Maximum advance

[James Montebello]

Lower octane does not equal higher burn rate.  Octane is strictly
a measure of resistance to detonation.  Lower octane fuels TEND to
burn faster, but this isn't true in all cases.  Anything you can do
to increase detonation resistance is a good thing, since it allows
a higher compression ratio, which means greater thermal efficiency.
If the price of higher compression is more spark advance, so be it.

Racing teams also can't just use any fuel they feel like.  Low buck teams
have to use what they can afford (pump gas), high buck teams have to
use what the rules makers allow them to use.  There's a certain amount
of inertia there.  Racers can be (and the successful ones usually are)
as conservative as anyone else.  I suspect current practice strikes
a reasonable balance between flame speed and knock prevention.  

As an aside, Keith Duckworth used to say that he considered any engine
that could acheive mean best torque with less than 30 degrees of advance
to be a good engine, and less than 25 degrees to be an exceptional engine.
The DFV typically ran best with 28 degrees on the fuels used when it
was introduced.

james montebello

On Tue, 10 Jul 2001, Jeff Bromberger wrote:

> i agree in theory.. but why don't we see race teams running 0 degrees of
> timing and 75 octane gas... with the much lower octane the burn rate will
> increase and you should reach your peak cyl pressure around 10 degrees after
> tdc... conversely, if you run 118 octane and have 35 degrees of advance
> you'll still hit peak pressure at the same crank angle... (numbers are
> arbitrary, but i'm asking the general question, why do people go towards
> more advance and slower burn vs. the opposite?)...
> 
> 
> 
> > More isn't better.  The more advance you have, the more the piston is
> > working against rising cylinder pressure on the upstroke.  What you want
> > is a peak cylinder pressure at a particular point in the downstroke that
> > provides the best leverage on the crank.  No more, no less.  If you
> > could somehow accomplish that with zero advance, you'd have the best of
> > all worlds.
> >
> 
> 
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