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Here in the UK it's common for petrol (gas) stations to offer both regular and "premium" unleaded petrol. The premium stuff is (naturally) more expensive, and I've often wondered what the actual difference is. Would I need to fill up with premium every time to enjoy these benefits, or does treating the car to a periodic tankful help in some way?

Edit: It looks as though there are regional differences with respect to fuels marketed as "premium". S_Niles says in the answer below that in the US, "premium" implies "high octane", pure and simple. Here in the UK, "premium" implies "high octane plus additives". (Here's an example) It's that "high octane plus additives" stuff that I'm interested in.

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    It's not clear what you are really asking. You provided the link to the detailed claims of the product in question (tesco.com/Momentum99/productBenefits-specs.asp); do you doubt the marketing or what? It has a higher octane number (99 vs 95) and higher energy content ("higher MPGs"). Probably it is more refined and comes pre-mixed with a treatment formula. If you are skeptical, just don't buy it!
    – Potatoswatter
    Commented Apr 29, 2011 at 3:04
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    It sucks that all the good answers discuss the higher octane fuel. The fuel with additives is what's interesting. Gas companies claim that it increases mileage, would be nice to know how close to truth this is.
    – I have no idea what I'm doing
    Commented Sep 24, 2015 at 20:47

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There is absolutely no reason to use higher-octane fuel unless your car explicitly requires it.

The higher the octane, the more compression/heat required to combust the fuel. In high-performance engines (turbo-charged, high compression cylinders, etc), a higher octane fuel is needed so the fuel doesn't combust prematurely (knocking).

If you put this fuel in a "normal" engine, it may even have detrimental effects, since the engine will have a harder time combusting the higher octane fuel.

Even if your car requests a higher octane fuel, it may be possible to use a lower octane fuel because of variable timing and other magic. Your manual would state that. However, it will not have as much performance, since the timing is being retarded to prevent knocking.

Note: I am assuming that in the UK, premium = high-octane, the same as here in the States. Over here, premium is ONLY a designation of octane rating, and has nothing to do with the additive package, etc.

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    Thanks for that. This is the sort of thing we get in the UK under the "premium petrol" label: tesco.com/Momentum99/productBenefits-overview.asp
    – Simon Whitaker
    Commented Mar 8, 2011 at 8:36
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    Hmm, that's a lot of marketing speak that leaves me more confused than educated. It claims that it has a better additive package than their normal fuel, so in this specific case, the premium grade fuel may have a greater difference than just octane. If that is true, then the question becomes whether your car benefits from the additional additives. It's impossible to tell from the overly broad statements on that page.
    – S_Niles
    Commented Mar 8, 2011 at 16:03
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    Anybody have a reliable source of reference for this information? i.e. someone objective and believable where there is no conflict of interest in selling higher octane fuels?
    – Guy
    Commented Apr 24, 2011 at 6:00
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    @Guy, which conflict of interest are you pointing out? The fuel to air mixtures in modern ECUs are well understood and motivated. Knock is bad => reducing the probability of knock requires one or more of: increased octane, retarded timing or more fuel. You can purchase the first, the second is bad for emissions and power, and the third uses more gas.
    – Bob Cross ♦
    Commented Dec 1, 2011 at 20:36
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    This answer contains incorrect information: Higher octane gasoline is not "harder" to combust in a gasoline engine, which ignites the fuel using a spark.
    – feetwet
    Commented Mar 15, 2017 at 17:04
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High octane is required for some cars: e.g., a turbocharged car will likely require higher octane to reduce risk of detonation with higher charge temperatures. High octane is also required to get the optimal power out of a tuned car: properly advanced timing will enhance power and economy (obviously, not at the same time).

Some cars are capable of utilizing the highest octane available, depending on the ECU programming (this is not as common).

Some cars can tolerate a lower octane for an unspecified time by retarding timing. This reduces economy and increases emissions but will allow you to limp home on lesser fuel.

That said, check the owners manual and use the octane recommended. If you use octane that's too low, you're risking damage to your engine. If you use octane that's too high, you're wasting your money.

Follow-up with my specific example: my owners manual says 91 octane is required for daily operation with lower octane tolerable for short periods (2004 Subaru WRX). The gas stations near me only stock 93 octane so, technically, those extra two points of octane are wasted.

EDIT SOME TIME LATER:

I have sinced picked up an Accessport and reflashed the ECU to Cobb's Stage 1 specifications. With that new program, the ECU requires at least 93 octane so my previous remarks about two wasted points are no longer relevant.

The Accessport also provides real-time and average fuel economy by monitoring the actual fuel injected and the miles travelled. The measurements support the points in the first paragraph: the increased octane is allowing the car to produce more power at peak as well as higher economy when driven like a grown man with children and who should know better....

By the way, the car is a delight after the reflash. I was skeptical before and now am greatly annoyed with myself for not taking the plunge years ago.

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Octane rating is a representation of how much fuel can be compressed before before it explodes on it's own. The action of the fuel igniting spontaneously due to excessive pressure causes what mechanics refer to as knocking or pinging. Knocking has the ability to do some real damage. So avoiding it at all costs is a very good idea. Sometimes, knocking can be heard by the driver and sometimes it cannot.

What if you have the music up too loud to hear it? Don't worry there are sensors called Knock Sensors that are bolted to the engine block and they constantly listen for knocking / pinging. If detected they will attempt to correct the condition by instruction the ignition system to delay (retard) the spark timing (when the actual spark occurs). If unsuccessful it will illuminate the check engine light_

  • The lower the octane the less pressure it can handle.
  • The higher the octane the more pressure required for it to explode.

The octane rating for a vehicle can usually be found inside the owners manual (depending on your area you should see at least three of these as your average pump 87,88,89,90,91,93). The octane rating the vehicle comes with is determined mostly by the compression ratio of the engine. Other things are also taken into consideration such as aspiration type (turbos / superchargers / etc.)

  • High performance (naturally aspirated, high compression) vehicles will require a higher compression ratio.
    • Stepping up the compression ratio in an engine, will increase the total heat produced when the gas is compressed on the upstroke.
  • Vehicles with Turbos or Superchargers do not usually have high compression ratios. But due to the power adder they will require a higher octane.
    • The forced induction can be generate anywhere from 8psi - 19psi+ (numbers can vary depending on application) and will play a rather large effect on the final cylinder pressure.
  • Individuals that modify their automobiles. As power increases the octane used must be increased to prevent knocking.
    • Modifying components and install a largest intake manifold, larger diameter throttle bodies will have direct effect on the overall cylinder pressure.
    • Also things such as incoming air temperature, engine core temperature (coolant), and, valve size are all factors in maintaining cylinder pressure.

Check your vehicles manual, if it says to use 91 octane then that's what the vehicle was designed to use. And that is what should be use. If it does not specifically state this in the vehicles documentation then it's is a 100% waste of money to put the higher octane in the vehicle.

Note: If you can hear the engine knocking or pinging then at the next fill put a higher octane into it. Sometimes it will solve the problem and save you a trip to the mechanic.

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    Hi cinelli - while correct, this doesn't really add anything to the answers already given.
    – Rory Alsop
    Commented Mar 27, 2013 at 12:17
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    This answer is also missing discussion of temperature which is just as bad as compression in an ideal gas law sense.
    – Bob Cross ♦
    Commented Mar 27, 2013 at 13:00
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    @BobCross, We could sit and list things that could raise temperatures inside of the combustion chamber all day. That is why I used most importantly because it is such an important factor. Since when a fuel is tested to find it's octane rating it's ran on a test one cylinder engine where compression is raised until detonation occurs..
    – cinelli
    Commented Mar 27, 2013 at 13:44
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    "But due to the power adder they will require a higher octane": this is a specific place where compression and temperature are both important. The compression ratio on the Fiat Abarth is apparently 9.8:1 (higher than my old Mustang V8) with a peak boost of 18.1 psi. That means that without boost, the car should be able to handle regular gas. As higher temperature boost is added, low octane gas will become problematic quickly.
    – Bob Cross ♦
    Commented Mar 28, 2013 at 11:49
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    What's missing from your remark about "power adder"s is why they require higher octane. In the next bullet, you imply that any additional power will require higher octane. That isn't the case: a better intake and exhaust on NA cars will often improve power without impact on octane requirements. As written, this answer is incomplete, bordering on incorrect.
    – Bob Cross ♦
    Commented Mar 29, 2013 at 11:23
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The main benefit is to reduce knocking in higher-compression engines.

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I have heard anything from it does absolutely nothing to help your engine to it actually hurts your engine.

While I don't believe it could actually hurt your engine, unless your engine needs it, then I believe it won't do anything to help.

If you hear your engine knocking during normal driving you should try a tankful of the next grade fuel to see if the knocking quits. If you've upgraded to Premium and you still have knocking, there is something wrong with the engine and a mechanic needs to take a look at it.

The higher the octane, for all intents and purposes, only prevents premature ignition of the fuel in the engine. Premature ignition of the fuel is what causes the knocking sound.

Just for reference, the 'hurting your engine' comment came for a salesperson at a US Toyota dealer.

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    As discussed on a number of fuel related posts here, the wrong octane rating fuel can damage your engine...both too high and too low. Use the fuel specified in your manual. see mechanics.stackexchange.com/questions/1447/…
    – Rory Alsop
    Commented Jul 25, 2011 at 22:53
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    @RoryAlsop - This is the first claim I've ever seen that using higher octane gasoline can cause any harm. And I've never seen an argument or theory to support such a claim.
    – feetwet
    Commented Dec 13, 2016 at 18:32
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    You should google more:-) Higher octane than a car can use will not burn properly so carbon and other deposits build up. This is not good for engines.
    – Rory Alsop
    Commented Dec 13, 2016 at 18:41
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Premium will last longer sitting in a tank. Chevrolet recommends premium for the Volt for this very reason. According to Volt Vehicle Line Director Tony Posawatz (as quoted by gm-volt.com):

If people are not using the extended range capability a lot, the premium fuel does last a little longer,” he said

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    Seems more appropriate (and cheaper) to use a fuel stabilizer if this is your concern.
    – Matthew
    Commented Nov 20, 2013 at 19:20
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As far as I know premium fuel is (or was) better suited for engines with higher compression ratios (10:1 or more), since lower octane fuel is more likely to combust before the sparkplug fires.

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    Doesn't even have to be a '60's muscle car. Economy cars in the '90's didn't all have knock sensors either!
    – Brian Knoblauch
    Commented Nov 1, 2013 at 12:01
  • The Ford F-150 SVT Lightning pickup truck produced I believe 1999-2001 with a 5.4L Triton V8 with Eaton Roots Supercharger also did not have a knock sensor from what I understand.
    – Syclone0044
    Commented Apr 23, 2016 at 5:40
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It's not just performance that's suffering if you use a fuel less than recommended. You're doing unseen damage to your head gasket. By the time it's noticeable, it's too late and you've caused an expensive repair that could have been avoided. If your owner's manual says 87 or higher, go with the higher! Subaru is having a huge problem with their head gaskets leaking because they said their cars can run on 87. Their fix to this is a coolant conditioner that contains stop leak, which means they know the problem is already done to the head gaskets and they're putting a band-aid on the problem. If they said use higher octane fuel from the get go, they wouldn't have the big problem they're having. So if it says 87 or higher, and you want your car to last with less of a chance of a hefty repair bill, go higher.

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    Can you explain how running a lower octane in the absents of spark knock causes head gasket damage?
    – Move More Comments Link To Top
    Commented Sep 18, 2014 at 15:21
  • Agreeing with @Larry here. I know Subies have had head gasket issues, but haven't heard this was the cause. Please, if you can explain this, I'm all ears.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Sep 18, 2014 at 22:17
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This interesting subject has many variables. Most opinions have been based on the supposition the an engine will be operated at its extremes of power demand.
From the point of view of what energy is contained in the fuel, low octane wins. From the point of view of seeking most power for engine capacity, higher octane wins.

In any petrol engine, fuel that is too low on octane can cause damage through detonation, more quickly if it occurs at high engine load and revolutions. Too high an octane could also cause damage in similar circumstances due to the exhausting of still burning fuel.

In normal use, an engine can manage a range of fuel specification with its own management. But a loose nut behind the steering wheel could help prove this wrong.

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    Do you have references to anything you've said here? As written, this appears to be a bunch of conjecture and opinion. Please read How to write a good answer if you have any questions.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 20, 2015 at 11:20
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Octane is one of the more stable hydrocarbons found in a gasoline mixture, so the octane rating pretty much correlates with how stable/unstable the gasoline mixture is under pressure. The compression ratio of the engine, i.e. how much pressure the gasoline mixture is put under, has a direct correlation, then, with the recommended octane rating of the vehicle. High compression engines (performance cars) would cause low octane gas to combust under the pressure alone rather than from the spark plug firing, resulting in knocking, which is very bad for the engine.

Contrarily, using a higher octane than what the vehicle is rated for would mean the gasoline mixture would not combust as readily as the engine needs it to. That would result in lower fuel efficiency at best and potential overall performance degradation or even stalling at worst.

Long and short, use the octane rating your vehicle is designed for, which you can find in your owner's manual or on the manufacturer's website.

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