Effects of Altitude on Octane Requirements

retiredpop · Jan 13, 2011

This is for all the deep thinkers out there. Do you require less octane as elevation increases? I think you are definitely not getting as much overall air and fuel in cylinders at higher altitudes in a normally aspirated engine so detonation would not be as likely with lower octane. Also because lower octane fuel is actually more volatile than higher octane (i.e a faster more violent combustion) would you actually get a performance increase with lower octane? I'm interested in your thoughts about this.

JROD · Jan 13, 2011

Yes, you are right. An engine requires more octane at lower elevation due to higher air densities. Due to the burn rates of higher octane fuel being slower, if you run it in an engine that doesn't require a high octane fuel, you will see a performance loss and with a fuel of lower octane you will see gains.

crashnbash · Jan 13, 2011

You can get away with less octane the higher in elevation you go. The lower the air density the lower the cylinder pressures. Thats why a motor will show lower compression at altitude than one at sea level. The aftermarket and oems both know this, that is why some heads, pipes, and timing keyways are not recommended below a certain elevation on pump fuel alone.

X2Freeride · Jan 13, 2011

That being said the octane rating needed also depends on the porting and compression of the engine.

JROD · Jan 13, 2011

X2Freeride said:
That being said the octane rating needed also depends on the porting and compression of the engine.

and combustion chamber design

milehighassassin · Jan 13, 2011

Less octane is required at elevation because at higher elevations there is less barametric pressure. Standard pressure is 14.6 psi at sea level, around 11k feet that is closer to 10 psi. As elevation goes up, pressure goes down.

So your motor has less compression at say 10k feet than it does at sea level, everything else equal. At higher elevation since you do not have the compression you have at sea level you do not need as much octane to prevent (pre) detonation.

Of course this is also the same reason you can get a higher compression head for higher elevations than lower elevations. You are really just compensating for what you already lost with the elevation gain. As you increase cylinder head compression you need more octane.

At elevation you need less octane on a stock motor than you do on the SAME stock motor at sea level. Now if you increase the compression on that motor you will need more octane.

retiredpop · Jan 14, 2011

Sounds like everyone agrees less octane is needed at higher altitudes on a stock normally aspirated engine. So, do you think a lower octane fuel (87 or 89) on a stock engine at higher altitudes will give a performance increase?

RKT · Jan 14, 2011

retiredpop said:
Sounds like everyone agrees less octane is needed at higher altitudes on a stock normally aspirated engine. So, do you think a lower octane fuel (87 or 89) on a stock engine at higher altitudes will give a performance increase?

There is not more power to be had on lower octane fuel at elevation... Despite what most think... lower octane fuel does NOT burn slower than higher octane fuel.. The burn rate is the same... what is different is the flash point or the amount of heat it takes to ignite the fuel.. Once ignited, the rate of burn is the same.

There are tons of misconceptions out there regarding race fuel.
At elevation, your engine does not require premium fuel and will run fine on the lower octane fuel , will it run better? maybe.. but not because, with 91 octane, you have too much octane, but because your fuel is igniting later.
With the lower octane there is a good chance that the fuel calibration (FI or carb) will be better suited than the higher octane at altitude. So, take the same sled, lean it down, and run 91 octane and the power will be the same as stock calibration with lower octane fuel (maybe even a tad better). We run 91 octane in our stock sleds and mod sleds.. The mod sleds require it, the stock sleds do not. Could we run 87 in the stocker?? probably... will we?.. not likely.. Will this octane change make a noticeable difference on the hill pull?? not likely?

Compression ratio is the same no matter what elevation you are running.. Cylinder pressures are not.

I wrote a little tech article years ago that relates.. If anybody would like to read it.. here is the link http://www.2strokeheads.com/tech2.htm

Kelsey

LRD · Jan 14, 2011

Here is an interesting graph by SLP on the RUNNING compression and octane requirements with various mods, heads, pipe and twin pipes. Porting would apply also, fills cylinder more, lowers cranking compression and raises running compression, = higher octane requirement. What it gets at is the cylinder can be filled a lot more than stock with various mods changing octane requirements. Good thinking article. The opposite is higher altitude = less running compression = lower octane. Less atmospheric air pressure filling the crankcase at higher altitude.

http://www.startinglineproducts.com/technical.cfm?articleID=2

Good Luck

Dootimes2 · Jan 14, 2011

If you follow snowest much you will listen to Kelsey and the rest of these guys on this. Seeing how the performance will be equal or maybe even a little less ( although probably about 99% unnoticable ) why worry about deto at low elevation on the trail in for no gain at elevation. Run good fuel and enjoy the ride. If you want a gain get one of Kelseys heads for your elevation.

X2Freeride · Jan 16, 2011

RKT said:
There is not more power to be had on lower octane fuel at elevation... Despite what most think... lower octane fuel does NOT burn slower than higher octane fuel.. The burn rate is the same... what is different is the flash point or the amount of heat it takes to ignite the fuel.. Once ignited, the rate of burn is the same.

There are tons of misconceptions out there regarding race fuel.
At elevation, your engine does not require premium fuel and will run fine on the lower octane fuel , will it run better? maybe.. but not because, with 91 octane, you have too much octane, but because your fuel is igniting later.
With the lower octane there is a good chance that the fuel calibration (FI or carb) will be better suited than the higher octane at altitude. So, take the same sled, lean it down, and run 91 octane and the power will be the same as stock calibration with lower octane fuel (maybe even a tad better). We run 91 octane in our stock sleds and mod sleds.. The mod sleds require it, the stock sleds do not. Could we run 87 in the stocker?? probably... will we?.. not likely.. Will this octane change make a noticeable difference on the hill pull?? not likely?

Compression ratio is the same no matter what elevation you are running.. Cylinder pressures are not.

I wrote a little tech article years ago that relates.. If anybody would like to read it.. here is the link http://www.2strokeheads.com/tech2.htm

Kelsey

so what your saying is since the squish in the domes stays the same the compression ratio is the same no matter what the elevation you run at, but since the air is less dense the higher up you go in elevation the cylinder pressure is actually less. Correct?

RKT · Jan 16, 2011

X2Freeride said:
so what your saying is since the squish in the domes stays the same the compression ratio is the same no matter what the elevation you run at, but since the air is less dense the higher up you go in elevation the cylinder pressure is actually less. Correct?

Yes.. but squish has nothing to do with it... The head design has not dynamically altered itself..therefore; compression ratio has not been altered either.

X2Freeride · Jan 16, 2011

RKT said:
Yes.. but squish has nothing to do with it... The head design has not dynamically altered itself..therefore; compression ratio has not been altered either.

I just read your article it all makes much more sense now. I never took into consideration port height before. Very interesting.

Another question how much does the design of the pipe effect this? IE. race pipes, twins, singles ECT.

RKT · Jan 16, 2011

X2Freeride said:
I just read your article it all makes much more sense now. I never took into consideration port height before. Very interesting.

Another question how much does the design of the pipe effect this? IE. race pipes, twins, singles ECT.

Pipes are a player in what is known as dynamic compression... Certain pipe designs can over-trap the cylinder and this can raise the running compression (actually fill the cylinder to more than its intended volume).

As mentine din the article, port height is a major player in the compression testing ..so, all this talk of how anything over XXXpsi is too much is simply not true.. the cranking compression (via gauge) is a worthless number for determining octane requirements.. Good for health check.. nothing more.

Another big misconception is that squish clearance determines octane... Simply not true either...
Glad you found the article useful.. that is why I write them.

Feel free to check out all of them on the site.

Kelsey

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Effects of Altitude on Octane Requirements

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