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My 2019 Forester manual says to use 87 Octane fuel, but in Colorado and other high elevation states they sell 85 Octane gas as regular gas. 87 is Mid-grade and 91 is Premium. I'm in Denver.
If I use 85 Octane which is about $0.20/gal cheaper than 87 Octane I can't tell a difference in performance.

Any known reason to not use the 85 Octane gas?
 

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Any known reason to not use the 85 Octane gas?
On road trips (Moab) I've seen and used this 85 octane. All I noticed was a decrease in MPG but honestly did not calculate the cost difference. On a high compression engine, you might also notice premature ignition. Try it. If you have a noisy experience, step up to 87. Or, bump up to it if the mileage gain is worth the extra coin.
 

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2018 Forester XT Limited CVT
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There's a page in the owner's manual that answers your question. Here it is for the 2018 XT, but look in your owner's manual for your 2019.
 

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The octane number is a measure of the gasoline fuel's ability to resist pre-ignition or detonation in the combustion chamber. The higher the compression ratio the greater the need for higher octane. Turbocharged gasoline engines often have lower compression ratios than the normally aspirated versions. In simple terms, the reason for this is because the Turbocharger, or any means of forced induction, effectively raises the compression ratio, hence the need for even higher octane gasoline.

While you may not notice a difference in performance, this is most like due to your particular driving style combined with the simple fact that modern engines of the last two decades and more have incorporated "knock sensors" that adjust engine parameters, depending on the overall engine design and engine systems, to reduce and then prevent detonation in reaction times measured in milliseconds.

The AKI (R+M/2) octane rating of 87 is the MINIMUM octane fuel Subaru recommends (this is true of virtually every other manufacture) and is followed by the phrase written in bold print "or higher". If you find anything that says, if using a gasoline with a octane lower than recommend may be used you can expect lower engine performance, they are telling you that the engine in question has a limited ability to sense inferior lower octane fuels and will, therefore, reduce engine performance to minimize the chance of detention or pre-ignition "knock".


For reasons concerning fuel quality, changing fuel formulations, unknown compression ratios in rebuilt engines, and other automotive factors at the time, plus my college automotive education and training during the same period of time, I have not used anything but premium grade gasoline with a AKI of 91 or better in my gasoline powered cars since the late 1970s early 1980s. Until I moved to New York a few years ago, from my native Bay Area and Northern California, the brand of that gasoline I used was almost exclusively Chevron. In New York, I use Shell's 93 octane gasoline as that is their premium grade and because Chevron is unavailable.


I have never understood any reasoning behind trying to ignore the information provided in an owner's manual, especially pertaining to items like what octane fuel to use. Especially, given the cost of purchasing a new car. $0.20USD is a pittance given what the price of any cup of coffee on the go costs. Engine damage caused by cutting corners on fuel to save penny's per gallon is not.


For years I told my customers that they did not buy a car (new or used), they bought an Owner's Manual that comes with a free car.
 

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Gasoline refined for the Colorado Plateau, and other high altitude areas, are lower in octane by a couple points. EPA mandates numerous formulations for different US areas. Formulations that change with the seasons. It is what it is.

As I hit the plateau I see that regular is 85 octane and premium is 90. I've been continent crossing for decades and never had any issues with pre-ignition using gasoline from regular pumps, or the lower octane premium in my motorcycles requiring high octane, ever. Understand that all modern auto engines have knock sensors that adjust the timing. My BMW motorcycle engines never have a knock sensor, and the Colorado Plateau lowered octane never cause pre-ignition knock, ever.

At the speeds we ride, any sort of low octane pre-ignition knock, with bikes that don't have knock sensors, and are cutting edge performers, we would know if the lowered altitude octane was an issue.



Don't worry about it as your car will be fine.

As I come off the plateau, and the gasoline comes up from lower altitudes, the regular pumps offer 87 again.

 

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If you only purchase Top Tier gasoline, with 5X detergent, carbon buildup inside the combustion area might not be an issue. It is these tiny bits of carbon that hold the heat and can ignite the fuel/air before the spark plug does its thing, pre-ignition, knocking.

https://toptiergas.com/

I add carbon cleaning gasoline additives from time to time to keep any buildup within the combustion area under control. In the remote rural areas I frequent, top tier gas is but a dream.

You may have noticed that the new BMW boxer motorcycle engine, the highest performance version of this engine ever, uses regular gas.
 

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The late Gene Berg, who founded the Volkwagen Air-Cooled Engine tuning and engine build company in the greater metro Los Angeles, California area, that bears his name and still exists today after six decades, spent much of his career debunking common myths that were accepted throughout the VW aftermarket industry and among VW enthusiasts with well documented research and testing, especially on the race track.

One myth he helped lay to rest late in his life was the argument that "high performance" street engines needed to be high compression engines. His argument at the time, well before the era of "Laptop Tuning", in the early 1980s was that the quality of gasoline at the gas stations was much lower than during the muscle car heydays (with or without lead) and that when building a new stock or high performance VW engine of the quality his company is well known for, then one should adjust the engine's compression ratio downward from, in many cases what VW originally designed engines to have, to match the fuels available. As I recall, he proved that by adopting 7.0-7.5:1 compression ratios, that were lower than an otherwise identical engines with high compression ratios, air-cooled VW Type-1 and Type-3 based engines ran cooler and preformed as well, or better, than their higher compression ratio counterparts simply because the engine compression ratios were matched to the fuel available.

Today, vehicles are designed to adjust to reduced performance when their end users fuel them with gasoline that meets the manufactures' minimum or lower than minimum octane requirements. It's that simple.

To fit into the American market's demand for low mileage vehicles to run on the cheapest gasoline available, Volkswagen states on the fuel filler door that I can use 87 AKI (R+M/2) Regular gasoline in my 2018 VW Alltrack's Turbocharged 1.8TFSI motor. It says the same thing in the owners manual. It also states in the owner's manual that if I want my engine to perform at its maximum rated performance than I should use a gasoline with a octane rating higher than the recommended minimum.

My 2010 Subaru Forester's Owners Manual says pretty much the same thing.

American gasoline quality verses required octane challenge is one of the reasons, among many, that I much prefer passenger car diesel engines. But that is another conversation for another day.


https://www.bmwmoa.org/news/438324/Understanding-octane--AKI-MON-and-RON-oh-my.htm
 

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I completely agree that when demanding the highest performance from an engine, allowing the timing to advance with the use of higher octane gasoline makes sense. Few of us are able to demand that level of performance in our daily driving. Stop & Go traffic tends to warrant the safety features of Subaru instead.

The difficult part of switching between octane levels is that the engine management computer takes numerous restarts to advance the timing to take advantage of the premium fuel. That is without disconnecting the battery to force a reset.

My motorcycles always get the highest octane. My autos, even the ones labeled as requiring premium, get regular. All because 95% of auto driving is mundane and nearly all motorcycling is performance oriented. That two wheeled flash you have passing you may be me. Laser jammers and radar detectors aid my two wheeling adventures.

We all have choices when purchasing consumables for our Subarus. A significant benefit in this consumer economy. May we each find pleasure in the manner we choose to spend our income.
 

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Right now regular is selling for $2.43 and premium $2.83, roughly a 15% difference.

Will I get 15% more performance with that added expense?
Will my stop & go traffic experience be better with that added expense?

Nope.

That’s why I let the sensors adjust the engine management to run well on regular gasoline. Then, I add additives every 5 refills to make sure the injector and combustion area stay as clean as I can keep it. Purchases of 5X detergent gasoline from Costco just aren’t convenient for most refills.
 

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@RawLand,

To date the fastest I have been clocked driving in the US is 92 mph, by a very friendly Nevada Highway Patrol Officer from NJ in fully loaded with cargo 2012 VW Golf TDI. We averaged 38 MPG on that cross country trip by the way. So you may have to work at it on the interstate..... Driving is never mundane, though I do drive as if I am on the Autobahn where abiding by the rules of the road and common courtesy are not an option.

Here in my adopted state of New York, as with my homeland of Northern California, ULSD tends to compete with 87 AKI to be the lowest priced fuel at the pump on any given day. The stability of fuel pricing in Europe is refreshing by contrast to the three card monte marketplace here.

This is not an issue of high performance, but standard performance and the false economy of striving to "trip over dimes to pick up pennies". Nor is it about training the ECU to accept premium in fuel systems that are closed loop and cycling the fuel back continuously to the tank to help keep it cool as long as the engine is running. I have a pretty good ideal what my former automotive department college professors would say about all this, but, sadly, most of them have passed on or long retired to ask.

We'll save the discussion about the mythology of "consumer choice" for another day. Enjoy your bikes!:thumbsup:
 

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A group of us on identical straight 6 motorcycles, same production period, same color too, no anti-knock sensor, rode several multi-thousand mile loops together in different western states. Monitoring gas mileage and performance is easy when range and passing ability really mattered. We found that a few points of octane didn't matter as much as local blending of gasoline. Our best performance and range are consistently in Northern California. Even more so when we can use Chevron gasoline.

Modern auto engines have anti-knock sensors which adjust the timing, which makes it harder to determine whether a couple octane points really affect performance. In addition, the weight of the vehicle and isolation from engine sounds further remove the driver from what is occurring.

So, my two wheel experience with all sorts of gasoline over hundreds of thousands of miles of spirited riding influences my experience.

Subaru recommends regular gas. As I cross the Colorado Plateau a couple times, towing a trailer, with my still new 2019 Forester, in July, I will see how the 85 octane regular affects performance and mileage.

Maybe I will find that it is indeed prudent to setup up to mid-grade to keep the 87 octane. Maybe not. Mid-grade and premium sits in the fuel tanks a lot longer because most purchasers choose regular gas.
 

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IMHO, so long as you are meeting the manufacturer's minimum octane requirement, the most benefit will come from using Top Tier Gasoline:

[url]https://www.consumerreports.org/car-maintenance/study-shows-top-tier-gasoline-worth-extra-price/[/URL]
Top Tier Gas, with 5X EPA minimum required detergent added, is indeed a good option, when available.

Sadly, when crossing the continent it isn't an option to make that purchase option. In fact, I tend to look for the freshest gasoline by selecting the busiest gas stations, because I have to assume that is my best purchase option. 90% of the time, the busiest gas stations are truck stops with EPA minimum gasoline. So, every 5 refills I add some additive to the gas tank.
 

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The octane is lower because of high altitude gives you lower air density, effectively lowers compression ratio in your NA engine, thus 85 in CO will resist knock same as 87 on sea level.

85 will be fine, just don't fill up before you leave.

Turbo engine on the other hand resist changing air density much better and probably should use the same octane regardless.
 

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Turbo engine on the other hand resist changing air density much better and probably should use the same octane regardless.
Excellent point.

I wonder if the boost is the same at altitude?

Found the answer:

stock STi):
-at 6000ft 25.9 / 29.2 = 88% so you have 12% less pressure going into the engine
-at 14,380ft 22.5 / 29.2 = 77% so you have 23% less pressure going into the engine

NOW same comparison for a NA car in terms of how much % less air:
-at 6000ft 11.4 / 14.7 = 77% so you have 23% less pressure going into the engine.
-at 14,380ft 8 / 14.7 = 54% so you have 46% less pressure going into the engine.
 

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When complaining about the higher price of higher-octane gas:

How do you get more engine performance?

- use a higher-performing engine and higher-octane gas, more expensive
- use a bigger engine and more gas, more expensive
- use a more-economical engine and put your foot down harder, use more gas, more expensive

I think if you bought the XT, put the right gas in. If you want to put lower-octane gas in, you could have gotten away with the normally-aspirated engine. Or, you could have gotten a vehicle with a bigger engine. If you want to use the XT and lower-octane gas, you may have to live with compromises.

If "regular" gas is 85 octane in some areas, I'd use it (if I had a car that used 87). I wouldn't use a lower-octane gas than recommended by the manufacturer, because there's no telling when I might need to tow a trailer or accelerate quickly. And I wouldn't use a higher-octane gas than recommended, because that would be added expense without reason.
 

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I'm also in Denver and believe 85 shouldn't be run in these (2019+) engines with the higher compression.

I've ran both and notice a calculated MPG and a noticeable driving difference between the two. In fact I filled up with high test this weekend and got over 38 MPG on a 100 mile interstate trip with some stop and go sections and AC on. That is way over what I've seen.

I don't know if 85 will hurt the engine over the long term, but it does affect performance at Denver elevation. Don't think I'll pay for high test all the time, but 87 will be my minimum (as the owners manual directs).

Again the new GDI has higher compression. I think it is more susceptible to knocking with 85 even at elevation compared to older, lower compression engines.
 

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1,200 Colorado Plateau miles with 2019 pulling a trailer with motorcycle using 85 octane without pinging. Additional 600 miles with 87 octane and no pinging.
 
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