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Okay so my Grandfather has an 07 Forester XT & a 96 SVX both of these cars require 91 octane or better, but he uses 87 octane regular, and claims that neither of them knock and quotes he can't tell the difference! Well I think other wise, and that he should follow what the factory recommends, especially with a turbo charged engine! I'm looking for some evidence that would support my argument, for him to use 91 instead of 87. What are some key points I could use?? what are the long term affects of using lower octane gas?? or anything else I can use to convince his stubborn old ass! lol

Sounds like he drives . . . like a grandfather. If he didn't, he'd almost certainly tell the difference.

haha yes he does drive like an old man, but there are times hen he really gets on it! I just want to know how bad it really is to run a lower octane gas? or is it ok?

From my '04 Forester XT's Owner's Manual:

2.5 Liter (Turbo) models
Use premium unleaded gasoline with an octane
rating of 91 AKI or higher. If premium unleaded
gasoline is not available, regular unleaded
gasoline with octane rating of 87 AKI or
higher may be temporarily used. For optimum
engine performance and driveability, it is recommended
that you use premium grade unleaded
gasoline.

And from an '07 XT:

Turbo models
Use premium unleaded gasoline with an octane rating of 91 AKI or
higher. If premium unleaded gasoline is not available, regular unleaded gasoline with 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.

Subaru went from 'recommended' to 'required' for optimum performance.


This ARTICLE does a good job of making the case AGAINST using Premium, even where it's recommended, so it may not be a lot of help for your battle, but it may be useful anyway.

The article includes these charts that show the 2008-20012+ vehicles that are PREMIUM REQUIRED vs. Premium Recommended.

The '08 FXT is on the Premium Required list (along w/all Subaru turbo models). The '07 is functionally identical, so should apply, too.

There are no Subarus listed on the Premium Recommended list...that I saw.


As stated, the car's performance is compromised by lower octane. The ignition timing is 'retarded' to reduce/avoid knock. So, although the car may seem to run fine - even w/out pinging/knocking - it's not running @ its optimum. And if you have a high performance car, why wouldn't you take full advantage of its potential???

Something to consider is that if the ignition timing is retarded to compensate for the lower octane, then the engine is not running optimally for MPG, either.

Your grandad could make a case for running the cheaper, lower octane and getting lower MPG - It might even be cheaper overall. But running Premium will allow the ECU to fully advance timing for the engine, increasing performance and MPG.

Premium fuel actually gets cheaper, from a % stand-point, as fuel gets more expensive. It's always about .20 more than Regular in my area, regardless of how much prices go up.

I'll let you do the math.

Also, if he does 'get on it' on occasion, there's a chance, IMHO, that the car may not compensate for the lower octane quickly enough, and it only takes one bad knock event to blow the engine! Another thought... if he were to ever get a bad tank of fuel, his 'margin of error' is lower making the cars even more susceptible to a knock issue.

GL,
Td