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· needs more cowbell
95L, 07 2.5i, R6
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12,639 Posts
If you want to see if it will help, fill up with 90. Then when you get home remove the negative battery cable for a few minutes. (tap the brake pedal a few times to deplete any power) Put the negative cable back on and go drive the car for a while. If the ecu can utilize the 90, this will get it done. If you don't notice any difference after running a few tanks of 90, do the same sequence with the 87. Its not THAT expensive. I run 93 in my n/a 2.5.
 

· 4 please, to go.
Ver 7 STi GC, elderly L
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4,680 Posts
before I took my car in to be swapped I ran a couple tanks of 93 to make sure the tank was properly primed with it. When I was driving with the 93 I got about 50 miles extra to the tank. I did not change my driving style in any way. At the time, 87 was about 20 cents less than 93.

That means I was spending about 3 dollars more to fill my tank. @ 4 dollars /gallon and 29 miles to the gallon, it was actually cheaper to run 93.

I say try it!

As gas prices increase the benefit of running 93 will effectively increase assuming the price gap between regular and premium stays constant.
 

· Banned
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5,165 Posts
93 octane contains ethanol in most places. I know in Iowa all 93 octane contains ethanol. Not sure about other states. I usually buy 92 octane from BP Amoco which has no ethanol added.
 

· Registered
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17,115 Posts
I use 93 all the time. I switched from 87 to 93, back to 87 and back to 93 and I can tell you without a doubt in my mind that the motor runs much better on 93. I'll be getting a tune that includes a demand for higher octane soon, but even without it, you'll notice the difference. Keep in mind, there's no NEED to, but for $3-4 per tank, it might be worth it.
 

· Premium Member
'17 Impreza Sport, '15 OB 3.6R
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13,557 Posts
87. Period. End of story, have a nice day. :D Unless you can get 89 for less money than 87, which happens in the midwest sometimes, there's no point in going higher, on a stock ECU/tune.
 

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87. Period. End of story, have a nice day. :D Unless you can get 89 for less money than 87, which happens in the midwest sometimes, there's no point in going higher, on a stock ECU/tune.
The engine runs better on higher octane (within reason). You get better gas mileage on higher octane. You get peak horsepower (nothing crazy) on higher octane. Less hesitation with higher octane.
 

· Registered
2005 2.5rs Dark Blue
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242 Posts
87. Period. End of story, have a nice day. :D Unless you can get 89 for less money than 87, which happens in the midwest sometimes, there's no point in going higher, on a stock ECU/tune.
What about resetting your ecu like one guy said? Would that make your ecu take advantage of it? I hadn't ever thought about that before, but it made some sense to me.
 

· Premium Member
'17 Impreza Sport, '15 OB 3.6R
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13,557 Posts
I wouldn't think it would do much, I'm sure the program is set up to run optimally on a certain octane. Only with a reflash (like www.i-speed.us ... or open source...) would it be different.
 

· Meany Head
2000 RSC
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13,908 Posts
Run a few searches through here and you'll find gobs of threads about octane and our cars. What you'll find is that our cars are not setup to use 91/93 to it's full potential. Our ECUs are setup to advance our ignition only so far. I want to say it's something like 40-deg at high-vac cruse situations. If you do not get any knock on 87 with 40-deg, why run 91? So, in cruse situations, your getting no advantage with 91, and your burning the same amount of a more expensive fuel.

You might get a few degrees of timing added if your stomping on the pedal. I think the ECU runs in the area of 10-deg under full load and high RPMs. It might even be less timing. Perhaps 91/93 fuel will allow the ECU to run a degree or 2 more. Having never looked at our ECU code, I can't actually confirm this. But, at this point, your NOT getting better fuel economy because your romping on it. You might get a couple of more HP though. I emphasis might. This is where I suspect the I-Speed reflash really shines. More timing advance while hard on the go-button. More advanced timing and you need higher octane.

We can speculate all we want about what we "think" might happen, and what we "feel" is happening to our cars. In the absence of measurable and quantifiable data and a working knowledge of our the ECU is programmed, it's all speculation and conjecture.

So, to answer the OP question, no higher octane is not bad, it's just not necessary. In fact, you'd likely be throwing away money if you do. Also, higher octane fuel burns slower than the low octane stuff. This can impact performance in a car not designed to run the higher stuff.
 

· Registered
2005 Subaru Impreza RS 5MT
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4,588 Posts
I've been told by some people who've pulled blocks apart that higher octane can leave buildup in engines not optimized to run on it, mainly due to burning slow enough that crap gets left behind.
 

· Registered
2005 Subaru Impreza RS 5MT
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4,588 Posts
The engine runs better on higher octane (within reason). You get better gas mileage on higher octane. You get peak horsepower (nothing crazy) on higher octane. Less hesitation with higher octane.
Sounds like the 87 octane around you is just crap gas. I've switched back and forth a few times on various tanks and gotten no difference whatsoever. Out here, 89 is often cheaper or the same price because they add ethanol, and this actually decreases your mileage (ethanol packs less energy than gasoline).
 
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