[Diy_efi] The Hunt effect

[Adam Wade]

--- dh at busb.com wrote:

> The octane can't be taken out of gasoline and still
> be gasoline.

If that were true, then shouldn't it be impossible to
greatly change resistance to detonation using anything
BUT octane?  If you can provide adequate detonation
resistance with, say, tetraethyl lead, hen why would
you need octane in the blend?

"Overall a typical gasoline is predominantly a mixture
of paraffins (alkanes), naphthenes (cycloalkanes),
aromatics and olefins (alkenes)."  I have found
nothing that says that the alkanes in gasoline blends
MUST contain n-octane.

http://en.wikipedia.org/wiki/Gasoline has a lot of
good info.

> You say apple and I say oranges, for a given flame
> speed smaller distance is less time.

That wasn't what I said.  Smaller volume means a
quicker rise in end-gas presure, and thus end-gas
temperature.  This can be easily seen in two engines
that are scaled differently but are in perfect scale
to each other, fueled by the same air/fuel mixure. 
The one with the larger volume will make less power
for a given displacement unless you advance the
timing, and that is a combination of changes in flame
front speed and the physical distance being covered.

> But, consider the change in shape, especially with a
> true hemi between piston down (cylinder) and piston
> up (drooping pancake.)

A "true hemi" will have a hemispherical combustion
chamber (i.e., flat-topped piston).  That is what the
"hemi" refers to.  A piston that is domed does not
make a "true hemi", but is necessary if you want to
increase CR without increasing displacement and still
keep the rounded head contour.

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