FW: Driving an MSD Ignition Box

[Zublin, Bryan (SD-MS)]

This is my first posting to diy_efi.

> A scope trace diagram (without units) shows the multi-spark output
> versus the output of a conventional ignition. The MSD box is shown as
> producing 5 sparks for each one produced by the conventional ignition,
> at 1,000 RPM. Each of the multiple sparks is shown as having similar
> voltage and duration to the conventional spark. As RPM increases, the
> number of sparks decrease. I can't find anything in the literature to
> confirm this, but I have heard it said that above 3,000 RPM, the MSD
> boxes are only giving one spark.

I did some measurements a while back on an MSD 6A.  I looked at the output
with an oscilloscope while varying the input frequency (RPM) with a
function generator.  I plotted the number of sparks vs. RPM for a
4 cylinder engine.  I'll post the data hopefully tomorrow.  As I recall,
the number of multiple sparks drops off quite quickly as RPM rises.  And,
compared to 4 cylinder applications, 8 cylinder applications will drop
off twice as fast.  The only way I see of improving this is to use two MSDs
instead of one, with each firing half of the cylinders.

I also have a schematic of the MSD 6A, obtained the brute force way
(reverse engineering).  It is consists completely of discretes, no
ICs.  I guess I could send someone a copy if they were interested.


Bryan Zublin
General Instrument, San Diego, CA, USA
bzublin at gi.com