101 clarification

[Bruce Plecan]

>Light is slowly dawning.  Between Ludis' disassemblies and Bruce's
>emails, I'm getting this 747 thing figured out.  It's starting to light
>a fire under me to convert my '78 monte carlo to EFI...
>
>I have another question for the 747 gurus.  I'm adding Bruce's
>description of how to convert table values to real world values.  He
>wrote, in part:
>
>Spark Advance
>      data byte *  90/256);     (This give 0-90 degress)
>
>Bruce, in your tuning tips table you had some negative values in the
>spark table.  How does that work? 

There is a spark adder, called main spark bias, which moves the
entire table up or down in value.   So lets say your at 0 degrees
advance with a spark bias of 10 degrees.   If you drop the spark
bias to 0 then your initial timing of 0 becomes -10.   There are some
engines that are fussy about the initial timing, and spark bias.
Generally I find alot of engines like 6d initial, a spark bias of 6.
Now just to really foul things up/and/or confuse them there is a cold
spark bias.  Normally I have a fair amount there (15ish).  Then with 
the above 16-20d BTDC idle timing. 
    Whiether this actually pushes the timing to AFTC I don't know, 
till after I get the bench stuff running.  From test driving it seems to,
but I have trouble setting timing by seat of the pants. 

 Also, this is a newbie question, the
>0-90 value is before TDC, right?  ie a value of 30 means 30 degrees
>BTDC?

Yes, and yes.
>
>thanks,
>--steve
>
>
>--
>Steve Ravet
>International Meta Systems
>http://www.imes.com
>steve at imes.com
>