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Old 01-28-2011, 11:18 PM   #1 (permalink)
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Default Gas question high octane vs. regular

When I took my 01 Subaru to the mechanic, a specialist, he said that I could only put high octane (premium) gas in it, is this true? I just bought it, have not put any gas in it, but the dealer that I bought it from said that's all he put in it. My grandmother has a corvette, which also requires high octane, but she puts regular in it. So, a little help please? Thus far, I have not seen one thing about the gas, am I just that new of a Subaru owner and this is just something everyone knows, or was the mechanic pulling my chain?
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Old 01-28-2011, 11:21 PM   #2 (permalink)
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From your other thread I see you have the H6, so premium is suggested by Subaru for that engine. The car will run as designed on premium. Not everyone does this, but it is what I would suggest as well.
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Old 01-28-2011, 11:36 PM   #3 (permalink)
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Yea the higher octane is going to run cleaner and nicer than the lower octanes... but ocassionally i'll put low octane in if I need a bit more bang for the buck.... BTW, in NZ we have 98 (highest) , 96 and 91 (lowest)... I run my H6 2001 on 96 which I take is good enough? It is 'premium' gas but not the top of the range 98?
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Old 01-28-2011, 11:48 PM   #4 (permalink)
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I thought that was the case, the way this guy put it, it was like my car was not going to start if I put regular in it. Thanx! I plan on making this car last, so if I can afford it, I will put premium in it more times than not, but it's nice to know that if I am in a pinch, I can use regular.
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Old 01-29-2011, 02:31 AM   #5 (permalink)
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The basics of it. Your engine develops hot spots under load that runs the risk of preignition of the fuel when using lower octane fuel. So if your smashing up the mountain with a decent load you want the higher octane fuel. Putting around town you can probably get away with lower octane fuel. The hot spots cause the lower octane fuel to combust before the correct point in the cylinder stroke. The new 3.6 runs on regular low octane because subaru corrected the cooling and hot spot issue in the design. So your car will start and run on lower octane fuel but as you raise the load on the engine you increase the pre ignition risk with the lower octane fuel.
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Old 01-29-2011, 02:37 AM   #6 (permalink)
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When the the 3.0 h6 detects pre ignition which can be called a knock or ping the sensors pick it up and reduce the power through timing and fuel management to reduce the chances of pre ignition which can cause damage to the cylinder given the fuel ignition must happen at a specific time or it can cause an imbalance between cylinders Loss of power and damage to various engine components
Higher octane gas is the same base fuel with various additives that lower the burn point of the fuel making it more stable and less sensitive to hot spots allowing the engine to fire the cylinder when the spark is delivered vs the fuel firing off due to pressure and heat say mid stroke.
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Old 01-29-2011, 09:35 AM   #7 (permalink)
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I have used both 87 (our low end octane) and 93 ( our high end octane) in my OB,

I have checked milage in town only, and highway only, and both... The cost

difference was aprox. .60 cents on average between the two per tank of gas...

That being said, I was not saving anything by using cheaper gas, I was only spending

less at the time I filled up... and I would rather use a better grade of gas that

Might...and I repeat Might make my engine last longer....
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Old 02-01-2011, 10:08 PM   #8 (permalink)
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So for any year you recommend the higher octane gas?
I always use Chevron or Shell, but I put the lower octane in just because I thought it truly didn't make a difference. Does everyone here think the higher octane will help the engine last longer?
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Old 02-01-2011, 10:46 PM   #9 (permalink)
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Quote:
Originally Posted by socksfrommars View Post
So for any year you recommend the higher octane gas?
I always use Chevron or Shell, but I put the lower octane in just because I thought it truly didn't make a difference. Does everyone here think the higher octane will help the engine last longer?
For the 3.0L H6, Premium is strongly recommended. The knock sensor should compensate for lower-grade gas, but you're safest just using Premium. It's required for turbos. For the basic 2.5L, using anything but 87 octane is throwing money away. The 3.8L H6 takes regular as well, not Premium.
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Old 02-02-2011, 12:30 AM   #10 (permalink)
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Quote:
Originally Posted by subiesailor View Post
.... So if your smashing up the mountain with a decent load you want the higher octane fuel. Putting around town you can probably get away with lower octane fuel. .

the opposite is true.... as you go higher altitude, the air is thinner and therefore you don't need higher octane (or higher AKI) gas..... This is the reason they sell 85 octane gas (instead of 87)in the rockies but not in the flatlands(pretty much rest of the country).

Think of octane (or Anti Knock Index) as resistance to pre-ignite index....we are talking abt gas to air(oxygen) compression ratio here... Your engine are build run a certain compression ratio (i.e 9:1)...the octance (or AKI) in the gas state it will NOT pre-ignite(or spontaneously combust) before a certain gasoline-air(O2) ratio.... (ie 9.5:1)
(look up how stuff work website for details)

to the OP, look up what your car engine compression ratio. then look up the compresssion (AKI)ratio of Premium and regular gas are.... as long as the engine compression ratio is lower than that gas number, you are OK using that gas....

Without looking at it(the AKI numbers), if your manual state it need premium gas(91+ octane) gas, you will need premium. Yes, you may get by sometimes but you're taking a major risk.... Pinging (Pre-mature combustion when you piston is still in compression mode) is NOT a pretty thing for your engine.....(again, read howstuffwork website to get a pictorial explanation).....
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