Re: Timing Belt

[AHKai@aol.com]

In a message dated 96-10-12 00:21:06 EDT, you write:

This from R&T Tech Tidbits:

There are 2 different kinds of engine designs, free-running and interference,
these differing in what occurs if camshaft rotation becomes less than perfect
(ie, belt breakage).  If the timing belt breaks in either design, controlled
camshaft rotation ceases and the valve action winds down in whatever
orientation the cam dictates.  In the meantime, the pistons reciprocating
motion continues, quite independent of the valves orientation.

If the engine design happens to be a free-runner, then pistons and valves
keep out of each others way as the engine ceases operating.  The car need
towing, the timing belt needs replacing (like the '87 GTi in downtown
Detroit).

But I'll bet you recognize why an "interference" design has that name.

Yep.  If a timing belt breaks on an interfenrence engine, pistons, and valves
fight for simultaneous residence in a cimbustion chamber-and both lose.  So
do you.

-I can testify to this, while hanging around my ex-mechanic's shop waiting
for my car (pressuring him to get it done-maybe that's why it never got fixed
right...)I was checking out a car that had it's cyl head pulled after the
belt broke.  Not a pretty sight.  A few of the pistons were beaten up pretty
well, and most of the valves were bent.  This one didn't break at that low of
an rpm.  If you think you should wait to replace your belt, one look at this
would convince you otherwise.  It was some japanese car so it wasn't that big
of a deal though.-

Now, I hasten to add that there's nothing conceptually wrong with an
interference design.  Indeed, it might give the best combination of
compression ratio, valve timing, bore/stroke ration, combustion chamber
geometry and a passel of other things.

Nor are belts particularly inferior to chains for camshaft drive.  For one
thing they're quieter.  And, these days, they're durable as well.

Which brings me full circle to the point of all this.  Any automotive
component has an average life expectancy and some sort of statistical
distribution related to this.  Automakers evidently set replacement intervals
based on these criteria.

Typically, with a lot of cars the replacement interval for timing belts is
60,000 miles.  The cost is nontrivial, perhaps $300 or more (or more), as it
involves a fair amount of labor just to gain access to the belt.

So here's the game:  If your car is a free-runner, your stake is getting
stranded (BTDT), a towing fee and the attendant hassle.  If your car's engine
is an interference design and the belt breaks, you downside is immensly
greater.  Maybe you want to, er...cover it.

Here's a list of some popular interference engine designs, gleaned from
Beck/Arnley Worldparts newsletter, which in turn references Importcar
magazine:
(Just a select few)
-Acura 4,5 and V-6
-Audi 1.6 and 2.0 diesels
-Honda 4
-Infinity 3.0 V-6
-Porsche
-VW 1.5 and 1.6 diesels
-Volvo
-Yugo (why waste the money on a new belt, buy a new car)


<< Can anyone give a clear explanation of what happens when the timing 
 belt breaks on an interference type engine.  Also what are the 
 advantages of this engine design? >>