More Distributor thoughts - long and nerdy

Tue Dec 9 23:30:34 EST 2003

I'll start here, since I agree it's not the backend spark I'd be worried 
about.  Great report on the distributor measures Dan.  My concern in reality would 
be a few fold.  First, you have accounted for no slop in the distributor gear 
OR the cam chain (remember, the distributor fires off the intake cam which is 
connected to the exhaust cam, which is *then* connected to the crank via a 
belt.  Think slop.  Add all slop together, I propose it allows very little for a 
plastic gear.

My next concern would be how crossfire might manifest itself.  One might 
propose that if those contacts in the rotor center and the distributor fingers get 
corroded enough, then I believe the same problems Bosch encountered with the 
99cent "platins" might actually manifest themselves.  At 40k distributor 
volts, I would expect the (next) spark to jump to the point of lowest resistance.  
IMO, that just might be the leading edge of a rotor to the outside (non 
contact portion - read clean/uncorroded) edge of the next finger in the distributor. 
 Similar to the "platin" problem in that as pitting developed the spark 
tended to travel up the side of the electrode (remember the platins were just a 
coating of ceramic, not a "casing" per sae.  I won't use the term "carbon 
tracking" but it sure is tempting boys...

Historically, when the points ignition systems went HEI (read:  GM and Ford), 
the Distributors got huge/wide and the design of rotors became part and 
partial to reducing the propensity for crossfire isssue when doubling ignition 
voltage.  VW-audi/Bosch, on the other hand, freely changed from low energy spark 
points based ignition systems to high energy ignition systems, with NO 
appreciable change in distributor cap OR rotor design.   A 6mm trim up of the rotor in 
1991 20vt only: a bandaid really.  A really bad one when you add in slop.

Take out the slop, any slop, the problem of crossfire *should* be less.  
Remember, crossfire happened without a *broken* plastic gear necessarily.  That 
indicates to me that the right combination of slop factors in the right 
circumstances could very well add up summarily to disaster.  I suspect that addressing 
<all> slop in unison is the safest thing to do (physically replace and 
inspect tension on TB, Cam gear chain, and Distributor/gear).  IMO audi went away 
from distributors for good reason, they wanted to keep the slop in the rest of 
the design parameters that ultimately resulted in distributor slop, in a bad 
distributor design.

I also think you will see a progression of less slop in Motronic up to the 
Camshaft position sensor = multi coil fire ignition.  If you look at the v8 for 
instance, you will see that the distributors are run from the exhaust cam, 
which is directly connected to the Timing Belt and auto TB tensioner (auto ign 
slop adjustment for the service life of the belt + no cam chain slop included).  
Then as the AAN (new 20vt motor) came about, the CPS is also connected 
directly to the timing belt  (albeit with manual TB tensioner) via exhaust cam.  I 
see these designs as progresive ways to not only reduce ignition related slop, 
but also eliminate cam chain tension slop.  Needed btw, if the distributor and 
rotor system design remains 30+years old in a HEI.

Ah, you might say :)!  What about the 30v v6 cars and the 1.8t?  Their 
respective CPS are back to intake cam driven.  Yes, they are, but the 
electro-hydraulic cam chain tensioner all but eliminates (or should before *it* fails - 
don't ask) the problem of chain slop.  IOW the computer can compensate for it.  
And NO distributor.

I'm no expert on any of this stuff.  I just enjoy watching how engineers at 
audi do things progressively and/or regressively.  In the case of crossfire and 
motronic, they addressed it in the v8 and not the 20vt for those first couple 
years.  I believe this is because audi already knew that direct fire ignition 
was on the way, and future designs (up to and including today's multivalve 
heads) were already addressing the problems inherent to crossfire ignition.

Summary:  Motronic turbo finally dictated a 30 year old distributor cap and 
rotor design obsolete.  Redesign the cap and rotor or go multicoil?  Thank 
goodness *something* good came of all this.

My .02 arbitraged thru the peso

Scott Justusson
QSHIPQ Performance Tuning

In a message dated 12/9/2003 5:47:58 PM Central Standard Time, 
cord4530 at writes:
I did just have one more thought on the distributor thing.....

For engine damage to happen, the spark would realistically have to jump
past the current post, and light off the next one (leading edge). If it
were to light off the previous plug (trailing edge) that should be a
moot point, since that plug just fired 144° ago relative to the
crankshaft. It's still on the power stroke with the exhaust valve
opening soon.

Firing off the *next* plug would probably be just after the intake valve
closes, which could be a problem...If this was the likely scenario, then
people would want to file off their leading edge :o)

It seems that the real solution to this confusion would be to eliminate
the distributor all together. Maybe I should design that coil-on-plug
conversion for the 20v after all :o)

Dan Cordon
Mechanical Engineer - Engine Research Facility
University of Idaho

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