are you going to use Regular old gas or do you care enough about it to put in Premium or Supreme? Maybe I’ll put in Plus. Plus what? There is nothing Plus about it. Gas stations and the delivery trucks only have 2 types of gas in their tanks, regular and premium/supreme. Plus is just mixed at the pump by pulling a blend from both tanks.
The 3 grades of gas only have to do with their octane rating. But the nomenclature gives the impression that there are 3 grades of quality. There are not.
Does it say in your owners manual, or on the gas cap or door, to use only Premium? Many high compression/performance cars do. If it doesn’t’ say it needs a particular rating then you are just wasting your money for no benefit to the car.
Regular is not a low grade of fuel. Premium is not a high quality fuel, and Plus in not really plus anything except a few more octane you probably don’t need.
I can’t speak to Telemark’s idea that higher octane is harder to ignite. The higher octane means it’s only harder to get it to spontaneously ignite just from compression alone. I’m not aware of any difficulty in getting higher octane fuel to ignite due to a spark though, and we are talking about spark engines here and not diesels.
I will say this though. While octane rating only refers to compressibility and not to burn rate, I have read that many (most?) higher octane fuels also tend to burn a bit slower, which may affect the timing in engines that aren’t designed for higher octane fuel.
Admittedly, I don’t have a freaking clue about how much this actually affects engines in general and if the slightly slower burn rate affects engine performance or not.
60 to 90 days? Nope. Not an issue.
From what I have read, blended fuels aren’t as stable in the long term, which means you need to look more at the ethanol content rather than the octane rating. If the lower octane has less ethanol, it will be stable for a longer period.
Worst case though it takes fuel many months to degrade to a point where the degradation is significant. If it sits for 6 months to a year, then you’ll notice issues.
Keeping the tank fairly full so there’s very little air in the tank for the gasoline to react with (specifically, the oxygen in the air) will also slow down the degradation.
… which is what you’d expect, since the M3 was designed to run on higher-octane fuel. The Fifth Gear test was interesting though.