Date:         Fri, 20 Apr 2001 18:10:20 -0600
Reply-To:     Damian Shaw <dts@XMISSION.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Damian Shaw <dts@XMISSION.COM>
Subject:      Re: vanagon Digest - 20 Apr 2001 - Special issue (#2001-478)
Content-Type: text/plain; charset=us-ascii

Blake,

The distributor is setup for basic timing and then there are secondary systems to adjust the timing based on other conditions.  In the case of your Digijet Vanagon you have a vacuum advance (and centrifical?) that advances the timing based on the intake's vacuum.  This is a cruder system than the modern setups.  Vacuum and centrifical advances, besides providing an
estimation of required advance, suffer from being mechanical.  They can jam.  Miss-fires that end up sparking into the distributor housing (common place for it) cause lumps and welds in the mechanical advances and either stop them from moving or cause them to no longer be smooth.  Also the vacuum supply line can get a leak in it.

The Digifant system is called an 'engine management system' for a reason:  it controls ignition and fuel injection together.  Rumour is VW named it Digifant where 'fant' means 'elephant' because it was such a huge undertaking for them to build.  It's a great way to get better emissions, fuel economy, and power.  In Digifant's case it isn't the best implimentation of
full engine mgmt system but more modern ones (Motronic) work quite well - apparently VW quit trying to build their own after Digifant and went back to using full Bosch systems again.  Much to most VW-enthusiasts' delight!

Most modern motronic cars have a TDC sensor on the flywheel and the computer can put the timing wherever it wants regardless of the distributor's position.  With this the timing is actually set with a computer link-up to the ECU!

Middle-of-road system were used on some VWs with CIS-E FI systems.  (A lot (maybe all) of the VW 16V engines used CIS-E with a knock controller ignition.)  In these the ignition advance was controlled entirely electronically but still independent of the FI system.  The use of a knock sensor with these allowed the ignition to be advanced as far as desired until knock
was detected and then retarded until it went away.  Effectively adjusting for fuel octane, engine temp, etc.  Very cool.

Distributor-less cars currently come in two flavors:  one coil pack per cylinder and 1/2 that many.  In the 1/2 that many type two cylinders recieve spark at the same time (from the same coil pack):  one that needs it at the top of its compression stroke and the other at the top of its exhaust stroke.  This was a way of economizing on the number of coil packs.  High
performance Chevrolet engines (and I'm sure others) have a coil pack per cylinder.  One of the biggest advantages to distributorless life is that the spark is not compromised by the distributor cap and rotor.  These devices break down and don't deliver as true or clean a spark as a direct coil-to-sparkplug setup would.  Plus with the coilpack you can get rid of the
high-tension spark plug wires, or at least shorten them.  The wires are another common source for ignition problems.

Enough already?

Damian