Re: Non-interfering valves - was timming belt.

[bbell@csn.org (Bruce Bell)]

>> An earlier post said that the 8 and 10 valve normally aspirated engines
>> were non-interfering.  What about the 10 valve turbo (MC) engine?  I
>> would hope that it's also non-interfering, since its compression ratio
>> is much lower than the normally aspirated engine.



For the 8 and 10 valve engines - turbo, or non turbo - compression ratio is
a non factor if you have factory style "dished" or concave pistons.  On a
TQC this dish is nearly 1/2 inch deep to provide the 7:1 Compression ratio.
What is an issue is the perimeter of the piston. This lip on the perimeter
is the highest part of the piston and is 8mm + wide. You will only have
problems if the valve and the and the piston collide at this point.  How can
this happen if the valve, when fully opened, is still completly inside the
head? Well, it's not supposed to, unless you are Dan Bocek then you get to
bend all 10. :(

In Dan's case as I remember the thread, fill in willya Dan, his engine had a
big cam lots of "lift" and "duration" which puts the valve closer to the
piston for a longer time. Also the TQC have very large valves relative to
the diameter of the bore. This helps put the valve in even greater conflict
with the lip on the perimeter of the piston. Finally Dan's motor made lots
of power at high RPM where, on occasion a weaker valve spring can allow a
valve to "float" when you are at the limits. 
 
So, I'm saying 8 & 10v engines are non interfering.  So dont worry about
replacing your timming belt. Heck I only had to walk 3 miles, the tow truck
gave me a good price and the wife wasn't too disapointed we couldn't go out
to dinner *NOT*.  And last but not least the risk and worry isn't worth not
replacing the belt earlier. And, yes I'm going to check to make sure I
didn't bend a valve.  

Do you bowl Dan? I still can't figure out how you got all 10 without leaving
a split.:)  


Bruce

'87 4000CS Quattro 

P.S. My jetta has (had,was) 82.5mm bore, 40mm valves, and a solid lifter cam
w/11mm lift and it was non-interfering. Oh yes, the block had been milled
.040" (1mm) and the head another .015" in order to achieve a 9.75:1 CR.