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Discussion Starter #1
Should we use premium gas in our cars?
This is the question that I recently asked myself after frequenting these forums. For the NA crowd, at least, it seems there is a general consensus that premium gas is just a waste of money.

I beg to differ.

My subjective view is based on my recent experience with a tank of gas at the local gas station. Since I have owned my '05 Outback, it has been treated to 93 (or at the very least, 91) octane.

However, after reading the opinions of others I decided that my next fill-up will be 87. The difference was almost immediately noticed. The next morning, I went to crank the engine and a "knock" was immediately noticeable. It lasted for about 1 minute until the engine warmed up. This happened on every cold start with the 87 octane gas.

Once it was time to fill up again, I went back to the 93 octane and the knock went away completely.

Coincidence? Bad gas? I don't think so. The gas station is a large brand, Mobil. Say what you want about Mobil politically, but I'm pretty confident with the quality of their gas.

Anyhow, I am just throwing this out there because it may help some others narrow down their knock problems. I know there are a few of you out there...

Mike
 

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I would attribute that to a knock sensor on it's way out or some other issue with your vehicle.

A vehicle designed to run on 87, and in proper running order, has no need for higher octane, and should not have responded the way your vehicle did.
 

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However, after reading the opinions of others I decided that my next fill-up will be 87. The difference was almost immediately noticed. The next morning, I went to crank the engine and a "knock" was immediately noticeable. It lasted for about 1 minute until the engine warmed up. This happened on every cold start with the 87 octane gas.
Of course it ran like crap. It takes more than a single tank of gas for the ECU to remap itself in a modern car to adjust engine timing, fuel mapping, etc.

EDIT: See this: http://www.cartalk.com/content/premium-vs-regular-1
 

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something for sure wrong with the car. Octane is a stability factor for fuel - the higher the octane the more stable the fuel. The standard non turbo subaru engines were intentionally designed to run on low octane fuel. Running higher octane fuel with a proper engine is simply wasting money - if the car runs rough or pings etc on the regular stuff you could have any number of possible causes which need to be fixed or in your case with more costly gas ignored till whatever is causing the issue becomes a bigger issue. LOL
 

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Of course it ran like crap. It takes more than a single tank of gas for the ECU to remap itself in a modern car to adjust engine timing, fuel mapping, etc.
^ this could be part of it However my understanding of how fast cars make major adjustments via the ECU is more like 100 miles or so which should be far less than a full tank of fuel - which case if your mileage is terrible then for sure you have a sensor some place fouling up the fuel system and could be causing your pinging and knocking also
 

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Discussion Starter #6
or in your case with more costly gas ignored till whatever is causing the issue becomes a bigger issue. LOL
LOL, maybe. If that's the case then this is all good info. I appreciate the criticism.

On these cars, is it possible to have a malfunctioning sensor and NOT have a code?
 

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Discussion Starter #7
Of course it ran like crap. It takes more than a single tank of gas for the ECU to remap itself in a modern car to adjust engine timing, fuel mapping, etc.

EDIT: See this: Premium vs. Regular | Car Talk

This is very interesting, thanks. You'll notice that towards the end of that page it says that if you are towing or climbing, etc., then you could benefit from premium gas. I use my car for work and it is oftentimes packed with quite a bit of stuff. Maybe that's the difference, I dunno.

In any case, I can try two 87 fill-ups (that's about 700mi) to see if the ECU remaps itself... that should be enough time, right?
 

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LOL, maybe. If that's the case then this is all good info. I appreciate the criticism.

On these cars, is it possible to have a malfunctioning sensor and NOT have a code?
Yes I recall one of the regular posters did a write up some time ago about the knock sensor that it can crack and give bad data to the ECU but still remain within a spec range where you don't get the engine light. The knock or pining could be one of many things.

Could be a fouled plug - bad wire - a sensor thats just not up to snuff etc. Really hard to say its one of those things where the car just needs to have a good look over by a tech to make sure that everything is actually working as it should.
 

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This is very interesting, thanks. You'll notice that towards the end of that page it says that if you are towing or climbing, etc., then you could benefit from premium gas. I use my car for work and it is oftentimes packed with quite a bit of stuff. Maybe that's the difference, I dunno.

In any case, I can try two 87 fill-ups (that's about 700mi) to see if the ECU remaps itself... that should be enough time, right?
Heavy loads for the 2.5 still should not matter. Given the engine is physically built to run on the standard lower octane fuels. The only engines subaru makes where higher octane fuel is suggested or recommended is the old 3L H6 which develops hot spots under hard use causing lower octane fuel to pre ignite vs higher more stable fuel will ignite at the proper time etc. Same goes for the turbo engines they run harder and develop conditions where more stable fuel is needed to make sure it ignites at the proper time.

Pinging or knocking is where the fuel ignites at the wrong time. Not a good thing
 

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Don't use higher octane on your standard 'Ru. It all has to do with the compression ratio inside the cylinders. High-compression engines (like 9:1 or 10:1) need higher octane fuel than lower compression engines (such as 8:1). Your engine is a lower-compression engine.

The fuel/air mixture gets compressed by the piston in the cylinder before it ignites. The reason you don't use lower octane on, say turbo engines, is because as the piston compresses the fuel/air mixture, the lower octane gasoline will pre-ignite, causing what's known as a "knock."

This is the same concept in a diesel. As you may or may not know, they don't have spark plugs. Diesels simply compress the fuel to such an extremely small volume that it causes an explosion in the cylinder (which explains the funny noise and explosive power of diesels.)

Now, as for going the opposite way, such as using a higher octane in a lower-compression vehicle, I don't have as clear-cut of an answer. However, I know that for the longevity of the vehicle and it's components, you want to stick with the lower octane. Sure, you may or may not notice a +/- difference in the short term, but I'd bet my two nickels that your knock sensor will give out. How do I know that? Because after only 5K miles of using premium fuel in my '98 LGT wagon, my knock sensor went caput.

In the end, it's your vehicle, do what you want. But you have been warned.
 

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Of course it ran like crap. It takes more than a single tank of gas for the ECU to remap itself in a modern car to adjust engine timing, fuel mapping, etc.

EDIT: See this: Premium vs. Regular | Car Talk
Actually, the 05 fuel map learns within a single drive cycle, about 10 miles of driving.

My 01, even before the supercharger, got worse gas mileage on regular 87 Shell than when I used 93 Shell. 87 got me 21 mpg, 93 got 24. The higher octane is better for the engine performance as well as easier on the CATs. Sure its a H6 high compression, but it also falls to my other Subarus and my one Chevy with a Saginaw 4 cylinder. They all ran better on Premium. No knock sensor issues, no piston issues, no valve issues, no CATs every 100k, and excellent fuel economy. I have owned Subarus most of my driving life and every one has gone over 200k miles without a major issue involving teardown of the heads or engine. Inlcuding the 2.5's.

It could also just be coincidence and its time to change your fuel filter. Pinging/knocking can be caused by lack of fuel also.

What does your owner's manual suggest? Top Tier gasolines at what RON?
 

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Agreed! Octane is just a rating of the fuel's combustable limit. Higher octane is ONLY needed in high compression engines. Read your owners manual and you will find that it tells you to run 87 oct or higher on the NA motor, it they wanted you to run 91 oct or higher they would put it in there...

Whether or not you are hurting your engine by running higher octane seems to be a bit unknown. You may be hurting it, you may not be causing any damage, there doesnt seem to be an exact answer. Regardless of what you decide to do, I would be a bit weary of your car running poorly with the recommended 87 oct. Maybe next time try to step down 93 - 89 - 87 between tanks and see if anything happens. Interesting topic
 

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Don't use higher octane on your standard 'Ru. It all has to do with the
compression ratio inside the cylinders. High-compression engines
(like 9:1 or 10:1) need higher octane fuel than lower compression
engines (such as 8:1). Your engine is a lower-compression engine.
Maybe you should check a few facts? Except for the Gen3 turbos, all
Gen3/Gen4 OB engines have compression ratios of 10:1 ...and higher.

It's been many years since octane requirements were so closely tied
to C.R. -- thanks to electronic ignition timing, fuel injection, etc.

BTW, in many cases where a manufacturer "recommends premium,"
the owner's manual also states that regular (87 octane) may be used
without risk of damage -- but with "slightly reduced performance."
(For example: 2.0L Miata GT, Mazda 626 V6.)

.
 

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Agreed! Octane is just a rating of the fuel's combustable limit. Higher octane is ONLY needed in high compression engines. Read your owners manual and you will find that it tells you to run 87 oct or higher on the NA motor, it they wanted you to run 91 oct or higher they would put it in there...
I haven't seen a manual for the 05 up lately. Does it say "or higher"?
 

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This is very interesting, thanks. You'll notice that towards the end of that page it says that if you are towing or climbing, etc., then you could benefit from premium gas. I use my car for work and it is oftentimes packed with quite a bit of stuff. Maybe that's the difference, I dunno.

In any case, I can try two 87 fill-ups (that's about 700mi) to see if the ECU remaps itself... that should be enough time, right?
Not enough time if it needs a tuneup.
 

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Discussion Starter #16
All great responses. Very helpful crowd here.

I recall one of the regular posters did a write up some time ago about the knock sensor that it can crack and give bad data to the ECU but still remain within a spec range where you don't get the engine light.
Scary stuff. Not what I wanted to hear. I'll locate the sensors and have a visual inspection. Maybe I'll go as far as testing their resistance/impedance if I can get a standard value from somewhere.


Actually, the 05 fuel map learns within a single drive cycle, about 10 miles of driving.

My 01, even before the supercharger, got worse gas mileage on regular 87 Shell than when I used 93 Shell. 87 got me 21 mpg, 93 got 24. The higher octane is better for the engine performance as well as easier on the CATs. Sure its a H6 high compression, but it also falls to my other Subarus and my one Chevy with a Saginaw 4 cylinder. They all ran better on Premium. No knock sensor issues, no piston issues, no valve issues, no CATs every 100k, and excellent fuel economy. I have owned Subarus most of my driving life and every one has gone over 200k miles without a major issue involving teardown of the heads or engine. Inlcuding the 2.5's.

It could also just be coincidence and its time to change your fuel filter. Pinging/knocking can be caused by lack of fuel also.

What does your owner's manual suggest? Top Tier gasolines at what RON?
I have to take a look. Ultimately, I am going down the same path you are. I am slowly collecting parts to supercharge it with a M62. I thought the engine ran tip-top until I came across this. I'll have to inspect further.

As far as better gas mileage with premium gas, I was also under that impression. That theory has held true for the last few cars I've owned and so I thought nothing of using premium in the Roo.
 

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This thread makes my head spin.

A few simple facts:

Higher octane will not hurt your engine. Using higher than your engine was designed/tuned for will not help anything either!

If using the correct octane that your engine was tuned for causes knock, then there is a problem with your engine.

N/A engine compression is not directly comparable to FI compression. The 2.5 turbo engine has a lower compression engine than the N/A 2.5, yet requires premium fuel. It has more to do with engine load than compression ratios.

Just use what is recommended in the manual!

Too lazy to quote specific posts right now...
 

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Modern "learning" computers, can benefit from higher octane fuel even in N/A Lower compression ratio engines. Better Performance and Economy.
The reason is because Ignition Timing plays a very important part in both. The computer can modulate timing based on feedback from the knock sensor. Advancing the ignition timing creates more power across the board. This extra power allows the engine use less fuel at part throttle which is 95% of driving. Some computers can even lean out the fuel trim at part throttle increasing economy that way.
Premium fuel doesn't harm knock sensors, the Hipo engines use the same sensors as regular engines do.
Turbo cars have lower 'Static' compression ratios because the the turbo increases the 'Dynamic' compression ratio.
 

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Modern "learning" computers, can benefit from higher octane fuel even in N/A Lower compression ratio engines.
That is a huge generalization. It's true, but in my experience, the software in the Subaru ECU doesn't try to push the timing to the point where any benefits are realized from higher octane fuel than is specified on the sticker next to the gas cap. It starts with a baseline, retards timing if necessary for conditions, and returns to the baseline periodically to try it all again.

I'm sure that someone could write a fuel map which assumed a more aggressive baseline, but that isn't what Subaru shipped.
 

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Before any ROM changes on my VDC I logged gear pulls and flat runs to gather information on how the ECM reacted. It does advance and retard timing to achieve what the driver requires. When you add a higher octane fuel that increases the power output from the engine, the ECM makes changes in the fuel trim and timing to compensated for the efficiency of the fuel. Through every drive cycle the ECM relearns trim bases for closed loop based on fuel efficiency and the driver's right foot.

After the second ROM reflash, before the SC was installed, the fuel economy jumped to 27mpg. Even with me running testing on the engine and ECM. And it continued to "learn" fuel trims on its own.
 
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