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Old 07-20-2006, 03:50 PM   #1
zender
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Default Wich gas to use??

I've made a lot of search but never really came up with a good answer for the NA Subaru.

Was just wondering wich octane of gas you guys use and if it helps in any way. I use octane 87 but was not sure if using higher octane could do any damage.

I've read alot of ppl saying that it helps power, quicker accel and stuff like that but i found just hard to beleive since they were using higher octane on an NA car.

I have a 2006 Impreza 2.5i .
No mod on at all. All stock.
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Old 07-20-2006, 04:32 PM   #2
MidnightBlackPearl
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It definitely won't hurt the car in any way to use a higher grade gas, but you're fine sticking with 87. The people saying that it runs better with better gas are probably (in my opinion) experiencing a placebo effect.
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Old 07-20-2006, 04:39 PM   #3
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Stick to 87.

The higher the octane, the more pressure and heat is necessary to get it to combust. Therefore, higher octane gas prevents knocking or small explosions in the fuel system. This is absolutely required with turbo engines and supercharged engines as unexpected mini-explosions can permanently damage pistons, rings and seals.

In lower compression NA engines, high octane fuel often fails to combust and leaves the engine intact. Thus, high octane fuel will result in lower gas milage but no increase in power.

The argument that higher octane gas is cleaner is only relevant to high compression engines. It does nothing for low compression NA cars.

Having had a moderatly modified RS for 4 years, I have found that if you use an intake, midpipe and exhaust; you will benefit from 89 octane gas. With my vehicle, I found that 87 octane gas had noticable knocking and lower gas milage while 91 and 93 octane gas was very smooth, but also with low gas milage.

If you engine, intake and exhaust is stock, stick to 87.
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Old 07-20-2006, 04:42 PM   #4
joecerv
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You can use octane 87 gas, no problems.
I have the same Impreza model, and once used octane 87 with 10% ethanol, it seems a bit slow in response.

PS: Update your profile, car, and location.
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Old 07-21-2006, 12:08 AM   #5
Babolat06
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87. Anymore is basically useless, the car was made for 87.
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Old 07-21-2006, 12:23 AM   #6
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Quote:
Originally Posted by Yebisu69
Stick to 87.

In lower compression NA engines, high octane fuel often fails to combust and leaves the engine intact. Thus, high octane fuel will result in lower gas milage but no increase in power.

The argument that higher octane gas is cleaner is only relevant to high compression engines. It does nothing for low compression NA cars..
i find this a little odd. my brother's protege5 has an intake and axle back exhaust and he uses 91 octane. his mileage has increased from around 316 miles per tank to around 385 miles per tank.

higher octane does help in most cases since its does burn more efficiently. you can't really go wrong with using 91 in a NA car. the only problem is the price... that being said, the car is made for 87 so using it won't hurt either.
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Old 07-21-2006, 12:27 AM   #7
GD6Noob
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the manual says to use 87, so stick with that unless you swapped it..
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Old 07-21-2006, 09:05 AM   #8
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I learned in college (auto/mechanic's school) that higher octane fuel does burn hotter and the actual flame propagation or explosion takes a few milli-seconds longer making it a more effecient/powerful burn. 87 will ignite faster and then die out losing some power. But either will work in our n/a cars obviously. I ran 91 for awhile and now run 87 due to cost and 91 gets a little better milage for me.
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Old 07-21-2006, 10:58 AM   #9
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Quote:
Originally Posted by littledrummerboy
i find this a little odd. my brother's protege5 has an

higher octane does help in most cases since its does burn more efficiently. you can't really go wrong with using 91 in a NA car. the only problem is the price... that being said, the car is made for 87 so using it won't hurt either.
What did the owner's manuals say was required octane for the P5? Probably was not 87.

Higher octane doesn't help anything unless you are pinging on 87 or you can get the engine to take advantage of the higher detonation resistance of 91 (advance timing). It burns less efficiently because it is harder to ignite (thus its resistance to predetonation). It isn't "better" gas, though the high-test might have more additives from a particular refiner than 87.

Also, running higher octane on an engine not requiring it can result in some bad things, namely additional carbon deposits on the intake valves and inside the combustion chamber. If you get a carbon deposit in the head and it gets hot and starts to glow, it'll cause predetonation all on its own. This is not likely, but it certainly is possible.
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Old 07-21-2006, 12:35 PM   #10
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Quote:
Originally Posted by littledrummerboy
i find this a little odd. my brother's protege5 has an intake and axle back exhaust and he uses 91 octane. his mileage has increased from around 316 miles per tank to around 385 miles per tank.

.
More than likely it was the mods causing the increase in gas mileage, not the gas itself. I had similar results with my GTI VR6 (catback exhaust, air filter/modded airbox).
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Old 07-22-2006, 12:43 AM   #11
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Quote:
Originally Posted by Julian
What did the owner's manuals say was required octane for the P5? Probably was not 87.
Exactly... there are NA cars out there that need a higher octane.

On another note, getting away from Ethanol (Arco) should improve your mileage.
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Old 07-22-2006, 11:40 AM   #12
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the P5's recommended octane is 89. i agree that the mods helped the car breathe and get better mileage, but the gas helped as well.

the higher detonation resitance of 91 octane means its more stable and has a more predictable burn. don't mistake stability and predictability with being inefficient. 91 octane does give a more powerful and efficient burn which is why its recommend for wrx's.

and i also agree that on turbo'd cars you'll get more of a gain with high octane than on NA cars. but to say that it gets no gains on NA cars unless you're pinging with 87 isn't a fair statement. and it won't make more carbon deposits either because it burns more efficiently. like i said too, 87 is fine for cars made for 87, but using 91 isn't bad either and you should see some kind of mileage gains. so in the end... take you want from this thread zender... and happy driving guys

Last edited by littledrummerboy; 07-22-2006 at 11:46 AM.
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Old 07-22-2006, 12:45 PM   #13
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I keep reading this thread and noting that one necessary note is still missing... the type of ECU you have. Some computers will compensate or even optimize for higher octane, some will not. As anybody with an older Focus should know (at least the Zetec owners) that ECU will NOT adjust and running higher than recommended octane has been known to cause issues. What our cars do with higher octane I have no clue, but blanket statements regarding "all" cars should be avoided.
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Old 07-22-2006, 01:49 PM   #14
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I noticed my car was knocking a bit a month or so ago...

I do have a lightweight flywheel and lightweight pulley, a larger diameter catback and a somewhat custom intake, so I decided to try some 91 along with reseting the ECU and running some seafoam through my motor.

The result: The engine IS running smoother now. Im noticing better mileage and less pinging. I dont really drive enough to notice a big increase in cost due to using 91 vs. 87 so its not a big expenditure over the course of a year for me. I think Ill stick with it, and perhaps go with some headers soon to smooth out the motor even more.
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