It's been a thought on my brain for a while: "How can I determine if I'm
using too much or too little timing? "
I remember someone joked about a "timing master" program, but that was a
while back. Well, a couple short circuits in my brain fired and I thought
of this. I read something like this on Bruce Plecan's page a year ago and it
took all this time for it to click.

These are a few things I believe to be true:
1. Higher vacuum equals better gas mileage, thus better engine efficiency.
GM uses MAP KPa to indicate the pressure of the intake plenum, so lower Kpa
means higher vacuum (everybody got that?)
By finding the spark advance which causes the lowest MAP value at idle we
find the top fuel efficiency. (make sense?)

2. More timing eats more fuel, so there will be a point where the MAP value
starts to rise again, finding the lowest MAP value will help us determine
that we're not using too much timing.

How to do it:
1. Warm the car up, write down the following data with the car idling
    1.MAP:
    2.Spark Advance:
    3.RPM:
    4.Value in matching closed throttle spark advance table (rpm and map):
2. Now, lower the value 4 degrees from the original at the idle RPM values
(800,1200) , re-flash the pcm, record the above data again.
3. Raise the value 4 degree from the original at the idle RPM values
(800,1200), re-flash the pcm, record the above data again.
4. Review the data, which direction gives you a lower MAP value? Try going
another 4 degrees in the same direction, does the MAP increase or decrease,
dial it in several times until you get the lowest possible MAP (best gas
mileage.)

Expand your thinking:
Now, given that we play with the idle first, this should start to get us
familiar with this methodology.
Now, let's realize we can do this for all the spark table (matching up the
RPM vs TPS, and the MAP value at that point.)
This can be done for the entire RPM and Map range, providing you can record
the values with any data logger, import the data into an Excel pivot table.
(note: avoid the overdrive gears (5th and 6th), as they may cause the engine
to lug, or skew things a bit.
1. drive for a while trying to hit the entire RPM range and TPS range, the
more data points the better.
2. import the data into an Excel pivot table (plot TPS vs. RPM and the MAP
value)
3. Scale the entire table down 5% - repeat driving and pivot table
4. Scale the entire table up 5% from original- repeat drive and pivot table

Again you are looking for the minimum MAP value at the same TPS% and RPM,
some may need more, some less, make changes in the needed directions, and
try again, repeat to optimize, the pattern will become clearer and the
timing should get nailed down.

Things to remember:
35 Kpa is generally where most cars cruise (stay same speed), below that the
car is either going down hill, or decelerating (slowing down the engine by
pulling timing at low MAP values uses the engine as a brake assist.)

Hope this helps get some gears turning, maybe Joe Georger can write a proggy
to manipulate this methodology too, but I'm thinking it will take a while.

Good Luck and share your results,
-Christian

Christian Millard
Application Developer
Crutchfield Corp
www.crutchfield.com <http://www.crutchfield.com/>