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Old 06-05-2003, 04:32 PM   #1
WAFlowers
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Default Octane for BD

Before anyone starts, yes I know what octane is a measure of.

From when I bought the BD used in February I've been using regular (87) and not thinking anything about it. When I last filled up I accidentally (really, it was an accident) grabbed the wrong handle and put in 89.

The surprising thing to me is that this NA car seems to be running better then before.

I wasn't expecting any improvement, but low-end hesitation is gone now.

Am I just imagining things or should I have been using a higher octane all along?

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Old 06-05-2003, 08:32 PM   #2
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Big arguments have been had over this.

I ALWAYS use 92 octane in my daily driver 2.2na . The outcome you have is very normal to what higher octane can do.
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Old 06-05-2003, 09:44 PM   #3
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Doesn't the owners manual suggested premium for the 2.5?
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Old 06-05-2003, 10:15 PM   #4
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To start, a higher octane will run better, based off it's octane alone. Do you need it? Not necessarily, but if a computer has been programmed to 87, 89 will run better. Plus, higher octanes have more detergents and additives to help keep them cleaner, and burn better/clean engine components better. (some people will call this a myth, but the petroleum companies advertise it as such, and false advertising IS still illegal).

Do you need it? No
Is it better for your engine? Yes
Can you run 87 and be just fine? Yes(even the petroleum companies, at least Mobil, tell you to only run what you need)
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Old 06-05-2003, 10:42 PM   #5
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Speedwagon: Be careful, if the same people who read the NA or Service forum read this your gonna get ripped.
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Old 06-05-2003, 11:14 PM   #6
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So what does the owners manual say on cars with the 2.5? If it does indeed suggest premium, then I would guess that your base timing curve is set for premium and using 87 causes it to retard your timing (with a noticable stumble on acceleration).

edit: I guess I'm making up things in my head about the reccomending premium thing... everything I found suggests regular. I guess that if your car has a knock sensor it might be advancing timing a little

Last edited by ws6fiero; 06-05-2003 at 11:22 PM.
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Old 06-05-2003, 11:20 PM   #7
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my '02 impreza manual says 87 for 2.5.
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Old 06-06-2003, 12:48 AM   #8
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The 96 2.5 was the only 2.5 that required premium.

That said, I thought my car ran a *little* better on 91, but not enough to justify the cost.
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Old 06-06-2003, 01:32 AM   #9
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Quote:
Originally posted by sybir
The 96 2.5 was the only 2.5 that required premium.
That's what it was! I'm not going crazy!

Unfortunately this doesn't explain your situation, WAFlowers. Hmmm... I think I'll stick to my advancing the timing answer
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Old 06-06-2003, 07:44 AM   #10
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Quote:
Originally posted by ciper
Speedwagon: Be careful, if the same people who read the NA or Service forum read this your gonna get ripped.
I don't frequent either of those(periodic visits only), so why do you say this?

edit: Went to the S&M forum ( ).. and now I remember.. both of us were in that thread.. hehe

Last edited by Speedwagon; 06-06-2003 at 07:52 AM.
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Old 06-06-2003, 10:18 AM   #11
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OK, some of what was said here runs counter to what I understood. Let's recap.

In simplified terms, octane (as I understand it) is a measure of the combustability of the fuel. The higher the number, the less combustable the fuel. In some situations, a highly combustable fuel may predetonate (aka knock). One of the ways to eliminate knocking is to switch to a higher octane.

However there is a limit to this: use too high of an octane and (as I was once told) you can get incomplete combustion -- not all the fuel consumed. This will result in a loss of power, increased pollution, etc.

So blindly saying "run a higher octane, always" may not be the best answer. Race gas in a NA 2.5 might not help, and might make performance worse, for example.

How am I doing so far?

So my concern is how high an octane is too high? How low is too low? And what is the "sweet spot"?

The book says a minimum of 87. It is possible that this recommendation is optimistic and works only because of a knock sensor pulling timing. But I don't know if that is true. I don't believe that the BD has a knock sensor!

Because I have no actual measurements or ways to obtain them, I have to go by my very unreliable and uncalibrated seat of my pants. My butt-o-meter says that 87 isn't good. On 89 it ran very well. On 93 it feels like it is stumbling occasionally.

I need to try buying from a few different vendors as well because maybe I just got bad gas (for the 93). I really did like how well it ran on 89 for one tank, and I have 4 months of experience with buying from various gas stations and running 87 to compare it to.

Does anyone think there would be any advantage to my resetting the ECU when I switch octanes?

I'm glad I don't hang in the NA or Service forums if they are as described. That's why I posted in this generally friendly forum.

Thanks, and keep those responses coming!
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Old 06-06-2003, 11:01 AM   #12
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Check out the third paragraph or so

http://www.babcox.com/editorial/ic/ic30332.htm
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Old 06-06-2003, 11:11 AM   #13
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This isn't completely necessary, but I'll say it anyways. Octane measurments are actually a measurement of heptane and octane. A true OCTANE is 100, because, well, then it is 100% octane. 87, for instance, is a mix of heptane(cheaper) and octane, to make it the equivalent of 87% octane, with the rest being heptane or some equivalent. RON(research octane number) + MON(motor octane number), divided by 2 is how we get our numbers in the states. It's not that's it's not combustable, it's that it resists detonation. Detonation, in essence, is spontaneous combustion under pressure/heat. Obviously, we only want it to combust on que.

Now, the reason octane is used, instead of heptane, is because heptane will ignite under very small pressure, unlike octane. And,since all this can be put on paper and figured out mathematically, any given octane rated gasoline has a certain compression ratio at which it will detonate. That being said, if 87 requires a CR of 10:1, and your engine runs at 11:1, you'll see detonation as soon as you hit 10:1 on your way to 11:1. On the other hand, if 91 detonates at 12:1, then you won't finish burning it, because you never reached 12:1. And because of this, you will never see the full potential of 91. BUT, it won't harm performance, it will just dump the unburnt fuel out the exhaust(possibly bad for the cat though). So naturally, you'd want 89, because it detonates at 11:1, and you'd get the full benefit of the gas.

Since we aren't going to figure all that out, we just run the lowest that makes it run the smoothest..

How'd you like todays science lesson? hehe

I hope it at leastedhelped a little..
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Old 06-06-2003, 01:21 PM   #14
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I would completely disagree with what Speedwagon says. If the ecu is programmed to see 87, assuming you have not modified the car significantly via bigger cams, increased compression or some form of forced induction, the higher octane fuel will do absolutely nothing for you performance wise.

All modern gas has detergents in it (more marketing hype, sorry to say). If you believe that the big oil companies don't stretch the truth, then your being way to kind to them. Sure false advertising is illegal, but they are not really telling you anything except it has detergents in it. Nowhere do they ever say that their 93 octane has more than their 87 or 89 octane.just that the 93 has it..big deal.

What WAFlowers said is right on the money...its trail and error. My Legacy loves Shell and Mobil, and is not so fond of Amoco..why? Who the hell knows...my ZX which has around 600 HP runs just great on Mobil 93 for the street, and I have a tank of Amoco 93 in it right now and the datalogs all look the same.

If it runs better on the 89, thats cool....maybe now try some different brand 87's and see what it does.

Good luck

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Old 06-06-2003, 02:01 PM   #15
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The issue all of you have to take into account is that when running the specified 87 octane the car is NOT running at maximum advanced timing throughout its rpm/load range.

Some RPM/Load conditions will not be retarded very much while others are. The benefit from the high octane gas depends on learning your driving conditions with that gas. It wont be an instant change just by trying a couple gallons.

If the higher octane meant unburnt fuel was sent into the exhaust then the oxygen sensor would see this. The vehicle would lean out and the exhaust temp would raise. This isnt happening.

Once combustion is initiated the energy in the flame is greater than any resistance that 92 octane would have.

Maybe 89 octane is the best balance though.

Too many of you give up easily. You think you are right, which you may be, but you are just going with what you always have been told without looking into it.
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Old 06-06-2003, 02:14 PM   #16
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Quote:
Originally posted by Z1 Performance
I would completely disagree with what Speedwagon says. If the ecu is programmed to see 87, assuming you have not modified the car significantly via bigger cams, increased compression or some form of forced induction, the higher octane fuel will do absolutely nothing for you performance wise.

All modern gas has detergents in it (more marketing hype, sorry to say). If you believe that the big oil companies don't stretch the truth, then your being way to kind to them. Sure false advertising is illegal, but they are not really telling you anything except it has detergents in it. Nowhere do they ever say that their 93 octane has more than their 87 or 89 octane.just that the 93 has it..big deal.

What WAFlowers said is right on the money...its trail and error. My Legacy loves Shell and Mobil, and is not so fond of Amoco..why? Who the hell knows...my ZX which has around 600 HP runs just great on Mobil 93 for the street, and I have a tank of Amoco 93 in it right now and the datalogs all look the same.

If it runs better on the 89, thats cool....maybe now try some different brand 87's and see what it does.

Good luck

Adam
Allow me to retort: (just had to use that word )

Actually, Mobil's website does state that their premium fuel has more detergents in it. I only state Mobil, cuz that's all I have looked up in the past.

As for your car not liking Amoco.. that would be detergents/additives. Because, all the gas is the same. It is more economically feasible for the petroleum manufacturers to make the gas to standard, and put it all in the same pipelines to ship it wherever. Amoco, Mobil, Shell all put some 87 octane in on one side, and pull some 87 octane out the other. That doesn't mean they pull out THEIR 87 octane, which is why there are standards. The differences lie in the detergents/additives after they get it to the regional location.

And, it's quite possible they do stretch the truth. But, from the research I have done, I cannot find any good reason to waste my time trying to prove them wrong. So, I will go with what they say. If my car runs better on 89 than 87, and Mobil instead of Amoco.. well, the proof is in the pudding then.

And just so I know, which points are you disagreeing with? You can't possibly be disagreeing COMPLETELY.. cuz we aren't saying the opposite of each other here.
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Old 06-06-2003, 02:16 PM   #17
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All I know is I use 89 because that when it feels like it runs best for me.
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Old 06-06-2003, 02:17 PM   #18
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Quote:
Originally posted by ciper
The issue all of you have to take into account is that when running the specified 87 octane the car is NOT running at maximum advanced timing throughout its rpm/load range.

Some RPM/Load conditions will not be retarded very much while others are. The benefit from the high octane gas depends on learning your driving conditions with that gas. It wont be an instant change just by trying a couple gallons.

If the higher octane meant unburnt fuel was sent into the exhaust then the oxygen sensor would see this. The vehicle would lean out and the exhaust temp would raise. This isnt happening.

Once combustion is initiated the energy in the flame is greater than any resistance that 92 octane would have.

Maybe 89 octane is the best balance though.

Too many of you give up easily. You think you are right, which you may be, but you are just going with what you always have been told without looking into it.
How do you know this isn't happening? Have you seen it for yourself, or are you basing this off theory? That might come out wrong, so don't take it wrong. It's just a curiousity here.
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Old 06-06-2003, 02:33 PM   #19
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Ciper - actually, what you said in your 3rd paragraph is precisely what happens...the ecu sees the higher octane, likes it for a time, then goes right back to where it was before. Datalogs of mine and many a customer car have verified this. As for running maximum advance, again, I can confirm via datalogs to my car, that max advance did not change., only the advance at certain rpms did. However, once the ecu figured it all out 9which does not take long mind you..in my case, around 75 miles of my normal driving, which is a mix of highway and local), then it scaled timing back to the old map.

As for the detergent issue Speedwagon, I hear what your saying, but then again, my GVR4 will literally run on whatever gas I give it (I always use premium, as its a turbo car). it does not care if its 93 or 94, and it does not care what brand...again, I have countless datalogs to show it. The only gas that madea difference was race gas (which I can run since the car is catless)...it then runs quite rich once its cycled through the lines (whcih happens after around 10 minutes of leaving the car to idle). The resultant changes in timing, etc then are the result of my having to lean the car out on the VPC at idle, and the AFC at other rpm points, whcih in turn causes a change in timing.

At the end of the day, I really just wish Subaru would hand off all their ecu work to Mitsubishi to handle......the Subaru ecu's are so finicky its sickening, and they rarely run consistenly day to day. Even the new STi is plagued by this as well (at least on some cars)

I do agree Speedwagon, that it comes down to testing what your car likes...and going with that. My disagreement was that if the ecu calls for 87, it will not run better on 93 for the reasons described above....at a certain point, the ecu scales things back. But maybe what we are finding is that the 87 is of such marginal quality, that the step up to 89 keeps things happier....could be.

Adam
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Old 06-06-2003, 03:08 PM   #20
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Yes.. well, I might not always articulate myself well, cuz I do agree that running 93 is pointless if your car wasn't meant for it, and that it will 'fix' itself to see no benefit.
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Old 06-06-2003, 03:36 PM   #21
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Quote:
Originally posted by Z1 Performance
Datalogs of mine and many a customer car have verified this. As for running maximum advance, again, I can confirm via datalogs to my car, that max advance did not change., only the advance at certain rpms did. However, once the ecu figured it all out 9which does not take long mind you..in my case, around 75 miles of my normal driving, which is a mix of highway and local), then it scaled timing back to the old map.
I really wanted to hear somebody who had tried this. I personally have always run the reccomended octane in all of my cars and have never seen anybody with solid proof that it's beneficial to do otherwise. The only thing I buy premium for is my lawnmower with a milled head I'm glad to see that you took the time to look into this, Z1.
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Old 06-06-2003, 04:41 PM   #22
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OK, thank you to everyone who contributed (no sarcasm here). It confirms that I do understand what is going on even if my extremely simplified description of what "octane" is measuring was misleading. I had enough chemistry way back when that I could have given the entire treatment that Speedwagon present, but I just didn't feel like typing that much.

You've also confirmed that what I felt might be real and, contrary to popular belief, I might not be crazy. Of course, the verdict is still out on that one.

So I'll try 89 from a few more sources and maybe some more 93 and see what feels best. After all, I want this beauty running her best when I take to the autocross course!
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Old 06-06-2003, 07:25 PM   #23
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My BG runs better on premium (91/93) than on regular (87). The engine sounds smoother and IIRC, I had better power at low rpms. But it also dropped my mileage, by about 3 mpg.

In the end, I switched back to regular. I'm not racing or doing anything that requires maximum performance out of my wagon, it's just my daily driver. And if I can achieve the performance I want with 87, 87's what I'll stick with.
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Old 06-09-2003, 03:07 AM   #24
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IMHO, the final analysis has to be made on a car that is running flawlessly (perfect spark delivery, perfect sensor system: knock H20 temp, O2, TPI, etc., perfect ECU maps, perfect fuel delivery, perfect electrical path tying it all together. As was mentioned, the Subaru ECU is pretty finicky about all of these parts working just right or it will start retarding/advancing timing to try to make it work right. Some of us see this in the low end of the RPM range, some of us see it in the mid and upper RPM registers.

The butt dyno is very subjective at best and, for that short burst while the ECU figures out what we've done to "affect" its internal maps, will probably indicate a "boost" in power related to the increased octane. Again, as was mentioned, that's only going to be temporary while the ECU relearns and returns what we've done to its normally programmed state. And it's pretty insistent on doing this.

Without beating a tired horse, I think one of the best ways to help the "system" to do its job is to make sure all the electrical pathways are free. I was tired of the mid-range performance dips and poor off-idle acceleration so I made sure that all the typical stuff (plugs, wires, throttle body, MAF,etc.) was working to factory spec and then started noodling around with different octane "blends". I'd reset the ECU, run two tankfulls and then compare the results. I never ran 92 octane (highest that's available here in SoCal) but the numbers just didn't change between 87/89 so I saw no need to pay the extra bucks for the 92 octane. My mileage and range remained consistent across the different gasoline mfrs and grades.

Once I did the single point ground (SPG), however, I had two immediate results. Smoother, consistent power delivery and increased mileage across all five Subes all running on Texaco (now Shell) 87 octane. I'm convinced that the sensors (particulary the knock sensor, O2, TPI and MAF) can now get their signal to the ECU more effectively and this results in more stable engine performance, smoother power and better mileage.

Hope that's not too off-topic but wanted to share my corollary to your octane thoughts.

Br, Dale
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