Fuel Tips: The Science of High Octane

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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mad_machine
mad_machine 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

USGUYS
USGUYS New Reader
5/15/19 9:39 p.m.

In reply to mad_machine :

Premium fuels don't alcohol added.  That's cause of the difference in MPH you're seeing. MBTE blends are even worse.

15f80
15f80 New Reader
5/15/19 9:51 p.m.

Gasoline is different stuff depending on which refinery it came from. Adding the variable of ethanol is a big wild card on top of that.

F1 supposedly uses fuel with an octane number around 87. My impression is it's the best balance of energy versus combustion properties. Reading their rules, it sounds like they use blend of fairly pure components which should result in a product that has a longer shelf life. I wonder if the teams have one blend made and use it for most of the year, or have different blends for different tracks...

I have done my own tests in a  1994 Honda (supposedly pre-OBDII by 2 years). The recommended fuel is Premium. It gets about 1 mpg better economy on Premium, just enough that it is usually worth using the higher octane fuel.

15f80
15f80 New Reader
5/15/19 10:14 p.m.

In reply to USGUYS :

My impression is that ethanol is used in every grade of fuel in states with pollution control testing.

MTBE is at least partially banned in at least 23 states. I can't remember the last time I saw it listed on a pump (left coast driver). I am pretty sure it has a higher energy content than ethanol, so should get higher MPG.

alfadriver
alfadriver MegaDork
5/16/19 6:26 a.m.

E10 is the nominal national standard.  

And MTBE has been banned from fuels for a long time, way longer than it's been used for certification fuel around the standards it was written for.

Also, gas being "different"- it is, but within a very tight bounds of what is allowed.  The real factor in why gas is different is that regional blends are allowed in various parts of the country to try to meet some air quality issues.  Which really isn't refinery related, but area related.  And, in these days, isn't as helpful as it used to be.  Getting rid of the various blends and resorting to a fixed set of seasonal options (like California states) would  be a net benefit for the entire country.

One other thing- octane does NOT indicated how combustible a fuel is,  it's a measure of how stable it is when subjected to conditions that can result in knock (aka- spontaneous explosion). 

F1 is trying to push engine efficiency for normal road cars by spec'ing THE nominal fuel available around the world.

z31maniac
z31maniac MegaDork
5/16/19 7:13 a.m.
rslifkin said:

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.  

I'm glad you mentioned the tanks. When I worked for MerCruiser, the rubber bits weren't the issue. It was the fiberglass tanks from some companies. 

frenchyd
frenchyd UberDork
5/16/19 7:35 a.m.
alfadriver said:
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.

That and water absorption   Sitting in the tank there is a path for outside air to infiltrate and carry some water molecules with it, but it’s the same path that allows the lighter ends to escape from.  

Unless there is a molecule cop on that pathway it won’t work both ways in and out at the same time.  

Besides gasoline has always had water in it.  Look under the hoods of cars from the 30’s -40’s- 50’s    You’ll see glass water traps to show you how much water is in the fuel. 

A little water isn’t a bad thing. Assuming the wiring is up to snuff cars always run smoother in the rain.  And help prevent knock while cleaning the combustion chamber.  

frenchyd
frenchyd UberDork
5/16/19 7:40 a.m.
USGUYS said:

In reply to mad_machine :

Premium fuels don't alcohol added.  That's cause of the difference in MPH you're seeing. MBTE blends are even worse.

Even premium has 10% ethanol added. Except for non oxygenated fuel which isn’t legal in newer vehicles.  

rslifkin
rslifkin UltraDork
5/16/19 7:49 a.m.

In reply to frenchyd :

Good point on the water absorption.  Prior to ethanol being common, it was common to see people add stuff to fuel to soak up the water.  Ethanol already does that and for the most part, the water passes through the system harmlessly.  It's only an issue if you get so much water in that the fuel separates, but that would be approaching a problematic amount of water sitting in the bottom of the tank without ethanol anyway.  

iceracer
iceracer UltimaDork
5/16/19 5:56 p.m.

In reply to rslifkin :

And gas stations still sell the alcohol in a can and people still put in their tanks.  The cars continue to run fine. I guess.

tcora
tcora
5/19/19 9:24 a.m.

I call BS on pretty much this entire article.

 

Mr Santner might be a Technical Specialist, but he doesn't bother to mention that the number he is talking about is the AKI, not the octane. And his recommendation that you should run 93 AKI in a car which specificies 87? Sheesh.

iceracer
iceracer UltimaDork
5/19/19 1:22 p.m.

In reply to tcora :

Isn't that what octane rating is.

rslifkin
rslifkin UltraDork
5/19/19 1:29 p.m.

AKI is one measure of octane.  It's the average of research octane and motor octane (2 different methods of determining octane). 

iceracer
iceracer UltimaDork
5/19/19 6:01 p.m.

In reply to rslifkin :

In other words,   pump rating

alfadriver
alfadriver MegaDork
5/19/19 6:27 p.m.
iceracer said:

In reply to rslifkin :

In other words,   pump rating

More specifically, US pump rating. AKI =RON+MON/2.  Whereas the rest of the world uses RON.  Not sure how that invalidates the entire article, though.

johnlumer20
johnlumer20 New Reader
5/19/19 11:17 p.m.

Great information. Very helpful.

StuntmanMike
StuntmanMike New Reader
5/20/19 8:31 a.m.

I wouldn't say that premium should be ran in all 87 recommended engines, but it doesn't hurt and I think it will clean it but I only run 93 maybe a couple times a year in my "87" vehicles, and run them hard with that tank!

It's always funny talking to non car people (or the car & driver mag readers that think they are car people) that say 93 is all hype and you don't need it even if it says recommended on the cap. And when you try to explain engine compression and how most modern engines are high compression, and that most companies have to add the 87 fuel maps to pull timing since they know people will be cheap and put swill in the tank. Any of my "premium recommended" cars make a big difference in how they drive, even the transmission shifts at better RPM's in my Yukon.

frenchyd
frenchyd UberDork
5/20/19 8:51 p.m.

In reply to StuntmanMike : what I fail to understand is why focus on 91 or even 93 octane when ethanol is 114 octane.  

Yes I understand there is less energy in alcohol then gasoline  but there is less energy in gasoline then diesel.  

So is octane the holy grail or is energy?  Either case gasoline loses .  

 

LT_Rusty
LT_Rusty New Reader
7/11/19 11:59 p.m.

In reply to mad_machine :

I spent quite a lot of time watching the engine on my old Dodge Dart 1.4 turbo with the Torque app for a variety of unrelated reasons, and I do have to say that running 91 octane vs 87 made a BIG difference. The difference in gas mileage was pretty significant (3-4 mpg), but the amount of boost was also a lot higher. That extra boost really made a difference with the butt-dyno. (And if I hadn't enjoyed that so much I'd probably have seen even more improvements in the gas mileage.)

frenchyd
frenchyd UberDork
7/12/19 8:40 a.m.
mad_machine said:

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

When dealing with octane and mileage, please remember that ” your mileage will vary”  That’s just a polite way of saying there are some lousy drivers out there.  If you get better than reported fuel mileage how much is due to your skill? And how much due to the fuel? 

 

frenchyd
frenchyd UberDork
7/12/19 8:50 a.m.
StuntmanMike said:

I wouldn't say that premium should be ran in all 87 recommended engines, but it doesn't hurt and I think it will clean it but I only run 93 maybe a couple times a year in my "87" vehicles, and run them hard with that tank!

It's always funny talking to non car people (or the car & driver mag readers that think they are car people) that say 93 is all hype and you don't need it even if it says recommended on the cap. And when you try to explain engine compression and how most modern engines are high compression, and that most companies have to add the 87 fuel maps to pull timing since they know people will be cheap and put swill in the tank. Any of my "premium recommended" cars make a big difference in how they drive, even the transmission shifts at better RPM's in my Yukon.

We are ignoring the elephant in the room here.  That is cost per mile.   If the difference in price exceeds the increased cost then you are not gaining but losing.  

Right up to the point where performance priority exceeds the priority of cost.  On track performance wins. On the street, at least when mundane tasks such as commuting, grocery shopping, taking kids to soccer, etc. cost must win. 

At least if you are on a budget like most people. 

frenchyd
frenchyd UberDork
7/12/19 9:02 a.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.

Valid if storage is your priority.  In the past I filled my tank often more than once a day. Today in semi retirement it’s reduced to almost weekly.  My collector cars and my race car are stored with little or no fuel and what fuel there is is treated with fuel stabilizer. 

So fear of Ethanol loses any impact with me. 

I use math*  to decide if I want to save the money with  the lower cost of ethanol.  Although to be honest there are times when the added power offered by Ethanol wins out just because I enjoy turning  the vehicle into a giggle machine. You’d think at my age (71) I’d grow out of that by now.  

Right now 92 octane non oxygenated is $3.29 a gallon and E85 with around 100 octane is $2.09 

E85 costs me roughly 2 mpg (depending on how much I want to be cheap and how much I want the performance improvement). 

So I I save $24 a tank but can only  360 miles per tank instead of 400. 

Floating Doc
Floating Doc SuperDork
7/12/19 9:52 a.m.

Lots of anecdotal evidence here, so I'll add my two year experience with a 3.4 Tacoma running a TRD supercharger.

No fuel or tuning modifications, since it was dealer installed. At that time, the official line from Toyota was that the blower would require no tuning, so if I did any tuning modifications it would have voided the warranty.

I tried multiple different brands of 93 octane gasoline and it had a part throttle ping with every one, except Shell. It ran perfectly fine with Shell in the tank.

BTW, after I sold the truck, Toyota added a 7th injector to the intake ahead of the throttle body as part of the kit.

frenchyd
frenchyd UberDork
7/12/19 11:39 a.m.
Floating Doc said:

Lots of anecdotal evidence here, so I'll add my two year experience with a 3.4 Tacoma running a TRD supercharger.

No fuel or tuning modifications, since it was dealer installed. At that time, the official line from Toyota was that the blower would require no tuning, so if I did any tuning modifications it would have voided the warranty.

I tried multiple different brands of 93 octane gasoline and it had a part throttle ping with every one, except Shell. It ran perfectly fine with Shell in the tank.

BTW, after I sold the truck, Toyota added a 7th injector to the intake ahead of the throttle body as part of the kit.

While localities differ around here Shell gas comes from either the Koch refinery or the Ashland refinery.  Those are the only two refineries within hundreds of miles.

  Only 5 refineries are located in the US but the site didn’t point out which refineries produced gasoline and which produced chemicals and other products.  

So buying fuel via the Brand is not likely to get you any difference since there are plenty of gas stations even in my city that sell Shell. 

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