Spark Advance Engine POWER!

[John Brewer]

Given the close relationship between ignition timing and fuel metering, I
decided to post this to this list as well as the OffRoad Mailing List, hope
you all don't mind.  Comments would be appreciated.

I've been doing a little light reading (always dangerous) on Internal
Combustion, Spark Ignition Engines.  I have discovered that, theoretically,
for peak power and fuel economy, the spark should be advanced (degrees
BTDC) as far as possible until the engine begins to knock, then of course
backed off a little- you don't want it knocking.  Also, for peak power and
performance, the vacuum and centrifugal advances should be in good working
order and should advance the spark under load or RPMs.  However, in
American cars, the vacuum advance is (and has been since about 1972) a
vacuum *retard*.  This does reduce NOX emissions but it is detrimental to
power and mpg.  Now, I noticed on my '78 Toyota FJ40, that the spark
'advance' assembly has two ports.  Typically a spark advance unit looks
kind of like this:

                       /---\
                      /  :  \
                     /   :   \
vacuum connection---|    : o  |---mechanical link to dist.
                     \   :   /
                      \  :  /
                       \---/

The dotted lines in the middle are where the rubber diaphragm is inside.
My FJ40, and I'm guessing everyone elses as well, has a second port
pointing straight up where the 'o' is in the figure above.  This port is
plugged with some kind of rubber sealer.  I'm guessing that this port leads
to the plenum on the other side of the vacuum advance unit and I'm guessing
that if you removed the plug, plugged the other side, and hooked manifold
vacuum up, voi'la, you'd have yourself a real spark advance in addition to
being in violation of federal emissions laws!  *Neat*  I suppose Toyota,
since they sell TLC's all over the world, and only the US requires a
retarded vacuum advance, manufactured only one unit with two ports.  They
plugged one of the ports depending on to what country they were shipping
the vehicle.  Of course, I'd never monkey with the assembly as being in
violation of a Federal law is unthinkable to me but I was simply wondering
if anyone else has tried this.  Of course you would never keep it that way,
it would be purely for experimental purposes and switched back to SOP as
soon as you were convinced that there was a difference :)

John Brewer


"Any man who would trade liberty for security deserves neither." -  Ben
Franklin