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Has any 2020 XT owners experimented with 87 octane versus 93 octane. Notice possibly any performance gains or increase fuel economy.
 

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ZERO performance increase with running higher octane unless you get a tune for that higher octane. Cobb already did that analysis and posted it in the Ascent forum.
 

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ZERO performance increase with running higher octane unless you get a tune for that higher octane. Cobb already did that analysis and posted it in the Ascent forum.
Cobb's own website says:

Subaru truly calibrated the ECU to use an 87 octane fuel and testing with higher octane fuel on the factory calibration did not impact the overall power of the engine, though it did clean up occasional minor knock events.

So using 87 octane produced minor knock events that using a higher octane did not. Given that fuel quality may vary, my choice is to use higher than 87 just to avoid those occasional minor knock events. Subaru says 87 or higher so I'm just following guidelines.
 

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Discussion Starter #5
Cobb's own website says:




So using 87 octane produced minor knock events that using a higher octane did not. Given that fuel quality may vary, my choice is to use higher than 87 just to avoid those occasional minor knock events. Subaru says 87 or higher so I'm just following guidelines.
I agree with you. I traded into the onyx from a 17 STI so I was accustomed to putting 93 in the tank. Throttle seems more responsive with less lag imo.
 

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I agree with you. I traded into the onyx from a 17 STI so I was accustomed to putting 93 in the tank. Throttle seems more responsive with less lag imo.
I went from a 2011 WRX to the Onyx. I have a bluetooth ODB thing so I guess I could try logging for knock. I'm scheduled to get the ECU update in February so maybe after that I'll do some logging. It would be interesting if there were some knock events on 92 octane (the premium in my area) - if so, then it would demonstrate that despite Cobb's results in their area, in my area the 92 octane might be more appropriate. Ideally you eliminate all knock events, but ECU's adjust by trying to maximize timing while minimizing/eliminating knock, but they can only do that by increasing timing until a knock event is detected.

If Subaru's ECU is hard-limited in its ignition timing such that it will never encounter knock on 92, then the engine will simply run at the maximum timing advance all the time. It won't advance timing to take full advantage of the premium fuel, but neither will it be pulling timing.

Cobb's analysis is that even on 87 the Ascent isn't pulling any timing, though I'm wondering what the ECU did with those knock events - were they ignored?

I can say that when running premium fuel, I am not seeing significant fuel efficiency gains. There was some discussion that instead of pulling timing, knock events would cause the Ascent to richen the mixture instead. In either case, the only downside to premium is higher cost for minimal if any benefits.

Premium grades may also come with increased or enhanced additive packages - not sure how much benefit they provide, but there's that argument as well.

If it's a matter of pure logic and financial self-interest, we'd be driving the naturally aspirated 2.5. For me, the added power of the Onyx is a luxury I don't mind spending more for, including more frequent oil changes, higher octane fuel, or whatever old-school logic tells me I should do.
 

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Additional notes:

The 2.4T has a high 10.6:1 compression ratio.

Torque (android app) saw 14.7 pounds of boost spike using MAP method on my car in very limited testing. That amount of boost in a 10.6:1 compression engine doesn't sound like 87 octane to my old school mindset.

An Ascent owner reported much higher boost:

peak boost for ascent is 14.4 after doing some research. On stock air filter I saw boost spikes up to 16.8psi, using AEM intake filter I saw boost spikes up to 15.7psi. at least its better than the wrx. I had boost spikes all the way up to 23.1 and i think peak was suppose to be 15.5psi

So for me, using higher than required octane, even if I don't get a performance boost, gives me peace of mind that to me is relatively cheap in the scheme of things. Some people spend thousands of dollars on paint protection, aftermarket sounds systems, wheels, whatever. I spend it on fuel, filters and oil.
 

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So for me, using higher than required octane, even if I don't get a performance boost, gives me peace of mind that to me is relatively cheap in the scheme of things. Some people spend thousands of dollars on paint protection, aftermarket sounds systems, wheels, whatever. I spend it on fuel, filters and oil.
That’s my philosophy as well. I’ve tried to get my wife to consider at least running a tank of premium every now and theN in her Onyx but so far she’s refusing. That was the one thing she always complained about with her previous Outback , having to use premium. I think an occasional fill up with premium doesn’t cost that much and doesn’t hurt anything even if there’s no obvious benefit.
 

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Using 93 octane in an engine designed for 87 octane would seem to be a waste of money. But, people do it.
Exactly. But if they feel it's warranted and their wallet will support it, that's just fine. :)
 

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Using 93 octane in an engine designed for 87 octane would seem to be a waste of money. But, people do it.
This.

As for peace of mind, people using higher octane are actually setting up the engine for carbon build up. Stick with 87. People often claim 93 gets them more MPG, again that is all in their head.
 

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One other related aspect...there are folks who live in higher elevation areas where "regular" is only 85 octane. Subaru says you must use "87 octane or better" and not to use the 85 octane fuel. (Again, extensive discussion on this in the Ascent forum)
 

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That’s my philosophy as well. I’ve tried to get my wife to consider at least running a tank of premium every now and theN in her Onyx but so far she’s refusing. That was the one thing she always complained about with her previous Outback , having to use premium. I think an occasional fill up with premium doesn’t cost that much and doesn’t hurt anything even if there’s no obvious benefit.

Cognitive dissonance going on there. Using Premium in a car not designed to use it, is NOT good for the car in any way.
 

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Using Premium in a car not designed to use it, is NOT good for the car in any way.
I think that is not an accurate statement. Using higher octane "premium" fuel isn't going to hurt the engine. Some premium fuels do seem to have higher levels of helpful additives for engine cleaning, etc., although the real benefit may not be there, IMHO. Someone mentioned carbon buildup...maybe; maybe not. The one thing that is absolutely true that does hurt, however, is that higher octane fuel...costs more.
 

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I think that is not an accurate statement. Using higher octane "premium" fuel isn't going to hurt the engine. Some premium fuels do seem to have higher levels of helpful additives for engine cleaning, etc., although the real benefit may not be there, IMHO. Someone mentioned carbon buildup...maybe; maybe not. The one thing that is absolutely true that does hurt, however, is that higher octane fuel...costs more.

I noted that.

Go ahead and use 93, in time your car will have to use it, and your car will no longer run well on 87.. The higher octane gasoline in a car not designed to use it causes incomplete burn and lower mpg, hence the carbon build up. This will then increase the compression ratio, then you've self inflicted yourself requiring a higher octane to run w/o knocking. There is no difference in energy between 87 vs 93, so again wtf do people want to argue this.
 

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...so again wtf do people want to argue this.
We one of the most prevalent things I have seen running this site for now 16+ years is the propensity of having the same or similar questions asked time and time again. It is nothing to be bothered by, I just accept it and move on.

In the case of this thread and this oft asked question I would myself defer to those leaving it up to personal choice. It is your money that you are spending.
 

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I think I am going to make 89 octane my minimum octane for several reasons:

Cover the possible octane inconsistencies in fuel.
Hopefully cover the possibility of Low speed pre ignition.
Cover the possible solids being re introduced by the PCV system.

My Gen 1 Ecoboost F150 has a very poorly designed PCV system and as a result, I have had a dual stage catch can on it since almost new. In the Summer I empty very little actual fluid, but it's remarkable how much fluid I empty out of it in the Winter. It's a mixture of mostly condensation mixed with oil. It looks like coffee with cream in it. I have read that this mixture will indirectly lower the effective octane in the combustion chamber, in addition to gumming up the intake valves, etc.

Hopefully, there will soon be some good technical info on the PCV system on the new motors, other than the fact they might come apart and put pieces down into the engine.

In Western PA, we only have 87, 89, and 93, with the exception of Sunoco, which has 91 as well. Although being a Tier One fuel, Sunoco stations are kind of scare around here. As a result, I usually go to BP, our most common Tier One fuel.
 

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QUOTE. "Cobb's analysis is that even on 87 the Ascent isn't pulling any timing, though I'm wondering what the ECU did with those knock events - were they ignored?"

Yes I believe so. They events have to be more often before it starts pulling timing. I ve had a Cobb on my WRX and it s interesting to watch the handheld numbers but it will drive you crazy seeing all the knock events and then further seeing or not seeing pulled timing. It s crazy how involved it is.
IMO the XT Outback would have a great improvement in drivability , power and fuel mileage with a Cobb tune but that s with a higher octane being used. FYI There are other tuners that do the same.
 

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BTW as mentioned above, if you are running 87 octane and get a "bad batch" the ECU will just pull some timing to adjust so no engine damage occurs.When the fuel is acceptable again it will slowly add the timing back. So if that s the only reason for running higher octane you may want to re think.
 

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BTW as mentioned above, if you are running 87 octane and get a "bad batch" the ECU will just pull some timing to adjust so no engine damage occurs.When the fuel is acceptable again it will slowly add the timing back. So if that s the only reason for running higher octane you may want to re think.
I come from the land of WRX owners where "the ECU will pull timing and protect your engine" doesn't compute.

If we had a naturally aspirated 9:1 compression ratio engine, yeah premium makes no sense.

What we have here is a 10.6:1 compression ratio and turbo boost to at least 14.4 PSI. In old-school logic, the only way to run that is with low timing advance or very rich mixture or both when using low octane.

I'm not pressuring anyone to use premium gas - just stating my reason for using it on my car. This is my fifth turbocharged car, and third turbocharged subie. Certain ideas have been ingrained in my mind from experience. It's difficult for me to wrap my mind around purposely choosing a lower octane fuel in a high compression turbocharged engine. The manual says 87 or higher. I'm doing what the manual says.
 
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