91 vs 93 Octane 2010 STI
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This is a discussion on 91 vs 93 Octane 2010 STI within the STi Engine Performance forums, part of the STi Technical Forum category; Just got a 2010 STI, I was reading in the manual that 93 octane is recommended. Unfortunately, there is no ...

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    91 vs 93 Octane 2010 STI

    Just got a 2010 STI, I was reading in the manual that 93 octane is recommended. Unfortunately, there is no 93 in my area. 91 is as good as it gets. So I bought a bottle of octane booster and put it in with my last fill up, and man this car really woke up. Now I'm getting better than S mode performance out of I mode on 91 fuel. I live at 3200 ft ASL. The power I am now getting is more like what I expected from this car, being the top shelf model.

    Can anyone else confirm this experience between fuel grades? The manual say that 91 will not harm the engine, but to expect a decrease in performance. I have had several vehicles that suggest high octane fuel, and some that would knock without it (my 01 silverado would knock without at least 89), but I have never experienced a major increase in performance before just by putting a bit of octane booster in the fuel. The other vehicles would stop knocking, but there was no real increase in performance.

    I was thinking of chipping the car with a Cobb unit, but now I think it's cool the way it is. Before, frankly, the thing was a slug in I mode, not much acceleration at all, even if you dropped a gear, which was the only way you could get it to accelerate in I mode. But now it's pushing me back in my seat in I mode, in the 3-4K range, it seems a bit better than it did in S mode with 91 before.

    Has anyone else had this experience?
    Last edited by macshooter; 08-29-2010 at 11:54 PM.

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    1) Don't waste your money with the octane booster.
    2) If 91 is the highest octane fuel available to you, run that. If you find a station with 93, run that.
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    Registered User viperuws's Avatar
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    I read somewhere using octane booster boosts it to around 91.2. That "waking up" was all in your butt...and head.

    If I also recall correctly, there is no difference between the 2 octanes unless you tune for them as far as performance goes but your engine might be happier.
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    You know COBB does have maps for 91 octane... I would rather run a 91 octane COBB map, than factory map that requires 93.
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    Quote Originally Posted by wrxdrvr View Post
    You know COBB does have maps for 91 octane... I would rather run a 91 octane COBB map, than factory map that requires 93.
    Factory map requires +91. It will make less power but it will be fine on 91 vs 93. The difference becomes apparent during modification. A stage 2 STI on 91 will make 20 whp less.

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    The Default One SeattleJeremy's Avatar
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    Quote Originally Posted by macshooter View Post
    Just got a 2010 STI, I was reading in the manual that 93 octane is recommended.
    USDM Turbo Subaru's are tuned from the factory to run on 91.
    The manual should state that 91 is recommended.

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    Quote Originally Posted by jeremy96gase View Post
    USDM Turbo Subaru's are tuned from the factory to run on 91.
    The manual should state that 91 is recommended.
    Quote Originally Posted by brfatal View Post
    Factory map requires +91. It will make less power but it will be fine on 91 vs 93. The difference becomes apparent during modification. A stage 2 STI on 91 will make 20 whp less.
    Well that's not how it reads in my manual... It says 93 is recommended but it "can" run on 91...If you have to use 91 because you can't buy 93. Thus my statement that it should be "safer" to run a 91 octane specific map...
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    The Default One SeattleJeremy's Avatar
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    Seems to me I remember it differently, but you're right.

    Here's the 2010 Legacy/Outback Owners Manual, the wording should be the same on the Impreza.

    Section 7 - 2
    https://www.subaru.com/content/downl...OBK_Manual.pdf

    Turbo models:
    The engine is designed to operate at
    maximum performance using unleaded
    gasoline with an octane rating of 93 AKI
    or higher. If 93 AKI fuel is not readily
    available in your area, unleaded gasoline
    with an octane rating of 91 AKI may be
    used with no detriment to engine durability
    or driveability. However, you may notice a
    slight decrease in maximum engine performance
    while using 91 AKI fuel. Use of
    91 AKI fuel will not affect your warranty
    coverage. If premium unleaded gasoline is
    not available, regular unleaded gasoline
    with an octane rating of 87 AKI or higher
    may be temporarily used. For optimum
    engine performance and driveability, it is
    required that you use premium grade
    unleaded gasoline.
    NOTE:
    Be sure to use premium unleaded
    gasoline of 91 AKI or higher for a turbo
    engine model. If other gasoline (lower
    than 91 AKI) is used, knocking, reduced
    output and poor accelerator
    response will result.
    The point is there is no need to waste your money on oct boosters, just use 91 and it will be fine.

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    Quote Originally Posted by viperuws View Post
    I read somewhere using octane booster boosts it to around 91.2. That "waking up" was all in your butt...and head.
    Well I figured out what was going on. I just so happens that I was driving at night, and it was cooler outside, so I had turned my A/C off...

    Well that sorts it out. Thanks for the responses and inf guys.

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    Like people said octane boosters are a waste of money...But, you can... increase octant with Toluene. 10% Toluene added to every tank of gas should raise the octane to 93... That's about 1.5 gallons of Toluene.
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    Glad you got it soted out!

    All things being equal, a higher octane gasoline can actually decrease performance slightly if it's not needed.

    Only run enough octane to avoid detonation. Anything more and you're costing yourself power (albeit a miniscule amount).

    The blurb in the Subie manual about 93 vs. 91 (as it relates to performance) is nothing but marketing BS.
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    Quote Originally Posted by STImied View Post
    Glad you got it soted out!

    All things being equal, a higher octane gasoline can actually decrease performance slightly if it's not needed.

    Only run enough octane to avoid detonation. Anything more and you're costing yourself power (albeit a miniscule amount).

    The blurb in the Subie manual about 93 vs. 91 (as it relates to performance) is nothing but marketing BS.
    Marketing BS... So have you tried 87 octane and save some money? It's a turbo-ed car, I would think that if running lower octane was fine they wouldn't recommend you need/should use 93... They could sell more of these cars if these cars would run 87, I would say more than a few people didn't buy one because 93 octane was recommended... marketing BS...
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    Quote Originally Posted by STImied View Post

    The blurb in the Subie manual about 93 vs. 91 (as it relates to performance) is nothing but marketing BS.
    No,no its not.
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    Driving at night with the AC off really will wake the car up during the summer. Our cars are prone to heat soak it can make the car feel out of breath.
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    Quote Originally Posted by wrxdrvr View Post
    So have you tried 87 octane and save some money? It's a turbo-ed car, I would think that if running lower octane was fine they wouldn't recommend you need/should use 93...
    Look, I'm not trying to pick a fight, but let's keep the low brow comments under wraps, mmmkay? Thanks... I may be new to Subies, but you would be woefully mistaken to assume I am new to making HP with or without power adders.

    BTT. To clarify, the manual states you can run 91 with NO RISK OF DAMAGE. In other words, the engine is tuned from the factory to be safe on 91. Since this is the case, you have nothing to gain by going to 93. You are only running more expensive fuel that is unneccessarily more difficult to light off. A higher octane rating does not mean a fuel has more energy.

    As I said before, you will make the most power with the lowest SAFE octane level that the tune, motor, and about a thousand other variables will allow. In our case, Subaru says it's 91. I'm not arguing whether or not this is the truth in reality. That's a whole different can of worms.
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