[200q20v] dubious distinction?

Bernie Benz b.m.benz at prodigy.net
Sat Feb 2 14:54:14 EST 2002

Phil Rose <pjrose at frontiernet.net> wrote:
> I guess there's quite a bit of variability encountered in t-belt
> condition-- even for comparable age (and/or use). This might be due,
> in part, to the wide geographical variations in ozone level.
> I suspect that mileage correlates less-well with belt-breakage than
> does the belt age (unless the failure is induced by another failed
> component--like the distributor gear).  Bernie, you seem to be
> saying that it _will_  be OK to replace the plastic distributor gear
> _before_ it breaks?  You're  not saying this to just make us feel
> good, are you?

Of course, I'd do anything to help make you feel good, Phil.  But I suspect
that my help would be redundant, as apparently you have your "feel good"
thing well in hand.

Seriously, (if possible) I'd like to expand on the variability issue that
you raised.  I believe that the three cam drive(n) components, timing belt,
timing chain, and distributor drive gear suffer an interrelated variability
in reliability, caused by a (potential) resonant torsional vibration problem
in the cam drive system.

Why does the distributor rattle phenomium exist early, or at some point in
the life of some 20V engines and never in others?  IMO, distributor gear
breakage is allowed, not caused by the plastic gear material. The steel
geared distributor solves the gear breakage problem but apparently not the
rattle.  (A 20V lister has replaced 3 in warranty steel geared distributors
in attempting, unsucessfully, to eliminate the rattle.)  Maybe luckily, I
have been spared any resemblance of dist. rattle in either of my 20Vs.

Why do some 20V timing chains suffer (lists documented) wear to the point of
needing replacement, while others apparently never do?  Using the finger
thru the oil filler cap test technique, my chains are both as tight as a
drum at 150K.

IMO, both the distributor rattle and chain wear problems, as well as timing
belt life variability, are caused by a resonant torsional vibration problem
in the cam drive system.  The only varible among different 20V engines
affecting this torsional vibration system is the input spring rate, the
timing belt tension.  A low belt tension will both lower the system
torsional resonant frequency and also allow for a much increased torsional
vibration amplitude. This increased vibration amplitude and the associated
higher angular acceleration forces, IMO, is the root cause of this 20V cam
drive train problem. This problem is not evident in the 10V equivelent
system because it has only half the cam inertia, thus doubling the torsional
vibration frequency and pushing it out of the critical range, even with a
loose belt.

Thus, a tight timing belt should have a longer and more consistant life than
a loose one, while also preventing the other cam drive system rattle and
wear problems.

Keep your belts tight, kids!


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