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Old 05-11-2012, 12:08 AM   #1 (permalink)
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Default Octane requirements (2.5 XT)

Hi everyone. Still a relative newbie, but loving our Subie. We have an 05 XT 5-speed with 117k on it.
So what's the real dealio with the fuel requirements for these cars? Owner's manual specifies only premium but, depending who you believe, with the sophistication of the ECU it really doesn't matter what octane you put in the tank, so long as you're consistent. The car will figure out the mix, advance or retard as necessary, and I could be saving $5.50 a tank full by dropping down a notch or two.

Any good inputs?
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Old 05-11-2012, 12:19 AM   #2 (permalink)
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It's required because otherwise you'll get detonation. The ecu will pull timing but gradually add it back in until it knocks again. In other words, your engine will have consistent knock. Any money you save from using regular gas will be eaten up by engine rebuilds. Not a good idea. By the way, there are cheap tools to log what's going on in the engine. So it might be worth it to buy a $20 vag-com cable and monitor learned knock on a tank of regular gas. Try to stay out of boost on regular though since piston rings like to break from detonation.
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Old 05-11-2012, 12:19 AM   #3 (permalink)
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I have heard somewhere on this forum about this, I think that particular model is recommended 91 premium fuel, but not sure if its required or not.
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Old 05-11-2012, 12:20 AM   #4 (permalink)
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It's the h6 that recommends 91. The xt requires it.
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Old 05-11-2012, 12:22 AM   #5 (permalink)
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Quote:
Originally Posted by Seabass View Post
It's the h6 that recommends 91. The xt requires it.
I see.. makes more sense now.
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Old 05-11-2012, 12:38 AM   #6 (permalink)
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Got it.

tanks, mang. I guess no cheap fuel for me.
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Old 05-11-2012, 12:43 AM   #7 (permalink)
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Yeah, physics is kind of against you with that turbo equipment on board. As previously noted that H6 ECU can make adequate compensations, but it's just not gonna happen in the XT. Too much air & heat going in for the system to adapt for- you're doing the right thing sticking with 91 octane or better.

Focus on enjoying the acceleration of the car instead of feeling smug at the gas pump. It's the best way to make peace with an XT.
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Old 05-11-2012, 11:31 AM   #8 (permalink)
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Running regular in that XT would be baaaaaaaaaaaad.

Performance requires expense here. Premium fuel required, synthetic oil and short drain intervals recommended/required depending on the literature.
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Old 05-11-2012, 11:44 AM   #9 (permalink)
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$5.50/tank? Is 91/93 significantly more expensive than regular where you are?

93 is typically 20 cents higher than 87 here, and my Outback only holds 16.5 gallons (If I remember right...I doubt the Gen3 is too much more). For me, it's only around $3.00. I bought my H6 knowing that I should run higher octane in it, and would have known the same thing if I bought an XT. It's a few bucks, it really doesn't bother me.
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Old 05-11-2012, 03:54 PM   #10 (permalink)
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Quote:
Originally Posted by Seabass View Post
Any money you save from using regular gas will be eaten up by engine rebuilds.
Quote:
Originally Posted by rasterman View Post
As previously noted that H6 ECU can make adequate compensations, but it's just not gonna happen in the XT.
Quote:
Originally Posted by AWDFTW View Post
Performance requires expense here.

What everybody else said, and somebody here will correct me if I'm wrong, but the real day-to-day economical issue is that if the ECU is retarding timing constantly, then it's wasting fuel (more advanced timing makes more power and is more efficient but risks knock with inadequate octane).

So, even if you're paying 5-8% more for premium fuel, you're probably making at least part of that back in better mileage.
Alternatively, if you try to save money by buying "regular" gas, you're probably burning more of it and saving little or nothing.
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