[Diy_efi] Timing Advance Curve?

[Bernd Felsche]

On Sun, Dec 22, 2002 at 01:18:29PM -0000, Hugh Keir wrote:
> With reference to pump gas, please explian why it is normal to run
> more advanced ignition timing on a high octane fuel, it suggests
> to me that the process of igniting and burning the fuel is slower
> than with the low octane offering.

Greater advance produces a higher peak pressure closer to (just
after) TDC that (generally) results in greater torque. 

Retarding the spark produces not only combustion into a greater
combustion chamber volume as it's later after TDC; but the piston is
also moving down at greater velocity providing a more-rapidly
expanding volume into which the end-gas can move as it tends to be
compressed by the advancing flame front.

The force required to move the piston further down a given amount is
also less due to the mechanical efficiency of turning the crank.

The result is that lower peak pressures are "seen" by the end-gas,
reducing the likelihood of detonation.

-- 
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