[AMC-List] FW: Re; AMC 6 Ignition timing --Got It!

["John Elle" <johnelle@xxxxxxxxx>]

Tom and all
 
Snip
And of course as you start pulling a load the vacuum advance will retard
the spark at any given rpm depending on the amount of load which
translates into reduced vacuum until you get down to zero at which point
the only advance would
be mechanical and at 2100 rpm that would be 16 -20 degrees.
 
Yup! And this case (no vacuum) is where performance tuning can be done.
The factories were very conservative with these
numbers to save themselves a lot of work and ruined engines, and gas was
35 - 75 cents/gallon...
Snip
 
Except in those cases which do exist when at extremely high rpm's
running the timing a bit later improves performance, however these
circumstances do not reflect street applications and are unusual in
themselves.
 
However performance tuning is a singular criterion. On the street you do
a lot of partial throttle acceleration and the timing needs are
different than at wide open throttle for optimum efficiency. For the
most part you can use more advance at a given rpm in order to optimize
fuel economy for those conditions. Where as wide open throttle at the
same rpm would require later timing.
 
For example the timing needs at a flat and level highway to maintain
say, 60 mph can be done with quite a bit of advance. This will give you
the best fuel economy too. However hit a 6% grade or any grade you will
have to increase pressure on the throttle to maintain 60 mph thus
changing the fuel ratio. As changing the fuel ratio will also change the
burn rate, the timing will have to be retarded a bit in order to not
ping. This scenario could repeat with an increasingly large hill until
you are now wide open throttle just to maintain 60 mph and all of this
time the timing would want to run progressively later in order to
eliminate pinging and associated piston damage. Keeping the timing at a
fixed number that would in and of itself eliminate pinging would cause
fuel economy to be less than optimized.
 
In the early motoring days when fuel was even cheaper, say 11 cents a
gallon the average automotive engine had a manual spark advance operator
control. Many times as a part of the steering wheel. There was no
automatic control, vacuum, centrifugal or otherwise. 
Even early tractors in the very early teens had a water injection option
available to minimize pinging and improve power. It used radiator water
to do so. Remember these were the days when compression was a heady 3:1
on a good day and the same engine could run gasoline, kerosene or
alcohol with about the same performance or reliability. It was not until
some where in the late 20's or early 30's that Eythel or high test
gasoline was developed to help support the "new" high compression
engines of about 6:1  
 
 
Snip
> Depending on the year of the car or the distributor, the recommended
> vacuum source could be either ported vacuum or manifold vacuum, and it
 
I have some TSMs, I should go look, but I bet t wasn't until
late 1960's/early 70's they started the "ported" business.
Snip
 
According to my  AMC TSM collection that appears to be 1970 although I
am sure other manufactures may have used different time frames for the
switch, or used ported vacuum right along for quite some time prior to
that. 
 
I'm still interested how the curve comes out. 
John. 
 
 
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