Eric Frisbee wrote:
> Perhaps it's a trick of psychology, but I could have sworn the car was
> running better and seemed to have more power. I could not quantify
it with
> hard scientific data, so we won't bother with trying to prove otherwise
>
> I can however quantify that higher octane gas DOES provide better gas
> mileage. Three cars no less... '67 Datsun 1600. 85 octane got
26.4 mpg,
> 89 octane got 28.9 and the 91 octane got 31.4 mpg. '95 Geo Metro. 89
> octane = 36 mpg 91 = 40-45 mpg '00 Buick Regal 89 octane =26.7 mpg , 91
> octane = 29.5 mpg. This data was obtained on my 158 mile commute
every day
> at 5280-8100 feet altitude and 90% was highway miles. The Geo and the
> roadster can both run on the cheap 85 octane crap, but what's the
point when
> you get better mileage out of the expensive stuff? :)
>
> Eric
That is often the case Eric. The higher the octane, the more ignition
advance you can run. And the more advance you can run, the better your
mileage will be (within reason). Also, the higher the octane, the less
likely the mixture is to burn uncontrollably. A more cotrolled burn
often means a more efficient burn, and better efficiency means better
mileage. The actual stored energy in gas is pretty constant and not
really related to the octane. But the quality of the burn, it's rate and
other factors relating to how the fuel will ignite and burn, are related
directly to the octane. And these quality factors will affect
performance, drivability, and mileage. It's a question of how much of
that stored energy can be converted into work by the engine, rather than
into waste heat. Gas that ignites too quickly will actually cause
pressure to build up above the piston prematurely, while the piston is
still moving upwards. This means that the energy released to casue that
premature pressure build up is not available to do real work and so
engine efficiency is reduced. There's lots more to all this, but the
bottom line is that a "slow" controlled burn is often the most efficient
burn, and that's what produces the best engine efficiency in most cases.
In an older car you will have to adjust for the better gas manually, in
order to get full benefit, but in many modern cars the computers will do
so automatically. However, even in older cars you will often see an
improvement in efficiency by just switching to a better grade or higher
octane gas.
--
Marc Sayer
82 280ZX Turbo
71 510 Trans Am vintage racer
/// datsun-roadsters@autox.team.net mailing list
|