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my 2001 grand cherokee runs on 87 octane in Texas where I live....flat land
driving and good power in metro driving in the Dallas area....now I took a
trip to Vail, Colorado and of course buying the cheapest gas at 3.29,it
being 85.... is it me or do you loose power to the lower octane and high
altitude....

Re: 85 vs 87 Octane

Mindy wrote:
> my 2001 grand cherokee runs on 87 octane in Texas where I live....flat land
> driving and good power in metro driving in the Dallas area....now I took a
> trip to Vail, Colorado and of course buying the cheapest gas at 3.29,it
> being 85.... is it me or do you loose power to the lower octane and high
> altitude....
>
>
Nope, it's not you, engines loose power at higher altitudes, that's why
piston engine planes have superchargers.

Jeff DeWitt

Re: 85 vs 87 Octane

Mindy wrote:
> my 2001 grand cherokee runs on 87 octane in Texas where I
> live....flat land driving and good power in metro driving in the
> Dallas area....now I took a trip to Vail, Colorado and of course
> buying the cheapest gas at 3.29,it being 85.... is it me or do you
> loose power to the lower octane and high altitude....

First, do not confuse power with octane rating, they arn't related.

The higher the octane number the harder the fuel is to ignite.
And higher octane fuel has no more power than lower octane fuel.

What your experiencing is lower air pressure at altitude. The same
thing that makes you tire more easily at higher elevations also means
your engine is not getting the same amount of oxygen with each intake.
Less oxygen means less fuel can be burned. Less fuel of course means
less power unless you can push more air into the engine with a
supercharger or turbocharger.

You want to use the lowest octane fuel that does not cause ping.

--
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Re: 85 vs 87 Octane

Re: Re: 85 vs 87 Octane

On Sun, 2 Sep 2007 20:05:19 -0500, "DougW"
<I.only.read.usenet@invalid.address> wrote:

>First, do not confuse power with octane rating, they arn't related.

Actually they are a LOT more related than you think in some engine
that have knock sensing timing control

>The higher the octane number the harder the fuel is to ignite.

No it will light just as easy, the differnece is that it has a higher
auto ignite tempature and a slightly slower flame speed but it is no
harder to light its fuse with a spark

>And higher octane fuel has no more power than lower octane fuel.

Actually it has a few less BTU in it but in a higher compression
engine it allows for a more favorable ignition and expansion cycle
than low octane fuel does (which has to use a retarded curve to
prevent to rapid of a pressure build up and auto ignition or
detenation)

>
>What your experiencing is lower air pressure at altitude. The same
>thing that makes you tire more easily at higher elevations also means
>your engine is not getting the same amount of oxygen with each intake.
>Less oxygen means less fuel can be burned. Less fuel of course means
>less power unless you can push more air into the engine with a
>supercharger or turbocharger.

Mostly true. In the old days you would advance base line timing about
2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees
or so. It did not restore all power but it does help some. Some modern
cars can advance timing a bit more automatically as well as lean
mixture out. At start up it reads MAP sensor just before engine cranks
to set elevation data. The problem with this is if you start engine at
4000 feet and drive to 9000 feet the computer does not know it is at
9000 feet until you shut it out and restart it.

>You want to use the lowest octane fuel that does not cause ping.

This is not cut and dried as it seems. This works with old fixed
timing engine but not with some modern engine with knock sensor
because many of them (especailly GM's ) will never knock no matter
what, performance will just suffer. BTW, octane requirements do
decrease generally with elevation and this is why above 4000 MSL or so
you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus
and 91 for premium. You do not want a tank of 85 when you are headed
to lower elevations especailly in hot weather (ambent temp effects
octane needs too) Generally if your engine has a CR of 9 to 1 or
higher you cannot burn 87 at sea level without some spark timing
trickery as 9 to 1 and above really needs 89 or more (especailly in
warm weather) for optimal timing curving. At 10 to one and above you
really need 91 or better unless you run a compromised timing curve
which also reduces efficency and power too.

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