Octane?

Reading some about tuners. But is there any advange/disadvantage using higher octane in a 2012 Legacy 2.5i NA.? Thanks from a new member.

Advantage, no..

I feel like a notice a slight increase in responsiveness in my 2009 2.5i...

Depends on your car's IAM reading, which can be checked via Open Source...

 

Without getting too detailed, the IAM value will range from 0-16 (increments of 4) on older Subarus, and 0.00-1.00 (increments of 0.25) on newer Subarus. The higher the value, the better (engine is knock-free and running efficiently).

 

My initial value was 0.50, so the shop owner suggested I run premium fuel on my next tank of gas, so I did. I've used a full tank of premium now, and I can definitely say that the engine runs smoother and feels more potent (feels like I restored lost power; inclines that usually required a downshift is no longer needed; acceleration is noticeably stronger) and my gas mileage has also improved during my daily drive to work. I went back to the shop to check the IAM value again and it's now at 1.00, so the car is definitely "happier" and operating at the ideal IAM.

 

The funny thing is, in Europe and Australia (and probably Japan), the Legacy 2.0i/2.5i is recommended to take 95 RON, which is equivalent to 91 octane. It seems SOA thinks recommending regular fuel for the N/A cars will help them sell more cars to customers who want to save money at the pumps? I compared receipts, and it's only an $8 increase for about 50 litres (13.2 gallons) between 87 and 94 octane. However, the cost difference will be offset by the extra distance you'll drive from the better gas mileage.

Well I stand corrected..

 

And interestingly enough, my old Jeep got better mpg using 89, but when I monitored mpg with premium it decreased again.

I'm doing some tests on my '09...

 

First tank of gas (87 octane), no use of Sport mode on my 4 speed: 26.4 mpg via my calculations, 27.2 according to my trip computer.

 

Second tank of gas is currently in process using no sport mode, and 89 octane.

 

I plan to do the next tank on Premium (91 octane) and no sport.

 

After that, I'm going back down and using sport mode.

 

Whatever you may be thinking... yes, I do have a life.

I ran 89 with no sport mode and got the following:

 

28.2 via Trip Calculator, 27.9 via my calculations.

 

That's about 5% better.

 

At $.10 cents a gallon more, and a tank of 16.9 gal. that is $1.69 more per tank, right?

 

446 miles per tank at 87 octane.

471 miles per tank at 89 octane.

 

That's 25 miles more per tank using 89 over 87.

 

At over $3.60 per gallon, you just save yourself $2 per tank of gas using 89 octane.

 

I'm currently driving a tank of 89 octane and using sport mode... mileage is way down. (Like down in the 25 range right now according to the Trip Computer).

 

And the experiment continues.

I feel like a notice a slight increase in responsiveness in my 2009 2.5i...

 

I have put a full tank of 91 octane and I also noticed a smoother engine as well as increase of responsiveness. 91 Octane it is from now..

On a n/a, octane is generally irrelevant and won't make a difference in power or consumption no matter what you use as long as you are comparing apples to apples in the sense that you are not taking into consideratin a mix of straight gas and then gas that has ethanol.

The only time higher octane will make a difference on mpg and power is when you use shit gas and the car knocks enough to decrease the IAM.

IF this happens, IAM can crawl back to maximum within a block or it may very well take more than a full tank of gas.

 

The point is, the only way to truly know if it makes a difference is to interface with the ECU and see what IAM is at, as a starting point.

You would have to use some spectacularly shit gas to get a 2.5i to knock to the point where it would have to make adjustments significant enough as to decrease power to a significant degree.

But I have been wrong before.

 

Keep in mind that the only effect octane has is, the higher it is, the less prone it is to pre-ignition. Higher octane gas doesn't create a more powerful combustion, or anything else.

Based on this I am inclined to believe any perceived throttle response is placebo.

The funny thing is, in Europe and Australia (and probably Japan), the Legacy 2.0i/2.5i is recommended to take 95 RON, which is equivalent to 91 octane.

I do believe this is completely incorrect.

Europe at least, posts the RON value.

US makes the average between RON and MON.

95 is equivalent to 87, 98 is equivalent to 89 and anything above such as 100 and 105 is 91, 93, etc.

Therefore the recommended grades are not different across territories

Wikipedia, among other sources, indicates 95 RON is equivalent to 90-91 AKI.

 

91-92 RON = 82-83 MON = 87 AKI

95 RON = 85-86 MON = 90-91 AKI

97 RON = 87-88 MON = 92-93 AKI

 

Canada and US use AKI.

Im tuned for 93 OCT here in VA I think with a tune you can reap the slight benefits of higher OCT.

The wiki more or less uses rough approximations. You wouldn't consider SOA speccing the wrong fluid for the US. If anything they would tune it for the gas here.

Again, the truth is in the pudding, meaning data logging.

Yes, data logging is where the money is. That's why I'm encouraging fellow members to check their IAM like I did.

Regarding different throttle response and overall improved drivability on premium; the reason I attribute this to placebo is the following.

 

When the car is happy and does not knock, it runs the best, maximum possible timing allowed by the tune. Running premium would not gain any power on top of this because the ECU is unable to add anything to the timing it is already told and allowed to run. This is why running premium is a waste of money.

 

In the event timing was in fact pulled due to knock to an extent that it was enough to make a noticeable change in power, the IMMEDIATE gain in power would be zero or not noticeable. This is because learned knock or in other words learned timing retard takes several driving cycles to decay. In other words, the car would not advance timing back to max right off the bat.

 

Regarding whether or not timing pulled can be noticed, I can tell you this.

When my stage 2 LGT loses 25% of power, I can't tell. Not right away. I can tell in time, or I can tell when that power is back, but the instant it is lost, this is not perceptible. And I can assure you my senses are not dull.

 

Having said all this, I still encourage datalogging and even tuning on N/A but not for reasons of power since there can't be had any to any significant degree, but for fuel economy and a better running car.

The Subarau 2.5i NA will not advance the timing in order to optimize the available octane as most vehicles will do now days. Is this what is being said? The 2010 VW 2.5 that I traded in would optimize according to octane.

But there is another benefit to most premium fuels.

Although an expensive way to do it, brand name premium fuel usually contain a stronger additive package.

Although - the lower octane's do have a minimum detergent composition as dictated by law.

SO! - May not hurt to run a tankful of premium every so often. I believe that Chevron has the most powerful additive package, with Shell right behind. Non name brands will usually run just the minimum additive package, required by laws, in all grades.

I would pay the extra expense for 93 if the Subu NA did optimize. But except for an occasional extra cleansing with a premium brand 93, I will run 87. But will not skip on oil. Full synthetic all the way! Amazing some will spend 32K for a new vehicle, then save themselves a few bucks with mart-store dino oil. These are usually the same types that believe wrestling is authentic and vote democrat. But I guess that is a different conversation.

The Subarau 2.5i NA will not advance the timing in order to optimize the available octane as most vehicles will do now days.

No vehicle does this, you have the wrong logical approach to what is being said. The knock control strategy works by pulling timing when knock is detected. If all is good, maximum timing is allowed.

Knock is learned and takes several driving cycles to unlearn. The ECU "tests" by adding back timing and listening for knock. If none is detected, it will continue to add timing back to what it used to be. But this process is not instantaneous.

The quality of dino oil is very good these days and many gas stations such as Shell and BP add the same amount of detergents to all their gas grades.

Why declare yourself a republican zealot? It adds neither value, nor credence.

I agree with fishbone as to how the ECM adjusts timing for engine knock. I wouldn't have said it was that slow in adjusting, but I'm not an expert.

 

I do like to run a tank of the highest grade Shell every 3 tanks or so. I don't notice much difference in gas mileage because I am usually running a Lucas additive.

 

Btw, I am averaging over 30 mpg all the time. I live in the country so there are no short runs and very little city driving.

Wishbone did clarify for me, the knock sensing operation. And thanks for that.

The knock sensor optimizes "down" from the factory setting, not up.

I had mentioned earlier that my 2010 VW 2.5i NA optimized up but was actually designed to run on premium, with the knock sensor allowing lesser octane. The gas cap label called for 93. But would run on 87.

 

As for my "zealant political remark", I believe most Suby owners are liberal by nature and I thought I would stir up a little hornet's nest. And it did strike a nerve with one member. No harm intended, just a little jab.

But there was no denial (from anyone) that the frugal maintenance types are also wrestling-mania believers.

I'm actually of the opinion both parties, at the moment, are equally asshats.

Knowing how few of us here still run cat converters in their system, I highly doubt any would fall in the stereotype of liberal treehugging birkenstock wearing hippie

 

Any n/a motor should handle regular gas just fine.

It's an entirely different matter to compare it to forced induction engines requiring premium. The engine is put at risk due to excessive knock. It's not a question of IF it will break, but WHEN.

High-compression n/a engines run best on premium but they will run just fine on any grade gas. The only loss would be power and possibly mpg.

 

VW knock control strategy in logic is the same, in application more than likely different. Any car will try to run the maximum timing allowed by the tune. There is no "midpoint". If a car stays at some sort of "midpoint" that's only because the ECU is not happy with what it is hearing from the knock sensor.

 

The reason it takes several driving cycles for the timing to go back up where it should be is if the engine would change it in an instant, the car would knock CONSTANTLY and would put the engine at risk, essentially negating the benefit of having knock control strategy to begin with.

i to put 91 octane in my 2.5i and noticed a huge increase in power and republican zeal. no wait. its still dogshit slow.

i to put 91 octane in my 2.5i and noticed a huge increase in power and republican zeal. no wait. its still dogshit slow.

 

no shit sherlock.

no shit sherlock.

 

You mean my sarcasm has made your butthurt?

You mean my sarcasm has made your butthurt?

 

pfff , not at all sir. Your just pretty good to state the obvious

Anyone throw in 85oct?