Dist Gear life and failure conditions Q.

Bernie Benz b.m.benz at prodigy.net
Sun Aug 4 15:54:55 EDT 2002

Dave, I'll add my .02 to Derek's .01.

If you read my 8/3 20V Distributor Gear Fab Program Summary sent to both 20V
lists, you know that the gear transmits no (steady state) torque and that
any material that can tolerate the enviroment for an acceptable period of
time is an acceptable gear material for this app.  Audi/Bosch elected to use
a poorly designed injection molded plastic gear for this distributor,
"poorly designed" being the problem, not the fact that it is plastic.
Further, Audi/Bosch elected to not sell parts for this distributor, only
full replacement distributors. (Apparently, at one time in the distant past
the plastic gear had a P/N assigned and price of $135, but to my knowledge
was never available from either source.)  Early replacement distributors had
the plastic gear, current replacements (using the same P/N!), all(?) have a
machined steel gear.  So apparently, they have wised up to their problem,
but continue to milk the dist. replacement business.

Stock plastic dist gear life IMO is at least partly dependent on the
condition of the particular valve train driving it.  Many 20V owners have
complained of a condition commonly known as "distributor rattle".  Dist.
rattle occours in the excessive gear mesh clearance between the plastic dist
gear and its driving cam gear, but IMO is caused by a torsional resonant
vibration of the cam drive system, i.e. the resonance between the timing
belt spring rate and the high inertia of dual cams. This causes a cam rotary
oscillation which peaks at the torsional resonant frequency of the system,
and causes an oscillating torque to be applied to the dist gear.  I've never
had the problem on either 20V, but as I recall, others have said that
"distributor rattle" peaks at about 2800 rpm?  Never had "distributor
rattle" I believe because I run a timing belt tension that is tight enough
to insure that the belt is operating at its maximum spring rate, resulting
in a higher resonant frequency and lower angular vibration amplitude, thus
valve actuating friction may be sufficient to critically damp the otherwise
resonant system.  This is my theory as to why I have never had "distributor
rattle" with the plastic geared distributors, while many others have.

More that you ever wanted to know?  Sorry about that.


> From: "Derek Pulvino" <geargp at hotmail.com>
> Date: Sun, 04 Aug 2002 11:32:09 -0700
> To: drk150 at psu.edu
> Cc: b.m.benz at prodigy.net
> Subject: Re: GP Confirmation and Payment
> Dave,
> Yes, to my knowledge it is related to using plastic on the gear, and that
> plastic getting brittle over age.  As to mileage and occurence, I think
> within the gp, about 10% of the respondants reported having problems with
> their plastic gear, although I don't have enough data to say a mileage this
> occurs at.  Age may have as much to do with mileage.  Quality of the
> replacement gear is as good as the stock metal gear, and better than the
> plastic that's is currently on your car (wrt to tolerances and consistency).
> We used a metal geared distributor as the model for the production run,
> and more or less replicated it-exactly.
> As to a recall, you'd have to ask Audi for their reasoning.  By my guess it
> is probably because a broken gear doesn't pose a safety hazard/injury
> potential to the owner/user of the vehicle.    Also, bear in mind that on
> all new distributors that Audi sells for the 20v turbo and NA engines, a
> metal, not plastic gear is included.
> I've also copied this to Bernie as he's a little more versed in the
> engineering world than myself.
> Derek p
>> From: "David Kraige" <drk150 at email.psu.edu>
>> Reply-To: "David Kraige" <drk150 at psu.edu>
>> To: "Derek Pulvino" <geargp at hotmail.com>
>> Subject: Re: GP Confirmation and Payment
>> Date: Sun, 4 Aug 2002 01:28:30 -0400
>> Derek-
>> I'd still like to know what mileage this tends to happen at, and how many
>> times it has happened.  It sounds pretty freak to me.  Is this basically a
>> result of the plastic getting brittle with age?  If it is a big problem,
>> why
>> has Audi not addressed it under a recall?  Finally, what is your confidence
>> in the quality of this new metal gear?  Is there any chance that it will
>> put
>> extra wear on the gear it bears against?
>> Any info would be great, thanks!
>> -Dave Kraige
>> '91 CQ, 89k miles
>> ----- Original Message -----
>> From: "Derek Pulvino" <geargp at hotmail.com>
>> To: <QUATTRO650 at aol.com>; <auditude at cox.net>; <tonygj at bww.com>;
>> <knotnook at traverse.com>; <theloves at localaccess.com>;
>> <thequattroking at yahoo.com>; <maximum at weetamoo.com>;
>> <Sales at IntendedAcceleration.com>; <tmullane at snet.net>;
>> <steve.mills at comcast.net>; <Eric_R_Kissell at email.whirlpool.com>;
>> <redrandy at pacbell.net>; <brett at cloud9.net>; <thebikefreak at yahoo.com>;
>> <Djdawson2 at aol.com>; <drk150 at email.psu.edu>; <termite at rocketmail.com>;
>> <wolff at turboquattro.com>; <BLEVINSBK at aol.com>; <chris at force5auto.com>;
>> <bfoster at bu.edu>; <SigDesign at aol.com>; <dfreed at bear.com>;
>> <info4mjs at worldnet.att.net>; <david at ingledene.fsworld.co.uk>;
>> <stevebednarski at mindspring.com>
>> Sent: Saturday, August 03, 2002 7:00 PM
>> Subject: GP Confirmation and Payment
>>> Hey all,
>>> All of a sudden there's a lot of you.  I've created an email group with
>>> everybody whose resonded for this next batch to ship so if you've seen
>> an
>>> email similar to this before, please disregard, and if you've already
>> paid,
>>> disregard as well.  This should cover most of the questions people have,
>> if
>>> you have a question that is not covered below, please let me know in
>> your
>>> response with the below requested information.
>>> -------------------
>>> The gears are still available.  Following should suffice.  Mailing
>> address
>>> is:
>>> Derek Pulvino
>>> 12517 NE 134th PL.
>>> Kirkland, WA
>>> 98034
>>> Cost is $42 per gear in the US.  I will send out instalation
>> instructions
>> to
>>> people as things are shipped.  I'm hoping the next batch will be shipped
>>> within 1-2 weeks from today.  With this many people again though, it'll
>>> probably be closer to two weeks.
>>> If you are going to move forward with this purchase, please respond via
>>> email with a confirmation of interest (we intend to hold you to your
>> word
>>> when you respond with a ready to buy), how many you'd like, and a
>> mailing
>>> address and I will reserve a piece(s) for you. This is then contigent
>> upon
>>> recieving payment.  For payment, a check at the above address or via
>> paypal
>>> (dbpulvino at hotmail.com) should work.  If you choose paypal I ask that
>> you
>>> include an extra $2 to cover their service charge. Had to do a "business
>>> account."  If you're interested, ASAP is turning into the M.O., as
>> response
>>> has been pretty good in the last day or so.
>>> Let me know if you have any other questions.
>>> derek pulvino
>>> _________________________________________________________________
>>> Chat with friends online, try MSN Messenger: http://messenger.msn.com
> _________________________________________________________________
> Chat with friends online, try MSN Messenger: http://messenger.msn.com

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