On Wed, 1 Oct 2003, Alex Towner wrote:

> 1) how is it possible for a Motronic computer - on a car with a
> distributor - to adjust the spark timing for each cylinder
> independently? The ignition rotor passes a contact for a given cylinder
> on the distributor cap, which in turn sends voltage to the spark plug
> for that cylinder - we all know this. How could the computer adjust this
> mechanical process for an individual cylinder?

The position of the distributor rotor does not set the timing in any
meaningful way.  It only has to be close enough for the spark to jump the
gap to the proper terminal.  If you look at a typical rotor, you'll see
that it has a wide terminal on the end of it.  The timing of the spark is
controlled by when the ignition coil fires, not the rotor position.

The reason that turning the distributor on a car with a points or
hall-effect ignition system changes the timing has nothing to do with the
rotor.  It's because you're changing the position of the base plate that
the points (or sensor) are attached to in relation to the lobes on the
shaft, and thus changing when the ignition coil fires.

Consider an ordinary vaccuum advance distributor.  You might have, say, 30
degrees of total advance.  What moves?  The cap doesn't; it's fastened to
the housing, which is bolted to the engine.  The rotor doesn't; it's
firmly screwed to a shaft, which goes to a gear drive.  The only thing
that the advance moves is the base plate inside the housing.

In the Motronic system, if I understand correctly, there is no hall effect
sensor or points inside the distributor.  The engine computer fires the
coil itself, depending on a crank position sensor for reference.  That
means it can time the spark whenever it wants, and even vary it from
cylinder to cylinder.  The distributor just routes the spark to the proper
cylinder, it doesn't control timing

David Brodbeck, N8SRE
'82 Diesel Westfalia
'94 Honda Civic Si