Boost and TDC and distributors

Bernie Benz b.m.benz at
Sun Jun 23 16:29:54 EDT 2002

Thanks for the correction, Mike.

Assuming that you are correct and I have no reason to doubt you, inasmuch as
I've not had to delve into it as yet, that the Hall sensor produces one
pulse per cycle, Then it is reasonable that the ECU uses this to select the
comp stroke TDC and then could/may very well switch to the speed sensor
pulse train for timeing thereafter.  A different system than I envisioned,
but this difference does not change the rational for Dan's proven ability to
run with the dist assy, rotor and housing, rotated 180 degrees from its
normal oriention on the engine.

> From: "Michael Sylvester" <mike.sylvester at>
> Date: Sun, 23 Jun 2002 17:54:06 -0400
> To: "Bernie Benz" <b.m.benz at>
> Subject: Re: Boost and TDC and distributors
> Obviously you have no understanding of this.
> The shutter wheel in a 20vt (10vt as well) has only one window.
> Rethink your theory.
> Mike
> ----- Original Message -----
> From: "Bernie Benz" <b.m.benz at>
> To: <QSHIPQ at>
> Cc: "200q20V mailing list" <200q20v at>
> Sent: Sunday, June 23, 2002 4:43 PM
> Subject: Re: Boost and TDC and distributors
>> Scott,
>> I've not tried it, but my intuitive logic says that you are wrong in
> saying
>> that the engine will not run if the dist is 180 off and the crank is 360
> off
>> relative to what had been Compression TDC, which I believe was Dan's case,
>> and which he proved that it would run!
>> My logic:  The ECU sees only two engine timing inputs,
>> 1. the timing reference pin sensor, which comes around twice an engine
>> cycle, once every engine rev.
>> 2. The distributor Hall sensor, producing 5 pulses per engine cycle, every
>> two engine revs.
>> From these two inputs there is only one point per ingine cycle at which a
>> distributor pulse is coincident with a crank pulse, and the ECU will
>> consider this unique occurence to be #1 cyl.Comp TDC, inasmuch as it knows
>> nothing of the relative physical placement of parts.  Thus, the ECU will
>> control and fire the engine on either crank TDC, always the one to which
> the
>> distributor is timed.  Therefore I believe, and Dan has so proved that
>> engine can function properly with the distributor located 180d form its
>> normal oriention!
>> Smile Scott, you can have the last word.
>> Bernie
>>> From: QSHIPQ at
>>> Date: Sun, 23 Jun 2002 15:37:41 EDT
>>> To: 200q20v at
>>> Cc: dans at, b.m.benz at, urq at
>>> Subject: RE:  Boost and TDC and distributors
>>> Gentlemen:
>>> If you ONLY removed the distributor and left the cam and crank TDC's
> intact
>>> (as described), you CANNOT put the distributor in 180 off, or it won't
> run.
>>> You can physically put the distributor in any way that fits, you just
> need to
>>> rotate the body until you have rotor pointing at number 1 (window plate
> hash
>>> mark and nick in dist aligned).
>>> If you are 180 out you will have the distributor 180 degrees from the
> nick on
>>> the outer lip when the crank shows TDC -0-.  When this happens, you
> can't
>>> start the motor, because the timing reference pin and the dist window
> need
>>> this condition for Motronic to fire the car.  AFTER the car is started,
> the
>>> dist window is ignored, so you can rotate the distributor and the car
> will
>>> continue to run, but should toss a code.
>>> Ned and I had several discussions on TDC windows over the years, mostly
> on
>>> the MC and early urqs.  At one of the Grattan track events in the early
> 90's
>>> there was an urq that wouldn't start when warm.  What happens in the
> urq's
>>> (and could happen in any car that is teetering on the edge of the dist
>>> window) is that the head warps slightly causing the distributor to
> misalign.
>>> Car still runs, but at hot shut down and restart the window was out of
> whack.
>>> Car cold, no problems.  Did this on my '84 when I first got it too.
>>> WRT 20vt stuff Dan, you could have too much slop in the gears (metal or
>>> plastic), or you were just on the edge of the window.  That said, when
> you
>>> install the distributor, many times you will find that the TDC reference
> hash
>>> are off a bit sometimes causing a "perfect" tdc reference to be not so.
> Your
>>> procedure of moving the distributor CW and CCW is what most of us end up
>>> doing with the same problem.
>>> I'm not at all with the thinking that you were 180off in timing Dan,
> but -0-
>>> on the distributor is +/- 2 degrees if service tech manual recall
> serves.
>>> That's a 1% rotational error.
>>> HTH
>>> Scott Justusson
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