Ignition Theory

[James Montebello]

Thanks for the detailed replies on basic ignition theory.  I was
more than a little embarrassed about forgetting all about vacuum
advancers, and characterizing engine load-sensitive ignition as
"new".

I've been fooling around in the motorcycle world for several years
now, and such things as vacuum advancers and even distributors are
nearly absent there, and have been for quite some time.  Direct
"distributorless" ignition has been the norm on bikes just about
forever.  Newer bikes do indeed use throttle-position sensors for
ignition control, even on bikes still equipped with carbs (which
means most of them).  Up until very recently (3 years), most
bikes still used a fixed ignition curve based strictly on engine speed.  The
changes brought about by the addition of load sensors
are quite dramatic, esp. in terms of fuel economy for a given
level of performance.  Prior to the change, a typical 600cc
sportbike would make in the neighborhood of 90-95hp (at the
crank) and get around 40-45mpg.  After the change, power rose
to 95-100hp (mostly a compression bump), and economy jumped to
50-55mpg.  All on 87 octane pump gas, with no pinging (and no
knock sensor).

Now that I'm working on a project car again, I'm having to relearn
a good many things, like how to deal with that big, whirling device geared
off the crank that has one big wire going in and six coming out. :-)

So, while it's obviously hard to make generalizations, it appears
as though ignition is, in fact, *usually* retarded as load (i.e.,
manifold pressure) increases (or advanced as pressure decreases,
depending on how you want to look at it).


james montebello