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This thread has been going on long enough that I'd like to comment on some of the posts. As to anecdotal reports like "my car just runs better on premium", to those I just have to say that it's all in your head. You may've gotten a tank of 'bad gas' that was adulterated by condensation, but the octane rating had nothing to do with that.

As to the guy who says his car "runs cleaner"; What? You took it for an emissions test and quantified that? Really???? Octane ratings do NOTHING for emissions. Nothing at all.

Premium has more power? No, it does not. Premium resists detonation longer, that's all. It actually contains slightly LESS 'power' per gallon than regular does. All premium does is allows higher-compression motors to extract more 'power' out of fuel without destroying the piston crowns with excessive per-detonation.

Ethanol-free gas: Older engines often have old seals and gaskets not designed to deal with the corrosive aspects of ethanol. Small gas appliances as well. Gasoline and air mix at about 18:1 ratio, ethanol and air at about a 15:1 ratio, so, the less ethanol in your fuel, the better fuel economy. Expect about a 2-3% improvement; not enough to make the $.50/gallon (or worse) extra cost be worth it. Also, many places with ethanol-free gas only offer it in premium grade. Others offer it as a 90 octane mid-grade. I buy a couple gallons of that each year for my lawnmower (also reduced 'vapor lock', but that's a problem with the air-cooled lawnmower motor that my car doesn't have).

In short, if you still believe that putting premium into your unmodified Impreza, then you are a sucker. Instead of wasting an extra $5 on each fill-up, just throw it out the window or flush it down the toilet; you'll get zero benefit from your money either way. Save the premium gas for your high-compression sports cars, your old muscle cars, and the like. They need it, you do not.
 

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ModFather
2019 Subaru Impreza Sport 5 Dr. Manual Shift
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Personally I use the fuel they say in the owners manual I've never had an issue. The only time I've used anything different is when I had a cobb accesport on my Forester XT and I could set it to use 100 or 110 octane I got at the drag strip.

This is worth a watch.

 

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A;so, I'd like to mention that the ECU is 'mapped' to work properly only on 87 octane. Cars which take premium are 'mapped' to work with the higher octane fuel. I once had a 1986 Omni Turbo with a less-than-legal ECU that was mapped for something like 97 octane, and I ended up having to mix 94 octane Sunoco gas 50%/50% with 100 octane racing fuel. But then, this car's ECU had been modified to produce considerably more turbo boost than stock (and was almost certainly not street-legal). I believe the previous owner had ran it on aviaton gas. I ended up putting the catalytic converter he'd removed back on the car, which actually made it run better.
 

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ModFather
2019 Subaru Impreza Sport 5 Dr. Manual Shift
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Our 2006 Tribeca, 3.0 H6 had premium fuel as "Recommended" not required you were able to run regular fuel. The ECU would adjust accordingly. The owners manual said for best performance and MPG's use premium. Performance wise honestly it was hard to tell any difference. It was not a light vehicle. We took a 14 hr drive loaded down with kids and gear. Going I used premium and returning I used regular. The amount of MPG increase vs the cost of the fuel s pretty much a wash. It's the only Subaru I've owned that had a recommended statement. Later they took that motor, made it a 3.6 and mapped it strictly for regular fuel. That was our Legacy 3.6R
 

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2019 Impreza Premium Hatch CVT
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I use 85 octane. I have for the last ~20 years that I’ve lived at high elevation. No pinging or knocking on any of my vehicles totaling a combined 400,000 miles on them. I typically run a car to about 200k before selling them.

The Impreza is averaging just under 34 mpg.
 

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Around 1999, Toyota made a 3.0l V-6, that it put in both the Camry and the Lexus GS. The Camry was recommended to use regular, the Lexus premium. The Lexus got 20hp better than the Camry, on the identical same motor; only the ECU was different. A lot of Camry owners started looking for Lexus ECU's at junkyards. I'm still not certain if an extra 40 cents a gallon was worth it just for an extra 20 hp (and on a Camry, yet!). I know that fuel economy was also just a bit worse with the switch.
 

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I run premium 91 default. 2 reasons mainly...
1. No ethanol in premium (tested)
2. Our engines are allegedly 12.5:1 compression ratio.... Any other car would ping on 87 octane... Why not ours?

I also run a few MLS of injector cleaner each tank (Lucas) and iv had ZERO fueling issues so far {50 k knock on wood)

I figure with our fancy direct injectors being so pesky a bit of care towards longevity can't hurt... Fingers crossed
 

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I run premium 91 default. 2 reasons mainly...
1. No ethanol in premium (tested)
2. Our engines are allegedly 12.5:1 compression ratio.... Any other car would ping on 87 octane... Why not ours?

I also run a few MLS of injector cleaner each tank (Lucas) and iv had ZERO fueling issues so far {50 k knock on wood)

I figure with our fancy direct injectors being so pesky a bit of care towards longevity can't hurt... Fingers crossed
Oh, man, it's not 1982 any longer! Actually, today a 12.5:1 compression is fairly common, yet almost none of then require or even recommend higher octane fuel. With a well-designed combustion chamber, you can easily run 14:1 without pinging. And, besides, cars today don't ping, because of ping sensors that will automatically retard the timing. Every_single_car made in at least the last 25 years does this.

But, your ethanol-free premium will probably give you about a 3% gain on fuel economy, but at what cost? Around me, ethanol-free fuels cost about 25% more, so, your choice of fuel is probably costing you about $5 more each fillup.
 

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I appreciate the insight on current compression ratio expectations.

Locally there's a tax free area so the addional cost of premium isn't there so I don't see why nol anyway, it's literally the same price as regular just down the rd :)
 

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2019 Subaru Impreza 2.0 Premium 5-Door Crystal White Pearl
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There's a reason I'm curious here. The Subaru factory states that the engine will run 87. But with a 12.5:1 compression ratio, that seems a bit high for 87. Engine does feel a bit smoother on the higher octanes. Just an observation.
 

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ModFather
2019 Subaru Impreza Sport 5 Dr. Manual Shift
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I like Scotty :)

 

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...... But with a 12.5:1 compression ratio, that seems a bit high for 87. ....
Once again, that's only because your mind is still stuck in the 1980's. Mazda manages to use 87 octane with a 14:1 compression.....

Seriously, instead of wasting the extra $5 on premium each tank, just give it to the first homeless guy begging at a traffic light. It will do more good there than in the oil company's pockets.
 

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2021 Impreza Sport
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Pre-ignition from high compression should not be an issue when the fuel is not in the air entering the cylinder, e.g. direct injection. The fuel spray actually cools the air somewhat, and there is no time for ignition to occur before the spark.
 

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Anecdotally, I had some pinging issues and switching to premium has eliminated them.

Ymmv and all that. ‘18 Base 5-speed.


Sent from my iPhone using Tapatalk Pro
 

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2019 Impreza Premium Hatch CVT
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Anecdotally, I had some pinging issues and switching to premium has eliminated them.

Ymmv and all that. ‘18 Base 5-speed.
What octane did you use before/after and what elevation do you live at?
 

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Since it's been discussed in the "Anyone do any Mods yet?" thread, I was curious to see what type of gas you use in your Impreza?
Although the manual is asking for regular, I tried upgrading to plus. It could be in my head but the power behind it, and smoothness of the entire ride feels better.
 

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OK, need more. NA or FI? Pulling hard in high temps, like the hill to Needles in July?
NA, bone stock. 55°f mornings, car is at operating temps, and something as simple as pulling out of parking lots or onto the freeway at around 50-70% throttle on level ground would induce the sounds. I’m at sea level in Western Washington.

Although the manual is asking for regular, I tried upgrading to plus. It could be in my head but the power behind it, and smoothness of the entire ride feels better.
I told myself if I didn’t see an improvement over the week I would take it to Subaru and ask what’s going on, but day 4 she felt butter smooth. No pinging/rattle/knock and she feels more responsive. It’s not night and day, no, but it’s noticeable enough that I’m having more fun just doing day to day tasks. It’s been about three weeks since I switched now.

Sent from my iPhone using Tapatalk Pro
 

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NA, bone stock. 55°f mornings, car is at operating temps, and something as simple as pulling out of parking lots or onto the freeway at around 50-70% throttle on level ground would induce the sounds. I’m at sea level in Western Washington.



I told myself if I didn’t see an improvement over the week I would take it to Subaru and ask what’s going on, but day 4 she felt butter smooth. No pinging/rattle/knock and she feels more responsive. It’s not night and day, no, but it’s noticeable enough that I’m having more fun just doing day to day tasks. It’s been about three weeks since I switched now.

Sent from my iPhone using Tapatalk Pro
Yup. Super smooth once the regular gas was cycled out. I won’t go back unless gas prices go up too much.
 
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