Octane question for those at high elevations

[SmashedGlass]

I just read an older thread and stumbled on to this:

 

"Another thing to consider about CO is that your car will run differently with the thinner air. You'll notice a little less power, naturally, as the ecu compensates. SOA suggests that you run 87 octane or higher gas above 5000 feet elevation. I usually have a blend of regular and plus in my tank since we're cruising around at 6700+ and rarely go lower in Durango. It seems to make the car more happy - i.e. run more clean and with more power." (member: Kanurys)

 

I was wondering this myself as I drove my new acquisition [spfi ea82] home over the weekend, as I had just put $10 of 85 in to get her home. Would running 87 octane really be beneficial? I've never understood why Colorado has 'regular' gas at two points below the rest of the country. I'm at just a hair above 6000' most of the time, and when I get up into the mountains I'd be above 8-9000'.

[Subarule]

I bought my '86 GL/5sp/4WD/non-FB carbed wagon in Denver brand new off the lot. It came with a high altltitude package on it. I lived at the time in a city that was another 1500+ feet higher than Denver. The sales guy, who had been at the dealership forever, said to NOT use higher-octane gas in it. It was the biggest Subaru dealership in CO so I believed he had to know what he was talking about, since they sold Subarus to people who lived at much higher elvations in CO. That the car did not need it & it would be a waste of money.

 

In '88 I moved to Portland OR (sea level) and didn't change anything as far as gas or my high-altitude pkg. went. No problems at all.

 

Keep in mind we are talking about 4-cylinder engines. Higher octane would not be any better for your car.

 

However, running Marvel Mystery oil through the gas tank all the time will be a plus.

[el_freddo]

I'm at about 5000 feet (~1800m) above sea level and have not noticed a change in the performance of the EJ22. The ECU should effectively reduce the amount of fuel used as the oxygen content reduces - it's can't be that much as I've not noticed anything by my "seat of the pants" dyno.

 

Cheers

 

Bennie

[Subarule]

I don't know about the original poster but my car does not have an ECU and I have used the lwer octane gas for 25 years.

[Dj7291993]

I think you are under a misconception of what "octane" is and means. Octane is the fuel's ability to resist detonation or pre-ignition. It is often needed in turbo cars, because the pressures caused by more air in the combustion chamber can create more heat, raising the probability of auto-ignition before the piston reaches the point where the spark is fired. In Colorado, because of the altitude, the air is thinner, which reduces the needed resistance to detonation, hence, a lower octane rating at each level (reg, mid, and premium). Higher octane fuel doesn't necessarily mean it burns more efficiently, it is used in performance engines to control when it burns.

 

In short, you gonna loose power because you have less air, meaning less oxygen. Changing your fuel grade won't change how much oxygen your engine has access to.

 

To others who have a better understanding of fuels, feel free to correct me if I've made any errors.

Edited August 9, 2011 by Dj7291993
Better wording

[Subarule]

 

I think you are under a misconception of what "octane" is and means. Octane is the fuel's ability to resist detonation or pre-ignition. It is often needed in turbo cars, because the pressures caused by more air in the combustion chamber can create more heat, raising the probability of auto-ignition before the piston reaches the point where the spark is fired. In Colorado, because of the altitude, the air is thinner, which reduces the needed resistance to detonation, hence, a lower octane rating at each level (reg, mid, and premium). Higher octane fuel doesn't necessarily mean it burns more efficiently, it is used in performance engines to control when it burns.

 

In short, you gonna loose power because you have less air, meaning less oxygen. Changing your fuel grade won't change how much oxygen your engine has access to.

 

To others who have a better understanding of fuels, feel free to correct me if I've made any errors.

 

 

I'm not sure which post you were addressing in your reply. When I was speaking of "octane" I meant what the labels say on the the gas pumps, to differentiate one level (and price) of fuel from another. Would "grade" be a better term?

[skishop69]

Grade and octane are the same. Each step up in grade is a step up in octane. That is all the grade rating is for. Changing your octane for altitude change does nothing UNLESS... You are advancing your timing by more than 4 degrees to compensate for the lost power due to 'less' oxygen. Then it would make a difference to reduce the ping from the advanced timing and the combination of both would yield an increase in power. Nothing major, but noticeable....

[SmashedGlass]

Lol, the 'original poster' here. So what I'm gathering is NO.....unless I make some adjustments such as advancing my timing (which I did on my old 200SX twin-spark piece of sh...beautiful classic ) and perhaps put a blaster coil in. I'll just keep running the cheapo stuff until then. Thanks.

[LPGsuperchargedBrumby]

the higher you go the thinner the air is....the thinner the air is the less of it there is to compress in you cylinders which lowers your effective compression ratio meaning you can run lower octane fuel without risk of pinging...lower octane= more BTU's per gallon which(hopefully) partly offsets the loss in compression

 

in the 70's mum and dad toured on a Moto Guzzi bike thru the himalaya's...as the altitude increased the octane of the fuel available decreased untill when they fueled up on the approach to the highest pass they rode across(18000ft) the fuel was marked as "60 octane benzine motor spirit"

[zyewdall]

The higher elevation you go, the less octane you need to prevent detonation. That's why Colorado's gas is at least two points below anywhere else.

 

I usually use the mid grade in the older subarus... seems to keep them from pinging on hills better. Maybe it has something to do with carbon buildup raising the compression from stock, or ??? But, they seem to like it better than the low grade stuff. Also... it's not just the octane rating that's different... the better grades have more additives to clean the fuel system usually. I find that in the company truck, which I usually use 85 in, it'll run a bit better after running a tank of 87 or 89 through it, even when it's back on 85 again. You could also add your own fuel system cleaner to the tank too...

 

Z

[cmill189]

I run 85 in my EA82 without issue; whether in the valley floor (~4000ft) or the highest I've driven in the mountains (~7000ft.)

[zyewdall]

I run 85 in my EA82 without issue; whether in the valley floor (~4000ft) or the highest I've driven in the mountains (~7000ft.)

 

If you want to experience "slow", drive a stock EA81 4wd over a 12,200 foot pass Using different gas won't make any difference

[coolskaterkid]

no kidding i drive around 6k-10k elevation in my ea81 all the time really gotta spin the poor thing to keep up with the semi trucks

[zyewdall]

For some reason, the 2wd EA81's seem a lot peppier up here -- I've only had one of them, and it felt distinctly faster than all the 4wd ones I've owned.

 

I do have a one vehicle that makes the EA81 seem fast... a 1976 ford courier 4wd... especially loaded with 1500lbs of wood or rocks..

 

Z

[subynut]

Let's take this from the other side of the coin.

I live in the high desert of southeastern Arizona (~5000 ft elevation), when I take a trip down to Tucson (~2400 ft elevation) in the middle of the summer heat running 87, my EA82, ER27, and the EJ25 pinged quite a bit more there than they do at home. Even though the air temps are the same between the two locations; with more available air, they are more prone to ping. When I go to Phoenix, the pinging is more pronounced, since I'm now at roughly 1100 ft. Assuming I don't switch to a higher grade fuel.

When I had my XT6, I could get away with running 87 at home with maybe a hint of ping when it was near 100 degrees outside. However, when I drove to WCSS10, I had to switch to premium in order to keep the pinging to a minimum while I drove though the Nevada desert. Now, part of that is because the desert floor temps were in the 120s, but also because I was near sea level in some of the areas.

I will most likely run mid-grade in the PandaWagon when I go to this years WCSS just to keep the engine happy.

 

So, yes, the higher in elevation you go, the lower the octane rating you can run since there is less air available. I would not apply this theory to turbocharged motors.