Re: Flywheels

[James Montebello <jamesm@domain.elided>]

On Sun, 13 Oct 2002 AlfaNeely@domain.elided wrote:
>        I just wonder how or why the tach counts the flywheel teeth.  There
> are 131 to count for each revolution whereas counting only one trigger per
> revolution would be easier even for a computer.

Computers are very fast, and most of them would spend a lot of time
waiting for the next flywheel tooth to come by even at 8000rpm (about 57
microseconds).  At idle, it would be waiting just about forever.  One good
reason to use a frequent signal is that you don't have to wait long to
know how fast the engine is spinning.  If you only get one signal per
rev, you have to wait two revolutions before you know the engine speed.
With the flywheel, you only have to wait for two teeth to go by to get
a reliable fix on speed, so you update the engine speed 65 times per rev.

The early Motronic systems probably didn't try this, but with that
sensor system and modern computing power, there are many interesting
things you can do with that kind of information.  By knowing how much
engine speed varies during one revolution, you have an indication of
how well the engine is running.  Misfires, for example, show up as
sudden changes in engine speed that would average out over the course
of an entire revolution.  Knock would also show up this way, allowing
you to back up the signal from the knock sensors (which are only tuned
microphones) to better identify actual knock (as opposed to valvetrain
noise, or something else).


>        I am also curious as to exactly how the trigger is attached to the
> flywheel.  How did you remove yours and how did you reattach it?
>        I have thought about moving it to get more ignition advance on my race
> car.  But it would probably be easier to switch to a normal distributor with
> a quicker advance curve and get rid of the ignition computer.

The trigger on the flywheel is just a bolt.  The sensor is (most likely,
I don't know for certain) a Hall-effect transistor backed by a magnet.
When a ferrous object enters and leaves the field of the magnet, it
triggers the transistor.  A small coil and magnet will do something
similar, except you get a sine-wave signal that varies with the distance
of the ferrous object.

Either way, you can advance or retard the ignition by moving the sensor or
the bolt.  "Getting rid of the ignition computer" isn't possible unless
you get rid of the injection computer, too (all one unit with Motronic).
You could also simply call up one of the many places that know how to
reprogram Motronic to have any advance curve you like installed.  While
you're at it, you can have the fuel curve changed, too.  Alternatively,
you can buy one of the aftermarket self-programmable systems like Haltech,
Electromotive, SDS, etc.

james montebello
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