Re: 5-Cylinder Timing Belt

[Peter Orban <ORBAN@nrcamt.iamt.nrc.ca>]

>> It may well be that the valves don't interfere with each other - but this is
>> surely quite normal on a single-camshaft engine.  Losing the belt on a single-
>> cam engine can never cause the valves to interfere, since the position they
>> adopt has to be a valid position defined by the cam.  What happens on a 20-
>
>Not really.  The valve timing of a piston engine is such that the valves are
>full open when the piston is away from top dead center.  At the top of the
>compression stroke, both valves are closed; at the end of the exhaust stroke
>both are slightly open (the "overlap" period).  It's easy and fairly common
>to design the engine where a full open valve will be hit by the piston, but
>it would only happen if the crank and cam get out of sync, as when the cam
>drive fails (belt, chain, gear, whatever).


I guess both of them are true, but it could be put a bit more clearly.
Valves can interfere with the piston, as the case with the 10V turbo engines.
The valves can also interfere with each other if the layout of the valves
allows for that, these are engines where the combustion chamber is
hemispherical. In this case the valves can be actuated by twin cams,
like Fiat Spyder, belt driven, or Alfa Romeo, older 6 cyl. Jaguar, chain
driven. If you loose your cam drive there, then the valves can hit each other
as well as the piston. There are also engines with single cam and hemi heads,
like the older BMWs. If your cam drive goes there, the valves will not
hit each other, though they still could hit the piston. If you over rev
an engine like that, and the valves start floating, i.e. they do not follow
the cam when they are closing, valves still could collide with each other.
The VW 16V head is twin cam, the exhaust driven by a belt, and the intake
cam driven from the exhaust by a chain. There usually the belt fails, the
chain not, and then the valves hit the piston. 16V owners religiously change
their belt at 60KMiles. I am not familiar with the Audi 20V head, but I
would not be surprised if it was like the VW head.

Hope it helps, Peter
orban@nrcamt.nrc.ca