"dyno" > wrote in message
...
> Daniel J. Stern wrote:
> > On Sat, 14 May 2005, dyno wrote:
> >
> >
> >>True. There's also the inlet temperature effect; temp up octane
> >>requirment up.
> >
> >
> > Oops, no. Ambient temperature goes up, octane requirement goes
DOWN, and
> > for the same reason that octane requirement decreases as altitude
> > increases: With both increasing altitude and increasing ambient
> > temperature, the intake air charge is less dense, therefore
cylinder
> > pressure is lower, therefore tendency to ping is less.
>
> Wrong. The change in density goes with the change in absolute
> temperature. But, the effect of inlet temperature is magnified
during
> compression. The compression effect dominates over the small change
in
> density.

I had a Tempo that would run at the high end of the temp gauge when
using 87 in the summer. If I switched to 92 (or better) the temp came
back down into the normal range. In the winter I used 87 for easier
ignition.

Ken (MI)