Re: Non-interfering valves - was timming belt.

["Dan Bocek" <dan@di.com>]

>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. :(

Actually, I didn't have any bent valves at all, much to the surprise of the
mechanics.  What happened in my case was that I had a valve spring
shatter on me in the head.  Some of the debris from the shattered spring
got underneath the cam follower bore, so on the next revolution of the
cam, the follower hit the spring debris so the follower then shattered, 
which generated even more debris, which .....

This resulted in broken cam followers running up and down in the bores,
which scored the hell out of the bores.  The head ended up being a 
complete loss, owing to the bores on #4 & #5 intake and exhaust being
chingered beyond repair.

Why didn't any of the valves hit the pistons?  According to the folks at
Sport Wheels, the turbo Audi engine *IS* of the interfering variety.  Those
dished pistons are only "dished" in the center of the piston.  It turns
out that the valves are of sufficient diameter (even in a stock head)
to hit the piston on the perimeter of the dish - where the piston height
is the same as (well, maybe even greater than) a normally aspirated
engine.  I didn't get piston damage because on the turbo heads (and maybe
normally aspirated ones as well) each valve has two springs - an inner and
an outer.  With the #4 intake inner spring shattered and the outer spring
broken in two, I guess there was still enough spring pressuer available
to activate the valve enough to preserve the piston.  I was extremely
lucky.  If, however, you break a timing belt on a turbo Audi, YOU WILL
GET INTERFERENCE.
-- 
                                                   
                                                    Dan Bocek
                                                    dan@di.com