Re: Re: rotor for 20v turbo engine

["Brandon Hull" <brandon@cardinalventures.com>]

Wait a second.  What we have here, I think, is an example of a post run
amok.   The issue of correct 20vt rotor size is just the sort of esoterica
that the list is made for.  However, the most recent posts indicate that the
thread has taken on a life of it's own, and departed from the very
interesting facts of this case.

Fact 1:

The proposition that the wrong sized rotor can cause cross-firing, leading
to catastrophic engine damage comes from one of the highest authorities in
Audi-dom:  Lehmann.  They replaced a blown engine which had been modified
with their components, citing cross-firing as the likely culprit.  They gave
the owner explicit instructions to use a particular rotor, and not
experiment with aftermarket ignition wires in this very high HP engine.

Fact 2:

Audi/Bosch once spec'd a narrow rotor unique to the 20vt, which they still
supply attached, when you buy a distributor, (Bosch) PN:  1 234 332 414 R1.
At some point, Audi/Bosch quit listing this number, and spec'd another
rotor, PN: 1 234 332 350, which thanks to Scott Mockry we know is also the
replacement part for the 1989-91 200, 1989-91 200 Quattro, 1985-87 4000,
1986-88 5000 Turbo/Quattro, 1988-90 Audi 80, 1990 80
Quattro, 1990-91 90 Quattro, and the 1990-91 Coupe Quattro.  It is also used
on:  the 1993-94 Hyundai Scoupe, several Volkswagen vehicles and one Volvo
application.
The "original" rotor, 414, is considerably narrower (by nearly half) than
the 350.  It apparently has no other applications, hence it's
discontinuance??

Fact 3:

Clearly, a too-wide rotor could cause cross-firing.  Just imagine a rotor
head so exaggeratedly wide that it spanned two contact points simultaneously
at the moment the ignition fired.

The theory:

I concur completely with the comments below, as far as they go:  Spark is
controlled electronically, using mechanical and electronic inputs
unconnected to the distributor.  Loosen the distributor retaining clamp and
twist the distributor, and everything will run fine, unlike the mechanical
ignition systems of old.  However a too-wide  rotor is a mechanical/physical
condition that the ECU doesn't have any way of knowing.

I haven't studied the firing order and valve train to see whether there is a
condition where the rotor may, for example, pass a cylinder on it's intake
stroke just before triggering spark at the next on it's power stroke.  But
it sure seem possible that at very high RPM, with the spark advanced further
and further, and a rotor which is too wide, a spark could leap to two
cylinders simultaneously.  (Firing one cylinder appropriately on it's power
stroke, and the previous cylinder inappropriately during it's compression
stroke)  Phil's comments on the resistance differences between rotors adds a
new dimension I hadn't realized, and may also be relevant to this theory.

Anyway, whether this position is right or wrong, this is the general field
of debate on the rotor question.  We are all in agreement regarding the
relative function of the ecu, timing pins, and distributor.

A final note:  The "wrong" rotor is running just fine on a lot of 3B
engines, including mine.  The risks here, if they truly exist, are probably
with very high HP engines with modified ECUs, higher rev limits, radical
timing maps, etc.

Brandon Hull
'91 ersatz S2

>> I believe the position of the rotor has absolutely no bearing whatsoever
on
>> when the ignition fires.

>Absolutely correct.  Timing is derived by complex >software algorithms
>that primarily use the flywheel pegs and teeth to >provide accurate
>information on the crankshaft position, and the Hall >Effect sensor to
>differentiate firing from non-firing TDCs.  The >distributor merely
>picks which of the plugs is going to get the spark - it >doesn't
>determine when.

>May I add a word during ignition discussions?

>A lot of the components specified by Audi have >resistive functions -
>some rotor arms and plug caps have 1k ohm or 5 k >ohm nominal resistance.