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Re: Octane levels vs...?

Brooks Ellis <brooks@fortnet.org> asks:

>        I have noticed, that here in Colorado, we get the lowest octane
>levels of many states I have visited recently. I have seen a MAX of 91
>octane here, where many other states will have 94 and 95 octane..
>        Why is that? It seems that for maximum turbo performance, we would
>need high performance gasoline too... Is it because of the altitude - the
>gas people figure that the air is thinner, and so engines ( normally
>aspirated ) will suck in less air, and thus be less likely to knock?
>        Or is it a.. conspiricy!?!?

     Yes, there's a very good reason why octane ratings are lower at higher
altitudes. It saves the gasoline companies money because aspros don't need
the higher octane.  ASTM has a specification for octane vs. altitude.  Road
& Track has a nice map of those states involved (R&T, Apr 1988, p. 190).
The specification varies depending on the average altitude of the region
involved.  The Rocky Mountain states lose out the most, with a drop of 3.0
points allowed for high octane gas.  The closest location to you without
any allowable octane reductions is Arizona.

    This comment is also connected to the recent thread on "Turbos at
Altitude."  If your boost is controlled _relative_ to atmospheric pressure,
you don't need any more octane either, because you suffer the same air
density reductions as an aspro.  So, no problem.  Only if your boost is
controlled absolutely, independent of air pressure, will you need the usual
higher octane you would find at sea level.  If this is the case, then you
could be in trouble.  The most important point is that there is no simple
way to tell how your boost is controlled, unless you have literature from
the manufacturer describing your turbo system works.  You can't tell from
your boost gauge, unless you _know_ it is an absolute gauge (and most
aren't).  If your turbo runs at 1.8 bar in Los Angeles, it will _read_ 1.8
bar in Denver irregardless of what type of boost control system you have.
Both types of gauges will read 1.8.  The difference is that the absolute
gauge will be indicating a true 1.8, whereas the relative gauge actually
means 1.8 - .17 = about 1.6 at 5200 ft.  Thus, actually a smaller mass of
air is pumped by the turbo; less explosion pressure in the combustion
chamber, and less octane required.

Terry Donohue
'71 BMW 2002Ti (1.0 relative because it's an aspro)
'90 200TQ (1.8 relative)
'95 BMW M3 (1.0 relative)