[Misc] Effect of timing belt vs. chain on overrev damage

[Knute Ream <knute@xxxxxxxxx>]

Michael Tarver writes in response to over-rev damage:
[...]
>Likewise, it is near impossible to bend the valves because there is a
>timing chain and NOT a timing belt on E36 M3 engines.
[...]

It seems to me that the belt vs. chain issue related to bending a valve is
limited to situations where the belt stretches or breaks (chains are less
prone to do so); valve-bending from overrevs is a different animal, and not
directly related to belt vs. chain.

Way back in the dawn of time (50's), one of Mercedes' impressive
engineering marvels was the 300SLR, which sported desmodromic valves.
These suckers had no valve springs whatsoever, and instead used cams to
push the valves in both directions.  This required constant maintenance to
keep everything in tune, but completely eliminated valve "float" and
permitted the engine to rev to unheard of RPMs.  (Other problems obviously
exist at obscene RPMs, but for this engine, valve float wasn't one of them!)

In modern engines, valve float is controlled by the stiffness of the valve
springs, but there's an obvious trade-off.  The stiffer you make the
springs, the less valve float there is, but stiffer springs rob valuable
horsepower.  (Note: that's one of the reasons that the "eta" engines have
such low redlines; they have soft valve springs...) Redline is not only a
good idea, but too far above redline on an interference engine means that
at some point the valve springs will not be strong enough to retract the
valve before it becomes part of the piston.

When this same subject came up on the digest about a year ago, somebody
pointed out that many older American V-8s (my old Ford Galaxy did this)
have a built in rev-limiter because the valve float robs enough power to
prevent the engine from revving high enough for more damaging symptoms to
appear.  ;-)

Anyway...

- -Knute
PizzaFest II survivor

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