Octane & Elevation Correlation Question

[lovcom]

I live in Orange County, Ca, however I am vacationing in New Mexico for two weeks at an average elevation of 5,500+ feet, which is about 4,500 fee higher then where I live. I normally gas up with 87 but regular in NM is 86 and I find that my car provides noticably less power around NM. I realize the altitude is one reason, so to compensate for the less power should I be gassing up with 88 or higher when in NM?

 

[MGregoir]

Less dense air means that the cylinder pressures are lower, which reduces temperatures and the chance for preignition.

It's likely you would make less power with higher octane gas.

The only way around this is a forced induction engine where it simply runs more boost at higher altitudes to even out the air density.

 

[lovcom]

Originally Posted By: MGregoir
Less dense air means that the cylinder pressures are lower, which reduces temperatures and the chance for preignition.

It's likely you would make less power with higher octane gas.

The only way around this is a forced induction engine where it simply runs more boost at higher altitudes to even out the air density.


So then if I understand you, using a higher octane might actually provide less power by the engine? Your explanation seems to make sense, given the air is lower in density.

 

[oilyriser]

liquid oxygen

 

[Canawler]

Also, higher octane gas actually ignites and burns slower than lower octane. That's why it is more resistant to pinging. To really take advantage of the higher octane you would need an increase in timing advance, which is something most computer controlled cars only allow to a very limited extent.

Back in the '80s, all my friends with the hot cars would fill up with racing gas before going to the strip, but they usually didn't bump the timing. They didn't realize they were potentially going slower because of the high octane gas.

 

[Bill in Utah]

My cars state in the manual that 87 octane is needed.

I run 85 since that is regular here.

No problems and plenty of power and MPG!

Valley Elevation is 4700ft up to 9000ft in the hills.

Bill

 

[lovcom]

Originally Posted By: Bill in Utah
My cars state in the manual that 87 octane is needed.

I run 85 since that is regular here.

No problems and plenty of power and MPG!

Valley Elevation is 4700ft up to 9000ft in the hills.

Bill


Bill, how often do you drive your cars in sub 1,000 feet elevation? I suspect your cars will provide higher power and higher MPG if you did.

Me going to 4,500 feet higher then I have at home has shown a very noticably drop in power, and MPG.

I was hoping I could mitigate this with a change in octane, but it seems this may not work.

 

[Titan]

Gentlemen: higher power does NOT directly correlate with higher efficiency. Sure, the air is more dense at lower altitude, which can result in an increase in Power due to more oxygen being available (in a naturally aspirated engine). However, the Coefficient of Drag also increases...depending upon your speed, the increased power from the more dense air may be completely cancelled out, and even negated, by the increased CD.

 

[c502cid]

Back in the days of the carburetor, I'd spend my summers in So Cal and the rest of the year in the Rockies. We'd advance the timing 5-6 degrees and change the jets with each trip up. Never had to change the fuel type though, you need less octane at altitude. Nowadays, the computer does it all for you. Your static compression ratio remains the same then and now.

 

[tropic]

Originally Posted By: lovcom
I am vacationing in New Mexico for two weeks...

Welcome to NM. Have fun. Enjoy your increased MPG. Don't underestimate the sun at higher altitudes. The red chili's normally hotter than the green. That's about it.

Originally Posted By: Bill in Utah
My cars state in the manual that 87 octane is needed. I run 85 since that is regular here. No problems and plenty of power and MPG!

Bill, you are SO tempting me to start running straight 86 octane in my 87-or-better car here at 6,000 feet.

 

[punisher]

Originally Posted By: Canawler
Also, higher octane gas actually ignites and burns slower than lower octane.


Common misconception. Octane and burn rate are two different and separate variables, independant of each other. Octane is only a rating of a fuel's resistance to detonation, nothing more.

 

[mstrjon32]

When I had my 2006 Monte Carlo in some higher elevations last year, including New Mexico and Colorado, I noticed that it had less power and that it dialed back its shift points. For example, at low elevations, full throttle shifts happen at ~6200 RPM, when passing a vehicle at high elevation in Colorado I noticed it shifted up at just ~5600.

 

[lovcom]

Originally Posted By: punisher
Originally Posted By: Canawler
Also, higher octane gas actually ignites and burns slower than lower octane.


Common misconception. Octane and burn rate are two different and separate variables, independant of each other. Octane is only a rating of a fuel's resistance to detonation, nothing more.


You may be right, but dosn't higher octane prevent detonation by preventing combustion before the sparkplug ignites?

 

[lovcom]

Originally Posted By: mstrjon32
When I had my 2006 Monte Carlo in some higher elevations last year, including New Mexico and Colorado, I noticed that it had less power and that it dialed back its shift points. For example, at low elevations, full throttle shifts happen at ~6200 RPM, when passing a vehicle at high elevation in Colorado I noticed it shifted up at just ~5600.


Interesting comment. My 4-speed auto-transmission Yaris shifts a little later in NM (5,500+ elevation) then when I am in So. Cal (100-200 feet).

 

[lovcom]

Originally Posted By: tropic
Originally Posted By: lovcom
I am vacationing in New Mexico for two weeks...

Welcome to NM. Have fun. Enjoy your increased MPG. Don't underestimate the sun at higher altitudes. The red chili's normally hotter than the green. That's about it.

Originally Posted By: Bill in Utah
My cars state in the manual that 87 octane is needed. I run 85 since that is regular here. No problems and plenty of power and MPG!

Bill, you are SO tempting me to start running straight 86 octane in my 87-or-better car here at 6,000 feet.


Thanks for the welcome! Actually, my MPG is worse in NM then in SoCal, and I was told that the green chile is hotter then the red...regardless, cheers!

 

[Bill in Utah]

Originally Posted By: lovcom
Originally Posted By: Bill in Utah
My cars state in the manual that 87 octane is needed.

I run 85 since that is regular here.

No problems and plenty of power and MPG!

Valley Elevation is 4700ft up to 9000ft in the hills.

Bill


Bill, how often do you drive your cars in sub 1,000 feet elevation? I suspect your cars will provide higher power and higher MPG if you did.

Me going to 4,500 feet higher then I have at home has shown a very noticably drop in power, and MPG.

I was hoping I could mitigate this with a change in octane, but it seems this may not work.


I go to San Diego once a year to visit family and have taken the corolla a few times. The gas in San Diego stunk and I got worst gas mileage until I could fill up in Las Vegas.

Last time, I filled up in Victorville and drove down to San Diego, drove around and back to Victorville and filled up again. Doing that the car ran much better and I got around 43mpg with the corolla loaded.

I think San Diego is below 1000 ft...

I average over 44 mpg here so I doubt being below 1000 ft would get me better MPG.


Bill

 

[Benzadmiral]

A former Colorado denizen here,

When I ran my first two Mercedes (an '84 and an '86) in Denver at about 5200 feet, I was advised that because of the altitude I could safely run the mid-grade fuel (87 octane there, I recall) instead of premium. So I did. Neither of my cars had modern timing adjustment systems, and neither pinged on mid-grade.

As far as gas mileage goes, I got better mpg at 5200 feet in the V-8 Mercedes than I did with it down here in The Swamp. But I drove more highway miles in CO, and stoplights were [censored] apart in Denver; here, it seems, you drive a block, stop, drive 2 blocks, stop. . . .

 

[punisher]

Originally Posted By: lovcom

You may be right, but dosn't higher octane prevent detonation by preventing combustion before the sparkplug ignites?


You have some terms confused. Ignition before the sparkplug fires is called "preignition". Detonation occurs after the sparkplug fires. Detonation occurs when the mixture ignites and the combination of pressure and heat causes the remaining mixture to literally detonate. Detonation occurs near the end of the burn after pressure and temp spike. Since the last part of the mix to burn is near the intake valve, that is where it is most evident. Detonation removes the chamber/piston boundary layer and parts burn up eventually.

If you have a big hole in the middle of your piston..........thats preignition. If you have a burned piston (upper ring land) near the edge by the intake valve....that's detonation.

 

[Shannow]

Originally Posted By: Canawler
Also, higher octane gas actually ignites and burns slower than lower octane.


No it doesn't.

Ignition and flame speed are essentially constant for petrol. Higher octane is more resistant to spontaneous (rather than controlled) combustion, particularly in the area away from the spark plug.

 

[lovcom]

Guys, thanks for the clearing up of this issue for me.