On Dec 12, 9:08 pm, clare at snyder.on.ca wrote:
On Thu, 13 Dec 2007 04:00:58 GMT, "Cy Galley"
wrote:

No extra heat in high Octane. Same hydrocarbon structure is both. Same
BTUs. Only difference is the speed of combustion is controlled so it is
slower in the higher octane to prevent pre-ignition.

WAY off base, Cy.
On ALL counts, except the no extra heat and same BTU's.

--
Posted via a free Usenet account fromhttp://www.teranews.com

Higher-octane rated fuels have about the same flame front
speed as lower-octane fuels, at around 100 feet per second. It's their
resistance to detonation, which involves flame fronts speeds of 5000
feet per second or more, that makes them necessary in high-compression
engines. As the combustion process begins in the cylinder, the burning
raises the pressure through the entire air/fuel mix, and low-octane
rated fuels will break down under that increasing pressure and
spontaneously combine with the oxygen, igniting almost all at once
instead of *waiting* (key word) for the flame to set them off in a
controlled chain reaction.
Detonation therefore requires time to develop. Low RPM with
high MP is pro-detonation. Lean mixtures burn more slowly, so they're
pro-detonation. Big cylinders take more time for the flame front to
cross, so they suffer more from detonation.
Preignition is something else. It's ignition of the mix
during compression by some hot spot in the cylinder, perhaps a glowing
bit of carbon in the head or on the piston. The burn begins early,
before the sparkplug was scheduled to fire, and gets way ahead of
itself due to the still-increasing compression as well as the
combustion, and detonation might happen. The damage is about the same.
Engines that aren't leaned properly will develop more carbon to cause
preignition. Too much lead fouling can do it. An oil-burner will
carbon up, too.
Octane ratings are different than octane content. In the old
way, fuel under test was fed to a variable-compression test engine and
the compression was raised until detonation began to occur. Then that
fuel was shut off and a mix of octane and heptane was introduced,
engine still running, and the octane/heptane ratio was varied until
the detonation point was established. An 80-octane fuel had the same
detonation characteristics as a mix of 80% octane and 20% heptane,
hence the rating. 80/87 reflected the detonation resistance at lean
and rich settings.
Fuels with ratings above 100 obviously have to be rated using
some other method, since we can't have a fuel consisting of, say, 115%
octane.

Dan