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Old 12-01-2012, 04:32 AM   #1 (permalink)
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Default Preferred octane gas for high altitude

Hello!

I have a '12 Forester X-Premium (non-turbo), and as you know the owners manual recommends 87 octane. I live at very high altitude (8500 feet), and almost never descend to lower altitudes -- I'm retired and spend my time playing in the mountains!!!

I've read tons of forum posts and it seems that everyone agrees that running 85 octane at high altitudes is fine. I've been doing so, but I have to say that I'm not impressed with the power of my Subi, especially when going up steep grades at high speeds (e.g. 60mph). In these cases, the trans always downshifts to 3rd -- at very high revs. I know this is due to the shitty four-speed -- but I couldn't wait another year to buy a 2014 model (which will have a CVT). I don't hear any probs with pinging or knocking.

Anyway, my question is this: Even tho it's OK to run 85 octane up here at high altitude, will my engine have more power if I run 87?? I'd sure love to be able to squeeze out a few more HP...

Thank you!
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Old 12-01-2012, 05:05 AM   #2 (permalink)
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you would most likely get less power if you went to a higher octane fuel and waste your money.
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Old 12-01-2012, 06:06 AM   #3 (permalink)
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Unless it is pinging, knocking or pulling timing there is no gain in running a higher octane fuel.

At 8,500' elevation you are looking at roughly a 40hp loss compared to some one at sea level in a NA Forester. So about 130hp. There simply isn't a way to gain that back or prevent that loss in a naturally aspirated engine. It is a function of air density.

There is nothing wrong with letting these engines wind out in a lower gear when it down shifts. You can also simply force a downshift by double tapping the gas pedal.
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Old 12-01-2012, 06:33 AM   #4 (permalink)
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Thanks guys, very helpful.

I guess I will just have to live with the high revving when it downshifts. Really only an issue when going at highway speeds (55+) up steep inclines at very high altitude (> 8000 feet). I sure wish this thing had another gear inbetween 3 and 4, tho!!! (Or better yet, a six speed auto...)

Other than the 4-speed trans, and the screwy sized wheels/tires, I absolutely love the car. Will be interesting to see how it does this winter (this is my first winter in CO, and the first winter for the car).
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Old 12-01-2012, 06:40 AM   #5 (permalink)
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I have a '12 Forester X-Premium (non-turbo), and as you know the owners manual recommends 87 octane. I live at very high altitude (8500 feet), and almost never descend to lower altitudes
The manual doesn't "recommend" 87 AKI fuel. Instead, it states things like:
Quote:
Fuel requirements
Non-turbo models
The engine is designed to operate using
unleaded gasoline with an octane rating
of 87 AKI (90 RON) or higher.
(ref 2012 OM page 7-2)

and:
Quote:
Fuel:
Non-turbo models
Use only unleaded gasoline with an octane rating of 87 AKI
(90 RON) or higher.
{ref 2012 OM page "Gas Station Reference" (last page)}

Higher elevation often leads to lower AKI requirement due to the lower air pressure. However, the Forester' engine's ECM will advance the engine timing somewhat to compensate for the lower charge compression, so 87 AKI fuel may be warranted to help optimize ignition timing.

Our 2000 Ford F250's OM is rather specific about this particular issue. In short, it says the engine is designed to operate on 87 AKI fuel, and that Ford "does not recommend" using lower octane "regular" gas with octane ratings of 86 or lower sold at higher altitudes. I never use less than 87 AKI fuel in the truck -- even at high elevations in the Rockies -- and it actually does better in the summer on 89 AKI fuel at low elevations, especially when towing.

Using fuel of lower than 87 AKI in your 2012 Forester may place your warranties at risk, so I'd advise against it for that reason alone. And, the very best fuel economies I ever saw out of our '06 Honda 5MT was 39-40 mpg on 87-88 AKI fuel at elevation in the Rockies, and 34-36 mpg in our '09 Forester 5MT running on 87-88 AKI fuel in CO and WY.

HTH,
Jim / crewzer
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Old 12-01-2012, 06:42 AM   #6 (permalink)
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...screwy sized wheels/tires...
225/55R17's? Not a problem for winter wheel and tires.

And, while you're at it, you might want to think about a full-size spare.

Regards,
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Old 12-01-2012, 06:55 AM   #7 (permalink)
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Interesting, I may give 87 a try and see if it makes any perceptible diff in terms of fuel economy. Obviously, I don't want to do anything to affect the warranty. I wish there was some concrete position from Subaru WRT elevation -- even among Subi service techs there is disagreement re which octane is best for high elevation...

I've got 225/55HR17 tires. I say screwy, as 55 profile tires seem more suited to a sports car rather than a high-clearance vehicle like the Forester (that is relatively well suited to marginal road use). The short sidewalls don't give the tires much "give", and from what I hear, you need to be careful re potholes, etc. The unusual size, and the really tight wheel well clearance (WRT using tire cables, which I need to have) really limits your tire choices (if you want something more aggressive and tough for "off road" use...)
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Old 12-01-2012, 08:03 AM   #8 (permalink)
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I do believe that 16" wheels will clear the brakes on your car, so that is always a good option for snow tires. It just means you'll need a spare set of wheels. As for fuel, we switched to 85 from 87 when we moved out here from FL. We have had no issues at all with pinging, mileage, knocking, or anything else. Our fuel economy dropped a little out here, but that's to be expected. FL is dead flat, and at sea level. With more hills, and less oxygen, I expected a little less fuel economy. And to be honest, it's only been 1-2 mpg change. I saw a bigger drop from my off-road tires.
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Old 12-01-2012, 08:37 AM   #9 (permalink)
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I do believe that 16" wheels will clear the brakes on your car...
Yes, they do. The base Forester X wheels are 16", and I sue 16" wheels for my winter tires.

HTH,
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Old 12-01-2012, 08:47 AM   #10 (permalink)
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The unusual size, and the really tight wheel well clearance (WRT using tire cables, which I need to have) really limits your tire choices (if you want something more aggressive and tough for "off road" use...)
You might want to double check CO's chain law. It's possible that M&S tires or "mountain snowflake" tires (such as my winter tires) with sufficient tread depth would be OK without chains or cables.

HTH,
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Old 12-01-2012, 10:38 AM   #11 (permalink)
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Should you be interested Official Colorado Meets some like minded individuals may be closer than you think, that's all I got to say.
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Old 12-01-2012, 12:30 PM   #12 (permalink)
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Yea I know I can downsize to 16 inch rims, just not happy about spending the money (4x rims and 4x tires). I'm surprised that out of all the models of Forester, at least one isn't more focused on "off road", e.g. with 16 inch wheels and more off-roady tires, side scrape panels, etc..... They should leave the low profile 55 tires for the Touring model!!

Re the cables -- CO doesn't require them if you have AWD, but I want them for safety sake in case I ever get in a situation I can't get out of with AWD. I keep a set in the spare tire storage tray. If I had more aggressive tires, I might not be so worried about having the cables -- but at least in appearance the stock geolandars don't look like they'd be so awesome in snow....
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Old 12-01-2012, 08:07 PM   #13 (permalink)
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Default 91 Octane

I use premium 91 octane fuel when I take my 2006 Forester 5 speed into the mountains. Why? The higher octane means I don't have to run the engine at higher RPMs before I have to shift. With higher octane gasoline, the engine doesn't start PINGING before I have to shift to a lower gear. This is one time that the engine doesn't ping because of high octane gas, the timing remains advanced, and the engine produces more power. Lower octane gas will ping, the knock sensor will retard the timing, or I shift to a lower gear, and the engine produces less power.
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Old 12-02-2012, 04:12 AM   #14 (permalink)
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Well, I appreciate everyone's help, but confused I am!!! Depending on which post above I refer to, the best octane would be 85, or 87, or 91!!!!!

I have to say that I'm surprised that Subaru hasn't officially weighed in on this issue before. This is a very popular question, and obviously a lot of confusion out there -- and a huge percentage of Subi owners live in the mountains....

BTW, is there any way the dealer would know if you were using lower octane gas than specified??
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Old 12-02-2012, 06:24 AM   #15 (permalink)
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Quote:
Originally Posted by Easy Fixer View Post
I use premium 91 octane fuel when I take my 2006 Forester 5 speed into the mountains. Why? The higher octane means I don't have to run the engine at higher RPMs before I have to shift. With higher octane gasoline, the engine doesn't start PINGING before I have to shift to a lower gear. This is one time that the engine doesn't ping because of high octane gas, the timing remains advanced, and the engine produces more power. Lower octane gas will ping, the knock sensor will retard the timing, or I shift to a lower gear, and the engine produces less power.
Well, I am prepared to be enlightened here. Logic (not experience) tells me that using a higher octane at altitude would not be helpful in preventing ping (I assume you are referring to engine knock) at altitude in a normally aspirated engine. Higher altitude = lower density = less oxygen = less power. Lower density should result in lower internal cylinder temperatures (because there is less air to compress). Premature detonation (knock) results from high temperatures which come from high compression. I have even read that you can get away with a LOWER octane rating at altitude than is required at sea level.
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