Re: TDC II

[QSHIPQ@aol.com]

In a message dated 98-12-10 12:06:04 EST, you write:

<< <<A couple degrees either side of TDC has been found by many, I have
>measured no more than 2, but that's plenty to create quite a difference in
motor >output.  >>
 
>> 	And the timing gear on the cam contributes to the confusion.  I have
several of >>the 10v gears in the shop.  Lining up the keyways and looking at
the teeth >>provides an interesting insight.  The teeth do not line up.  Some
are the same, >>some are 1/2 tooth off, which means some will "automatically"
be off by a certain >>amount.  Just wish I knew which cam sprocket was
"correct".
>> 	As it is now my current engine uses the "super" belts so I guess it
doesn't really >>matter if I know or not, huh?   ;-)
 
>> 	Later.
>> 	--ml<FONT COLOR="#000000" SIZE=3> >>

Ok, good point, lets list the potential variances to TDC in a given audi motor
*  crank bolt machining
*  flywheel bolt hole machining
*  flywheel tdc pin location
*  flywheel "O" location
*  Flywheel pin pickup location
*  cam belt stretch
*  cam gear tdc location
*  cam woodruff key tdc location
*  tdc alignment tooth location on transmission
   - motor to transmission mating tolerances
*  distributor tdc location

Bottom Line:  TDC is approximate at best.  To really get an "exact" tdc of the
crank, you would need to have the flywheel attached, the transmission on, and
the head off the car.  To translate that to camshaft tdc, you would need to
have the head on the block with the belt attached, and measure the cam lobes
for tdc.  The cam tdc would be a constantly floating target however, cuz each
time you changed the belt/water pump or even adjusted belt tension, cam tdc is
affected.

My own thinking is that audi accepted some of the variance already.  The
distributor window for tdc alone is a documented +/- 2 degrees from tdc.  

We do know that the car does run at flywheel "O" to distributor "O" (sometimes
that can give you the 2312 code though, btdt, so even that might be an
overstatement), regardless of the 'exact' accuracy of the cam tdc.  Given the
above factors, I'd say that TDC has a pretty wide production tolerance in a
given engine family.

Mike, I too have seen a couple degrees in the cam gear.  That's enough to have
one component on a turbo car explain the "that's a strong motor" comment I've
heard many a time.

My .02

Scott Justusson
QSHIPQ@aol.com

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