Premium fuel questions & Subaru WRX

I’m somewhat considering buying a Subarau WRX or a Subaru WRX STi within a year or two. I’ve heard they require premium.

I’ve never had a car that requires premium fuel, though, so I have a couple questions about premium.

  1. What’s the difference? Is it “cleaner”? Just more octane? What does octane translate to in laymans terms (can I think of it as a greater percentage of combustible material?)

  2. What’s premium fuel for? Is it simply for performance, or is it possible to damage the engine with regular fuel?

  3. What’s the cost/benefit? (I might be able to glean this from your first answers) If premium keeps the engine cleaner, will I have cheaper repair bills? Will the engine last longer? Will the engine get better mileage than it will on regular?

  4. Is midgrade fuel simply a compromise between the two?

Lastly, anyone have one of these cars and care to expound on it? I really like Subarus, and since I have about a 40 minute commute every day, I also like having something fun to drive while not going the full on sports-car route, and I’m able to get a little above the “civic” and “neon” range.


Higher octane rating, = greater resistance to knock/ping.


Yes. :slight_smile: The engine design dictates the need for high-octane fuel. Typically (always?), it’s a high-performance engine. Using lower octane fuel can result in engine damage.


The benefit is you get the performance the engine was designed for – it won’t run as well on regular – and you avoid damaging your engine.



In other words, if you choose this car, you choose to either consistently use premium fuel, or rebuild the engine prematurely.

Generally, the only difference between grades of gasoline is octane rating. Octane is merely a measure of a fuel’s resistance to detonation, or in other words, how slowly it burns rather than explodes.

Higher octance fuels burn more slowly. That helps the engine resist detonation (characterized by knocking and pinging). Detonation is bad for the engine. Higher performance engines usually have higher compression ratios or forced induction to squeeze more power out of each cylinder. The WRXs would ba a turbo’d example.

At higher pressures, the fuel is more likely to explode when burned. Higher octane fuel inhibits this detonation. So manufactureers can make more aggressive engines, but may brequire the higher grade fuel. Using a grade of fuel higher than you need will not have any appreciable benefit, because it doesn’t do anything to help a lower-performance motor.

In short, premium fuel allows cars to be made fast. It does not make cars fast.

If you use too low a grade in a car like that, you’ll get engine knock–fuel exploding in the cylinders instead of burning smoothly. To prevnt this, modern cars have knock sensors that retart the ignition timing to eliminate the detonation. But retarded timing has the effect of robbing power from the engine. So the car is sacrificing performance to save itself from damage. If you go with too low a grade, and the knock sensors can’t keep up, you’ll knock anyway. That can cause damage to the engine if it happens enough.

So, to summarize the answers to your questions:

  1. Octane is explained above.

  2. Premium fuel allows high-performance cars to do what they do, but it doesn’t make slow cars fast. Yes, you can damage your engine with regular fuel, but you’re more likely to lose performance.

  3. You need premium, so it’s a moot point. In most places, it’s only about $0.20/gal more, so that adds up to what, about $3.00 a tank? Small price to pay for getting the full performance out of your engine, and especially for not breaking your car.

  4. Yes.

Turbos generally require premium because the high compression at boost can lead to early detonation at lower octane levels. Also, turbos can generally take advantage of higher octane, putting the octane to good use (power/economy), where as normally aspirated cars just adjust their timing to compensate and off set high or low ocatane fuels.

Follow the manufacturers recommendations. When it comes to fuels, they don’t recommend more than you need, they recommend the minimum.

Two threads in the past month on premium vs. regular octane fuel:

Three Gasoline-related Car Questions

Another gasoline question

Basically, because your WRX is turbocharged (or supercharged?), the fuel-air mixture is pre-compressed before entering the cylinder, so it requires premium fuel, which is more resistant to pre-combustion (knocking). Using lower-octane gas can cause knocking, and the heat build-up can damage the engine in the long term. For 3), the mileage is not affected by fuel grade. In a WRX, your speed and driving style will have more effect on your mileage.

On preview, what everyone else said. :stuck_out_tongue:

Thanks all.

I actually did a search on “premium fuel” but nothing turned up.

Got to butt heads again with audilover again. High octane fuel does not burn slower.

The definition of octane rating is the resistance to knock. Knock happens when fuel that is in parts of the combustion chamber that the flame front has not reached yet decompose because of the heat, pressure, and light from the fuel that is already burning. As the fuel decomposes it can form species that spontaneously ignite at the temperature the mix has already reached, and so a relatively large volume of fuel mix will simultaneously ignite. This gives a large uncontrolled spike in the pressure in the combustion chamber and can be heard as knock. Preignition or pinging is something else and I’m not going to talk about that now. During normal combustion, as the flame front moves in an expanding circle from the spark plug to the chamber walls, the pressure will rise smoothly and then fall as the mix is consumed and the piston starts going down on the expansion stroke. When knock happens a lot of the fuel mix ignites at the same time when the piston is still very near top dead center, so the pressure can reach over ten times higher than normal, so the forces on the piston, rings, rod, rod bearings, crank, main bearings, chamber, head gasket, and everything else are also ten times normal, which is why knock breaks stuff.

If you take any fuel and simply heat it up rapidly in air there will be some temperature where it will spontaneously ignite. This is called the autoignition temperature and is the property that best correlates with octane rating. As the temperatue rises and the decomposition process begins there are a whole host of chemical reactions that occur that lead to combustion, and anything that slows one of those reactions will slow the overall process and raise the octane rating. Different additives function at different times in this process - oxygenates slow down the low temperature, early reactions, while tetraethyl lead slows down some of the medium-high temperature processes near the end.

As for “how fast high octane fuel burns”, I think that people are confusing multiple effects. It is true that higher octane fuel is harder to start burning (the fuel must decompose as part of the combustion process and we just said that high octane means slow decomposition). However, the speed of the flame front as it moves across the chamber is approximately constant for similar gasolines, no matter what the octane rating. So in that sense high octane fuel does not burn “slower” than low octane gasoline. Finally, the flame speed is different for different kinds of gasoline components such as alcohols, aromatics, and ethers such as MTBE, and each of those have their own octane ratings. That means that a gasoline composed purely of branched hydrocarbons will have some octane rating and flame speed, while another gasoline with straight chain hydrocarbons and lots of aromatics and MTBE could have the same octane and much different flame speed.

Just to add to your WRX or STi question - I own a 2003 wrx wagon with (gasp) an automatic tranny. The STi is a real bear to drive everyday - it is really stiff and you will burn yourself trying to drink coffee if you drive over the slightest of bumps. The regular WRX is a good comprimise between performance and everyday drivability, and the wagon is downright practical, in terms of versatility. I don’t think the Sti is worth the premium, especially if you are going to commute in it.

(bolding mine)
Look if a low octane fuel simultaneously ignites it stands to reason that it will take less time to burn than a high octane where additives **slow the overall process **
You yourself in your own post say that low octane fuel burns faster.
Now this next part However, the speed of the flame front as it moves across the chamber is approximately constant for similar gasolines, no matter what the octane rating I agree with up to a point. That point being, that when knock occurs and as you so nicely put it a large volume of fuel mix will simultaneously ignite. At this point the nice orderly burn is over and the entire charge goes boom all at once rather than the 2-3ms that it usually takes to burn a cylinder of gas. So low octane fuel no knock and high octane fuel no knock will take the same amount of time to burn. However raise the compression ratio, increase the intake air temp and the low octane fuel will simultaneously ignite or burn much faster. So under knocking conditions, high octane fuel does burn slower than low octane knocking fuel.

IMHO, audilover replied with a adquate answer considering the fairly non technical nature of the question in the OP.

There is a difference between an explosion, or deflagration, and burning. Fact is that when the fuel is burning, it burns at the same speed. If the high octane fuel detonates, I’ll bet it does so at the same speed as low octane fuel that detonates.

BTW, no flames here.
At this late hour, when the additives in my brain and body have been increased due to hi - octane cognitive supplements, my posts may not properly convey my intentions and meanings. The nitpick goes up (should not be a problem on this site) and my grammar goes down

Nitpick… deflagration is burning. Detonation is exploding.

And I purposely left out the great detail about knock. I gave the short and simple (albeit oversimplified) answer, because it’s suitable for the purposes of this thread. My graduate work in aerospace engineering made me suffer through a semester of combustion theory, so I’m not going to refute anything you said.

I just gave the quick and dirty answer that high-octance fuel burns slower, because that’s all the OP needs to understand. Someone who asks why they need to put premium in a car that the manufacturer says needs premium doesn’t need a lecture on combustion theory.

It’s like explaining airplane lift, and saying it’s caused by the Bernoulli effect. Totally inaccurate, but a satisfying explanation for the masses, nonetheless.

And the explanation I offered is totally inaccurate?

You should be able to answer this. Is the rate of poor combustion fast enough to be considered a true “explosion” or just a rapid burning? I know gasoline will cause an explosion like blast, but is it over 2000m/second or whatever the rate to be considered a true detonation/explosion? Nitroglycerin is detonated with a cap and burns at something like 8,000m/second - this is an explosion.

I believe the expressions used for the purpose of engines and burning fuel is more descriptive than proper. As in the grain elevator exploded; I just hit the button and it detonated. Yeah, but the grain burned at a rate that does not earn the proper term explosive. Kinda like the law calling a pipe bomb an expolsive device. Gunpowder/smokeless propellant is not an explosive, although it can be made to go BANG.

More hijack, sorry.
Also, did that Bernoulli guy do any work with probability?

Not trying to start a war, but you’re actually both incorrect as far as what pre-ignition is.

Pre-ignition is when a second flame front is created in the combustion chamber. The second front is ignited due to the fuel not being able to withstand the pressure and heat.

The knocking sound is created by the two flame fronts colliding and going supersonic.

This is bad for an engine on many levels. As has been correctly described above, pinging causes a pressure spike which places high loads on rod bearings. It also causes localized hot-spots in the chamber which (in extreme cases) can burn valves and even holes in the pistons.

Not directly related to OP about fuel, but I’ve got to say that my dad’s got a WRX. [beuller mode]I highly recommend that you pick one up, if you have the means.[/beuller]

I didn’t say that. I said that my explanation of “burning slower” was less than accurate, but it’s okay in this situation. Not unlike the Bernoulli explanation of lift.

But I wasn’t saying your description was inaccurate…just more in depth than the nature of this thread warrants.

And you’re correct, deflagration can produce an explosion. I kinda spoke to fast by saying that detonation is equal to exploding. They’re not the same thing. Detonation is best summed up by “supersonic burning.” Black powder cannot be made to detonate…smokeless powder can. Cite.

Explosion is just a rapid expansion, and can certainly be caused by a swift deflagration. Or even by non-reactive means, like pressurized gas.


Oh and the Bernoulli question (to get this hijack over quickly)

Daniel Bernoulli of fluid dynamics fame was one of a number of famous scientists from the same family. His father, Johann, and uncle, Jacob, were also pioneering mathematicians. Jacob is credited with great contributions to probability and statistics, and is responsible for Bernoulli Probability. Johann was another mathematical genius making many achievements in his own right. But he is also remembered for plagiarizing his son Daniel and faking the date of his 1738 book Hydraulica to appear to be published earlier than Daniel’s Hydrodynamica, written in 1734.

Pretty impressive family, really.


Is it so choice?

I’ve actually been mentally cataloging the bumps on the way to work and am slowly realizing the STi might not be optimal, but maybe the regular WRX will be fine.

But. . .the STi with different shocks on it is another issue.

Let’s just say the WRX is on the short short list right now. The question is probably not whether I get a WRX but rather, whether I get a new car at all.

Thanks, all.