"Sweet Spot"

[Robert Harris]

This is a mechanically fixed point by a combination of mechanical factors.
With influence from combustion.

If you set up a series of tests such that the only thing that varied was the
spark advance,  and you moved it such that the pressure peak varied from 10 to
20 degrees atdc, you could probably use acceleration or hp or torque or .. to
find it.  Without extensive testing, it probably is not work more than 1% or
so over just picking 15 to 16 degrees ( numbers that pop up in Heywood, Saab
etc  quite frequently ).   

Consistently placing the peak over a fixed point makes as much as ** practical
** without a research lab.  

One of the practical problems is that spark advance for mean best torque or
mean best power is through a relatively broad range of 3 to 5 degrees advance.
Generally set by factory or others to just a RCH  ( Red C Hair ) or a tab bit
retarded ( ~` 1 % power drop over peak ) to make maximum power and stay out of
denotations.   This relatively broad range reduces the sharpness of the point
to move peak to and gives you latitude to just stick it at 16 ATDC and be done
with it.  Not saying there might not be some more available - just think it
might severely tax most peoples resources to find it more bettor - for maybe a
limited return over getting it close and consistent.  

>----------------------------
>
>Date: Tue, 15 Jun 1999 22:45:16 -0500
>From: Tom Sharpe <twsharpe at mtco.com>
>Subject: Re: Combustion Pressure Sensor - revisited
>
>Can measuring crankshaft acceleration per degree dewtermine the sweet spot
>and how close you are to it???  TomS
>
>------------------------------
>