Distributor Geometry

[Dan Cordon]

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