Distributor Geometry

Bernie Benz b.benz at charter.net
Tue Dec 9 19:19:09 EST 2003

Nice work, Dan.
I was too lazy to do so, rather trying to goad the "narrow minded" into
performing the exercise.  But you took up the cause for them, may they be
forever indebted.

Removing the excess rotor width on the trailing edge, beyond the full retard
area seems pointless inasmuch as the correct plug would have relatively low
impedence under that condition, thus no tendency to crossfire to the
previous plug, which would be well into its exhaust stroke anyway so no
possibility of crossfire damage in tht cyl.

Thanks, Bernie 

> From: Dan Cordon <cord4530 at uidaho.edu>
> Date: Tue, 09 Dec 2003 15:38:13 -0800
> To: 200 List <200q20v at audifans.com>
> Subject: Distributor Geometry
> Not that I want to add any nitro to the fire, but here's some
> distributor geometry I did with dimensions from a 'gearless' (with
> narrow factory tip rotor) distributor I have.
> The ignition wire contacts in the cap are roughly 5mm wide. The radius
> to the contacts is right at 25.4 mm. With 5 poles in there (assuming the
> distributor cap is 360° around) that leaves 72° between contacts.
> However, considering the width of the contacts, there's really just
> 60.7° between the contacts.
> With a 11 mm tip rotor, there becomes a 35.9° sweep available between
> when the trailing edge of the rotor leaves a contact, and the leaving
> edge arrives at the next. But, for the spark to jump to the wrong plug
> wire, we're really concerned with the half-way point between the
> intended contact, and the wrong one...which still gives us 2 * (half the
> sweep) + contact angle. So, with an 11 mm tip rotor, this yields about a
> 47° window for tuning before the spark is statistically likely to jump
> to the wrong plug. Since this is distributor degrees, this is really
> about a 90° tuning window relative to the crankshaft .
> With a 17 mm tip rotor, the size does decrease significantly. It comes
> out to be 33.7° distributor degrees, or 67° relative to the crankshaft.
> All things being equal, these windows are *much* greater than the
> advance we're running on any of our audis. But there's far more that
> influences the decision of where the spark goes. You'd really have to
> look at the relative impedance seen from the rotor to the ground path to
> figure this out. It's here where high boost (harder for spark to jump)
> and a worn out wire could really throw things off. After all, the few
> cases of spark hop didn't happen all the time....more like 1 out of many
> million times. With those odds, the spark is almost equally likely to
> jump clean out of the car and shock the nearest pedestrian <grin>.
> One thing that I haven't heard noted are the potential advantages of a
> wider rotor. A 17 mm tip rotor stays in contact (within the air gap
> anyway) with a 5 mm post for almost 50° of distributor rotation. A 11 mm
> tip only has that contact for 36°.
> Since the higher modded cars are running *more* advance than stock, it
> seems logical to me that you would want to KEEP the wider tip on the
> leading edge - thus having the rotor tip closer to the contact when
> advanced. Then, since you're not any more retarded than stock, you could
> file/dremmel/sand/etc. the trailing edge off of the rotor. Doing this
> would net you ~7° more rotor-cap contact on the leading edge (or 14°
> crankshaft reference.....close to the amount of additional advance the
> modded engines are running).
> Anyway, I just thought I'd share my thoughts on the matter. After all, I
> haven't many reasons for 11 or 17 mm tip rotors, and I've definitely not
> heard *anyone* recommend removing just the trailing wing. Makes sense to
> me....make sense to anyone else?
> --
> Dan Cordon
> Mechanical Engineer - Engine Research Facility
> University of Idaho
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