Prem fuel? and question of octane rating at altitude

[WWV]

Ok, still lurking and will someday own one of these beasts but for the 1st time while looking at bike at dealer, I noticed a sticker on tank that said 91 Oct/Prem fuel only. I have Yami boat(newer) and part of their claim to fame is to NOT run on Prem fuel. So it through me that this bike did. More of a statement I guess, no real question on that part I guess.

Then the salesman made a statement when I expressed my surprise at Prem for this bike. He said that at altitude(Utah, won't say which dealer ;-)), that you could run lower octane fuel with no ill effects. Never heard this one before. Anyone confirm or deny this? Add their two cents......


WWV

 

[WWV]

Hmm, quick searches for the altitude question found this on Pop Science website. Learn something new everyday:

Q:
I live in Arizona and use 87-octane regular. In Utah, Idaho and Nevada, stations were selling 85-octane as regular gas. This forced me to pay more for midgrade 87-octane. Is this the latest petroleum-industry scam to get more of our money? Will my car run okay on this bogus 85-octane regular?

A:
Octane is the ability of a fuel to resist knock, and high-compression engines tend to knock more. The obverse of that is that lower-compression engines can run on lower-octane gas. Air is thinner the higher above sea level you go. Less air going into the cylinders means less pressure at top dead center when things go bang. It's a lot like lowering the compression ratio in the engine, reducing the need for high octane. Cars will run just fine on lower-octane fuel when they're well above sea level--and all of those states are. Hopefully, by the time you get back down to denser air, you've burned off most of the low-octane stuff, and can refill the tank with higher-grade fuel.

Read more: 30 Car Mysteries Solved: Low Octane Fuel at High Altitudes - Popular Mechanics

 

[MidlifeMotor]

Nice research. I don't have a scientific source, but I can say the ST seems to run different at altitude. I just spent 9 days in Utah at elevations ranging from 8-9000 feet. While the bike performed great, it was just a shade off it's normal "oomph" on the highway. I used 91 Octane the entire trip. I attributed it to the "thinner" air at altitude not providing the same density in the cylinders, thus less push after combustion. Based on the previous post, it seems to make sense. I guess you could call my contribution the "field work" of this research.

I would still pay the extra price and run 91 and less you were sure you are gonna burn the whole tank at altitude. To me it's not work the dollar saved on a four gallon gas purchase. I also promised myself I would not track gas mileage on this bike. Just want to enjoy it.
::021::

 

[3putt]

Lots of places in Canada and Alaska had only 87, I ran it with no ill effects, fully loaded. Think I actually got better MPG with it, but that would be hard to prove.

 

[stevepsd]

Nice research. I don't have a scientific source, but I can say the ST seems to run different at altitude. I just spent 9 days in Utah at elevations ranging from 8-9000 feet. While the bike performed great, it was just a shade off it's normal "oomph" on the highway. I used 91 Octane the entire trip. I attributed it to the "thinner" air at altitude not providing the same density in the cylinders, thus less push after combustion. Based on the previous post, it seems to make sense. I guess you could call my contribution the "field work" of this research.

I would still pay the extra price and run 91 and less you were sure you are gonna burn the whole tank at altitude. To me it's not work the dollar saved on a four gallon gas purchase. I also promised myself I would not track gas mileage on this bike. Just want to enjoy it.
::021::

At higher altitudes you need less Octane. I noticed in Colorado Springs that regular unleaded was 85 Octane, instead of 87 Octane at home.

And you did indeed loose alot of 'oomph' at altitude....an average 'guesstimate' for normally aspirated (non turbo or supercharged) motors like the ST is about 3% loss per 1,000ft gain in altitude. So going from sea level to 10,000ft you loose about 30% (actually a bit less) of your horsepower.

 

[tpak]

I have run my Vstrom up here in CO at 7000ft where I live, down to 5k and up to 11k on 85 and I get .... ready wait for it ... better mileage than with 91. I only have 400 miles in the saddle with my S10 and have filled it 2x with 91 (our highest at most pumps) and am tracking mileage and getting a feel for the bike. I will switch to 85 in a couple of tanks and track it and get a feel for it then too. I presume I will see the same results as with the Vstrom. Also - Nick Saunders and Davide Biga have taken these bikes up and down the Americas tip to tip and around the world respectively and sure as heck didn't get 91+ along the way ... and experience no material problems. In an interview with supertenere1200.com Nick Saunders indicated he got crappy gas in South America somewhere and it made the bike sluggish at high altitude but that was it. Read it here: http://supertenere1200.com/2011/09/15/nick-sanders-answers-your-questions/

So me, I'm not going to worry about it I'm just going tosee what gets better mileage, feels better (subjectively) and cost the leaset.

YMMV ::013::

 

[Bappo]

You should get better fuel mileage with lower octane than with higher. More BTU's per volume. I run lowest I can in all my vehicle without knocking. The S10 doesn't care in Idaho what octane I use but likes higher here in the UP.

 

[MidlifeMotor]

At higher altitudes you need less Octane. I noticed in Colorado Springs that regular unleaded was 85 Octane, instead of 87 Octane at home.

And you did indeed loose alot of 'oomph' at altitude....an average 'guesstimate' for normally aspirated (non turbo or supercharged) motors like the ST is about 3% loss per 1,000ft gain in altitude. So going from sea level to 10,000ft you loose about 30% (actually a bit less) of your horsepower.

Thanks for verifying this for me. I am going to look at it as gaining horsepower as I go to lower elevations (glass half full)

 

[markjenn]

I'll just chime in to say that this lower-octane-at-altitude effect is really simply to understand if you consider that a normally-aspirated (non-supercharged) engine is essentially running at partial throttle at altitude compared to sea level. The engine can't tell whether it is running at sea level and 80% throttle or 10K feet at 100% throttle - in either event the mass of air-fuel mixture in the combustion chamber is the same. So running at engine at 10K feet is EXACTLY the same as running an engine with a governor that prevents you from advancing the throttle beyond 80% of sea level power. Altitude is a essentially a built-in throttle restriction.

And most have experienced, engines tend to ping at full throttle, not at partial throttle. Big throttle openings at low-RPM are the operating conditions where an engine tends to ping.

The oil companies have figured this out and adjusted the octane in gas grades to reflect this. When I lived in Denver, I recall premium was typically 90 octane, while back E at sea level, it was often 92 or 93.

Personally, I still would run premium just to be safe, but you definite have more octane headroom at altitude. And precisely translating altitude to reduced octane requirement (e.g., at 5K feet, 89 octane is equivalent to 91 at sea level) is something that would require some testing.

- Mark

 

[bloodline]

Buy the best fuel you can get and ride. Relax.

As for altitude, I feel sorry for you guys (not really) that live where you can ride above 10K feet. HP is way down in those conditions but the scenery tends to make up for it.

 

[olie]

I'll just chime in to say that this lower-octane-at-altitude effect is really simply to understand if you consider that a normally-aspirated (non-supercharged) engine is essentially running at partial throttle at altitude compared to sea level. The engine can't tell whether it is running at sea level and 80% throttle or 10K feet at 100% throttle - in either event the mass of air-fuel mixture in the combustion chamber is the same. So running at engine at 10K feet is EXACTLY the same as running an engine with a governor that prevents you from advancing the throttle beyond 80% of sea level power. Altitude is a essentially a built-in throttle restriction.

And most have experienced, engines tend to ping at full throttle, not at partial throttle. Big throttle openings at low-RPM are the operating conditions where an engine tends to ping.

The oil companies have figured this out and adjusted the octane in gas grades to reflect this. When I lived in Denver, I recall premium was typically 90 octane, while back E at sea level, it was often 92 or 93.

Personally, I still would run premium just to be safe, but you definite have more octane headroom at altitude. And precisely translating altitude to reduced octane requirement (e.g., at 5K feet, 89 octane is equivalent to 91 at sea level) is something that would require some testing.

- Mark

... you are missing 2 important and modern things in your analysis:
1. the use of the the O2 sensor
2. the use of air mass meters with the fuel injection system.

Their obvious effects ensures proper correction of the air fuel ratio together with fast response as well any required "timing" adjustment, not only for altitude but also for air temperature and indirectly to humidity.

 

[hANNAbONE]

...I've used 85 & 87 in the mountains. Never a problem.

YMMV

 

[caveman]

My local fuel station sells unleaded in 3 versions 91, 95, 98 octane. The price goes up by 5 cents a litre for each rise. I have tried them all and find neglible difference. The performance is slightly enhanced with the 98 but fuel economy doesnt change across the 3 fuels.
This is a mystery as the higher octane makes a big difference to my WR450 and XR650. I suppose it backs up Olies statement, the bike seems to enjoy all of them equally, averaging 44 mpg. I now run the 95 as standard for no other reason than it makes me feel better but it looks like I am spoiling the big girl compared to the US fuels.

 

[jajpko]

My local fuel station sells unleaded in 3 versions 91, 95, 98 octane. The price goes up by 5 cents a litre for each rise. I have tried them all and find neglible difference. The performance is slightly enhanced with the 98 but fuel economy doesnt change across the 3 fuels.
This is a mystery as the higher octane makes a big difference to my WR450 and XR650. I suppose it backs up Olies statement, the bike seems to enjoy all of them equally, averaging 44 mpg. I now run the 95 as standard for no other reason than it makes me feel better but it looks like I am spoiling the big girl compared to the US fuels.

I wonder if your octane ratings are achieved in the same way, as those in the US? I don't think so??

 

[briang123]

Octane rating has nothing to do with Quality or Energy of gasoline. It helps prevent higher compression engines from experiencing pre-ignition. If your engine doesnt ping (pre-ignite) running regular, It will not run better running premium.
Wikipedia:
Many high-performance engines are designed to operate with a high maximum compression, and thus demand fuels of higher octane. A common misconception is that power output or fuel efficiency can be improved by burning fuel of higher octane than that specified by the engine manufacturer. The power output of an engine depends in part on the energy density of the fuel being burnt. Fuels of different octane ratings may have similar densities, but because switching to a higher octane fuel does not add more hydrocarbon content or oxygen, the engine cannot develop more power.

 

[snakebitten]

Octane rating has nothing to do with Quality or Energy of gasoline. It helps prevent higher compression engines from experiencing pre-ignition. If your engine doesnt ping (pre-ignite) running regular, It will not run better running premium.
Wikipedia:
Many high-performance engines are designed to operate with a high maximum compression, and thus demand fuels of higher octane. A common misconception is that power output or fuel efficiency can be improved by burning fuel of higher octane than that specified by the engine manufacturer. The power output of an engine depends in part on the energy density of the fuel being burnt. Fuels of different octane ratings may have similar densities, but because switching to a higher octane fuel does not add more hydrocarbon content or oxygen, the engine cannot develop more power.

With one caveat.

If an anti-knock sensor reacts to pre-ignition by retarding timing, and other adjustments that reduce power output, then in affect, the higher octane fuel results in higher power output.

I had a 1984 SVO Mustang with a switch on the dash for Premium vs Not Premium.
Basically, if you couldn't get premium and you put the switch in the proper position, the car reduced maximum turbo boost and less advance on timing too.

Kinda low tech compared to today, but higher octane allowed more horsepower.

Regardless, your Wiki quote is still literally accurate, I suppose.

 

[markjenn]

... you are missing 2 important and modern things in your analysis:
1. the use of the the O2 sensor
2. the use of air mass meters with the fuel injection system.

You're right, the FI system keeps them mixture constant, but this isn't relevant to the discussion. NA Engines have less tendency to ping at altitude because they run at reduced power due to the lower atmospheric pressure. EXACTLY THE SAME AS IF YOU RESTRICTED THE THROTTLE. Absolutely nothing to do with mixture, O2 sensors, etc.

- Mark

 

[olie]

You're right, the FI system keeps them mixture constant, but this isn't relevant to the discussion. NA Engines have less tendency to ping at altitude because they run at reduced power due to the lower atmospheric pressure. EXACTLY THE SAME AS IF YOU RESTRICTED THE THROTTLE. Absolutely nothing to do with mixture, O2 sensors, etc.

- Mark

You are NOT correct about "EXACTLY.....RESTRICTED THROTTLE". Here it goes a few more engineering basic statements from a well known Combustion Engineer that I know:
1. O2 input is used to adjust the air /fuel mixture in order to ensure sufficient O2 for proper combustion efficiency and emissions.
2. Air fuel mixtures are on the mass basis. Lighter air due to lower atmospheric pressure needs less fuel to keep the proper ratio.
3. The volume of air may be the same but the mass flow of air is lower==> cut fuel to maintain ratio
4. Similar cases can be made with high temperature air especially when in conjunction with high humidity.
5. All the above is applicable to the effect of compressed air from super /turbo charged engines as they become self adjusted when pressurized regardless (sort of...)of the altitude.
6. Yes, air /fuel ratios togeth with "timing" are fundamental to prevent pre-ignition or detonation to be more accurate.

Hopefully this is basic enough. No magic stuff from the net here... ::008::

 

[markjenn]

You are NOT correct about "EXACTLY.....RESTRICTED THROTTLE". Here it goes a few more engineering basic statements from a well known Combustion Engineer that I know:
1. O2 input is used to adjust the air /fuel mixture in order to ensure sufficient O2 for proper combustion efficiency and emissions.
2. Air fuel mixtures are on the mass basis. Lighter air due to lower atmospheric pressure needs less fuel to keep the proper ratio.
3. The volume of air may be the same but the mass flow of air is lower==> cut fuel to maintain ratio
4. Similar cases can be made with high temperature air especially when in conjunction with high humidity.
5. All the above is applicable to the effect of compressed air from super /turbo charged engines as they become self adjusted when pressurized regardless (sort of...)of the altitude.
6. Yes, air /fuel ratios togeth with "timing" are fundamental to prevent pre-ignition or detonation to be more accurate.

Hopefully this is basic enough. No magic stuff from the net here... ::008::

How is any of this contradictory to my basic statement that running a NA engine at altitude is equivalent to running the same engine at sea level with restricted throttle? My statement has NOTHING to do with mixture or A/F ratio. Probably not worth pursuing further. I don't think we're disagreeing, you're just inferring something in my statement that I'm not.

- Mark

 

[avc8130]

I was in an area that had only 87 octane around 1k-2k feet altitude. I had audible pinging if I did WOT runs like I normally do entering a highway or pulling onto the road. To my knowledge, this motor does NOT have any knock sensors. I don't believe this motor has ANY ability to adjust timing with regards to pre-ignition.

With both of these pieces of knowledge, I will be running premium whenever available. Any small savings just aren't worth the risk of engine damage due to detonation to me.

ac