(Re-posted from Honda_Civic_Hybrid on yahoo)

Hi all,

Next month, I'll be driving my HCH from Colorado to Iowa. The trip will reach as high as 6 or 7 thousand feet, and go down to just about sea level.

Anyone recommend things I should do to prepare for the change, or what I should do after I'm at sea level?

With other vehicles, I leave my tire pressure the same until I get down in elevation. Once there, I wait for the tires to cool overnight, then pump them up as needed. When going up in elevation, I let a couple of psi out before the trip, and re-check at higher elevation.

As for octane. It's possible that I can fill up at 85-octane, and be at low elevation before that tank is finished. Should I consider filling up with 87 before the trip? Or just wait till I get there and check for knocking as I drive down?

Thanks,
jinno

you can get 85 octane? i'd just stick w/ what you've got, if there is any problem the knock sensor should pick it up, you might just loose a few mpg's. but the cars should be made for all elevations and adjust the fuel amount (same as if the air was cold or warm)

Hidden Views Photography

Yup, Colorado sells 85-octane as its "regular" unleaded
gasoline. Mid-grade (or plus) is 87, and Premium is 91
(at least where I live). I know the manual for the HCH
says to use at least 86 octane, but I asked around in
the honda civic yahoo group, and other Coloradans have
no problem with 85.

That's a good point about the knock sensor. Thanks.
I should be able to get to our destination (Bellevue)
with 1.5 tanks. So when I fill up in Nebraska, we
should be at about 2000 feet, then I will fill up with
87 octane.

Thanks again,
jinno

i guess ya'lls elevation alows that. we get 87 as reg., 89 as midgrad and 91 or 93 (depending on gas station) for premium.

Hidden Views Photography

You probably know this already - but effectively, octane increases with elevation.

The reason for this is most easily explained by understanding (VERY basically) that octane is not really the ability of fuel to burn, but the ability NOT to burn.

As octane goes up, the fuel burns slower - not faster (this is a gross oversimplification, but it serves to illustrate...)

As altitude goes up, there is less oxygen therefore less tendency to burn - thus, higher effective octane. Therefore, a lower octane fuel (rated in a lab closer to sea level!) will burn as if it were a higher octane at altitude.

Coming down of course gives the reverse - but I really think a loss of two points really won't matter all that much. If you really want, putting in a mid-grade before the trip might be reasonable.

Jack

Eschew Obfuscation
Avoid pompous prolixity
If it ain't broke, fix it until it is.

Jack,

Great explanation. I knew the octane/elevation relationship, but
your short description is better than one I could've made.

I figure that I can go at least to Lincoln, Nebraska, on a tank
of gas (about 600 miles). I think the elevation there is about
1100 ft, so that would be a good time to get the "regular" 87
octane. I'll see if I can monitor any improvement/degradation
in MPG and performance as I go down in elevation with the
85 octane.

There'll be other factors besides the octane, of course. The
tire pressure should change. The air becomes less thin, so
I should experience more drag. Plus, the speed limits should
be really high on the interstates, compared to the 60mph I
drive on my commute.

Thanks,
jinno