[Vwdiesel] Help - messed in the head...Tolerances my boy...

[James Hansen]

Well, we're probably talking about gaps in the range of a hundred thou at
least.  Gotta allow for rod growth at rpm, piston hot expansion, valve
timing off by  few degrees in relation to the crank...  Crank acceleration
isn't that sudden in the grand scheme of things considering the volks has a
considerably large rotating mass in relation to the amount of power it
produces. If you could make the rotating assembly really light and balanced,
then it would be a lot snappier, but still, the valve train doesn't present
that great a load.  Boost isn't that sudden either, lots of time to
accelerate the rest of the system...  really, cam belts impress me with what
they can do for as long as they do.
New volks motors are using chain now. (just watched the '06 Jetta promo DVD
last night)
I am actually amazed just how many people change timing belts without
loosening the cam sprocket and indexing the cam to TDC and get away with it
completely. Tells me there is a lot of room in there (NOT!) or the cam has a
lot of lobe separation and no overlap at tdc,  which makes sense as more
separation gives better low end torque anyway.
But let's say you were right at the edge of cam being off and a few thou
away from valve hitting pistons.  You pull the head one day, and see valve
marks on the piston tops, but know that the engine has never had a belt
failure.  The marks are probably from when you ran it exceptionally hard up
a big hill with the valve timing off just enough but not enough to hit
normally- the piston grew just enough to tickle the valve without actually
destructively clobbering it.

I know a guy that built a race engine that ran a zero decked block, with
just enough room for the piston top governed by the head gasket thickness...
so he thought.  He wound up with enough rod growth at rpm, that the top ring
land cracked off over time.  Didn't help that they were Keith Black pistons,
that have the top ring very close to the piston crown.  Still it popped an
engine over a couple thou of rod growth... that sucked. Figuring out just
what happened was another thing entirely... it seemed to just get busted one
day.
-James

-----Original Message-----
From: vwdiesel-bounces at vwfans.com [mailto:vwdiesel-bounces at vwfans.com]On
Behalf Of Mark Shepherd
Sent: Sunday, April 17, 2005 1:56 AM
To: vwdiesel at vwfans.com
Subject: [Vwdiesel] Help - messed in the head...Tolerances my boy...


I'm in agreement with you James...
Here's a senario:
If we're only talking about a few thou gap and head skimming
reduces that...
How much can someone turn up the boost before the sudden
acceleration of the crank causes enough delay in the timing
belt to cause Le Crunch?
MTM(UK)
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