More Distributor thoughts - long and nerdy

auditude at auditude at
Wed Dec 10 11:26:41 EST 2003

Hi folks,

Where was all this knowledge and perspective about timing slop back when I was trying to develop a way to run standalone programmable engine management off the stock MAC11 (10vt) style crank triggers?

My posts are somewhere in the main list archives.  I wanted to use the combination of 62 degree BTDC pin, flywheel teeth, and distributor hall sensors to run a LinkPlus as opposed to using only the hall sensor and flywheel teeth.

My focus was eliminating the slop from the timing belt and cam gear, but I could hardly find anyone that agreed it was significant enough to bother.

I thought it would be easy (for someone who knows how to do this type of thing, not me at the time) to make a "black box" that would combine the timing pin signal and hall sensor so that a signal would be sent to the LinkPlus only when both were in sync.

But I gave up on it since it was only an idea, and later on Link's distributor here in the U.S. decided not to honor their published introductory sale price which I agreed to pay.  So to hell with them.  Now there's 034EFI IIC, and I've got a Motronic ecu anyway.

Sometimes I think it's funny how I can ask about something and get no response, or an opposite one, and then later theres a huge thread about it.  Sometimes "timing" IS everything, I guess. =)

Here's on of my futile posts about the stupid topic.  Can you tell I'm jaded?  :P


p.s.  I like the idea of something to run direct ignition off the 3B distributor.  I'm no engineer, but if I can help let me know.

QSHIPQ at QSHIPQ at wrote:
> 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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