The science of high octane fuel

David S.
By David S. Wallens
Oct 18, 2022 | Sunoco, Fuel Tips, Sponsored Content | Posted in News and Notes | Never miss an article

Higher-octane fuels are more resistant to engine knock than lower-octane options. This is why most high-performance engines require higher-octane fuels–in the simplest of terms, these fuels are more compatible with increased cylinder pressures, whether they’re due to the compression ratio, engine speed or boost pressure.

There’s another advantage of higher-octane fuels: They’re more stable when it comes to storage. To explain the science behind that fact, we’re going to crib from a post on the Sunoco Race Fuels website by Technical Specialist Zachary Santner:

87-octane fuels tend to be less refined and contain more unstable hydrocarbons. As the months pass during storage, these unstable components react to form gums, varnishes and lower-octane hydrocarbons. As a result, the octane can decrease within months for 87-octane fuels, especially when stored under less-than-ideal conditions.

93-octane fuels are more refined and contain more stable hydrocarbons. These stable hydrocarbons can last two to three times longer than those in 87-octane fuel. Even with proper storage, 87-octane gas can start to degrade in three months; 93-octane fuel should last closer to 9 months before degradation is noticeable. Keep in mind that 93-octane fuels are still susceptible to octane loss and vapor pressure decreases due to butane evaporation.”

So, other than cost, is there a downside to filling your tank with high-octane gas? “Filling up with premium when you don’t need it can help to clean the fuel system because it contains cleaner components than 87 octane,” Santner tells us. “No reasons to not use it, even in an 87-octane-minimum car.”

Race fuels, though, are a slightly different matter. Where street fuels are blended to meet a price point, race fuels feature better ingredients that lead to longer shelf life. If properly stored, Santner adds, Sunoco’s race fuels can sit around for two years or more without degrading.

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Comments
mad_machine
mad_machine GRM+ Memberand MegaDork
5/15/19 8:24 a.m.

It is my opinion that most modern cars can benefit from higher octane fuels. I know the manual and the flap over the cap says you can run the cheap stuff, but it has been my experience with even cheap cars that the adaptive engine management on new cars can make the most of higher octane.

 

For instance, I had a 1999 Hyundai Tiburon. With it's wheezy 135hp, it was not a big performer. It got epa rated to 30mpg. I consistantly got 35mpg, much more so than most people in the forum that was attached to the car. I ran 93 octane. I tried the lower stuff to test, and my MPGs would suffer as a result. In the end, I will admit, it was a wash as far as money to miles driven was concerned. Spend more at the pump and go further or spend less and have to fill up a little more often. The money evened out.

I never raced the car or dynoed it, but I always wondered if I had a few more ponies under the hood on 93 octane as the management more fully advanced the timing

alfadriver
alfadriver MegaDork
5/15/19 8:38 a.m.

In reply to mad_machine :

That really depends on the car.  Some allow knock control to advance the spark when it sees high octane fuel.  Some don't.  And even some that do are very conservative about doing it.

Which is to say that everyone should experiment with their own car to see if there's a difference.  And then, like you, do the math to see if it's worth it.

Pretty much every car that is made is knock limited at some point, as OEM's try to make sure the engine is as effecient as possible, so it runs as high a knock for the nominal region where it runs.

Larry
Larry New Reader
5/15/19 2:07 p.m.

The big factor ignored here is ethanol. Ethanol degrades fuel faster than the gasoline degrades, despite increasing octane equivalent rating.  93 octane pump gas is typically 91 octane gasoline with ethanol added. Ethanol-free 91 octane gasoline will last longer than 93 containing ethanol, despite the lower rating.

alfadriver
alfadriver MegaDork
5/15/19 2:19 p.m.
Larry said:

The big factor ignored here is ethanol. Ethanol degrades fuel faster than the gasoline degrades, despite increasing octane equivalent rating.  93 octane pump gas is typically 91 octane gasoline with ethanol added. Ethanol-free 91 octane gasoline will last longer than 93 containing ethanol, despite the lower rating.

A lot of people say that Ethanol is evil, and breaks down.  But I've never seen any evidence of that.  And my cars run fine with 5 year old E10 in them.

rslifkin
rslifkin UltraDork
5/15/19 2:27 p.m.

E10 can hurt rubber bits if they're old and not alcohol compatible, but I'll agree, properly stored and stabilized E10 has never been an issue for me.  My lawnmower sat with a tank full of it all winter, ran just fine for the first mow of the spring while I burned it off.  The tank of stabilized E10 in the Jeep is a bit over 5 months old at this point, no issues.  

alfadriver
alfadriver MegaDork
5/15/19 2:35 p.m.
rslifkin said:

E10 can hurt rubber bits if they're old and not alcohol compatible, but I'll agree, properly stored and stabilized E10 has never been an issue for me.  My lawnmower sat with a tank full of it all winter, ran just fine for the first mow of the spring while I burned it off.  The tank of stabilized E10 in the Jeep is a bit over 5 months old at this point, no issues.  

IMHO, for that specific case, that's more of a problem of who made it, unless it's an old part.  E10 has been the nominal US fuel for so long that there's no excuse to not be robust to it.  Let alone it's been available for somewhere around 40 years.

rslifkin
rslifkin UltraDork
5/15/19 2:39 p.m.

At least in the boat world, rubber bits from the early 90s and up (somewhere around 91 - 93) are usually alcohol compatible.  Older stuff typically isn't, so that's where you start to see issues.  Oh, and it can eat certain types of fiberglass fuel tanks.  But as long as issues like that are known and managed, it works just fine.  

Carsandbikes
Carsandbikes Reader
5/15/19 4:47 p.m.

I recently started using 93 octane at every 2nd or 3rd fill-up of my Crown Victoria to help stave off another round of problems with the fuel injection system.  This car acts like it would be okay running on Kerosene, but about 6 months ago I got a MIL and it turned out to be an injector going bad. The Shell brand has I use is supposed to help keep injection problems to a minimum.

I also switched to premium in anticipation of trading up to a Fiesta ST. 

iceracer
iceracer UltimaDork
5/15/19 5:01 p.m.

My FiST runs just fine on 87.    So this summer I will have to run a tank of 93, do some tests and report back.

Ranger50
Ranger50 UltimaDork
5/15/19 9:33 p.m.

A to B outlier test on octane. Same exact ignition map in my Avalanche and the SWMBO’s Suburban. On 87, which she runs, will pull 5^ due to knock plus that’s the limit I have programmed in to the pcm. I run either 93 from sams club or e85. I’ve done more recorded log files on my Avalanche and I don’t have any knock. Plus mine is way more fun to drive as it’s chirps the tires going into 2 through 4th.... laugh

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