[alfa] Timing chains versus cam belts

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Little Alfa content here but....................

A couple of years back Toyota introduced its new 4 cylinder engine for the
Corolla. In a local magazine there was a big writeup of how this was the first
Toyota engine for about 20 years that used a timing chain instead of a belt.
Among the many virtues extolled for the chain was, lasts longer, quieter,
lower maintenance etc etc.

I have a friend who works for the local Toyota factory who occasionally comes
and races with the local hillclimb competition.  I asked him for his opinion
on the new engine and mentioned the magazine article.  He laughed and said it
was written by the marketing guys as a selling ploy.  The real reason they
went back to a chain was because with the front wheel drive layout it was
necessary to loose 40mm off the length of the engine to get it to fit into the
engine bay.  The twin-chain was the easiest and cheapest solution!

As far as power robbing components of an engine I think the chain is probably
the lowest, taking into account the "peace of mind" factor.  Timing chains in
an oil bath is a very reliable method and with a correctly designed tensioner
gives long trouble free life.  Incidentally the AR 4-cylinder in line engine
does not have a self adjusting tensioner, the lock bolt fixes the chain
tension once the spring has pushed the chain to the optimum position.  Many
other engines that use timing chains have a curved spring steel element backed
with a nylon rubbing element that gives adjustable tension with engine
temperature.  This design was also used in the Jaguar Le Mans engines with
perfect success.

On my Fiat Uno Turbo, which has a single OHC with a toothed belt drive, the
idler pulley is also a locked tensioner.  The engine construction is typical
Fiat, it being a derivative of the old 128 engine, with a cast iron block and
an alloy cylinder head.  Timing belts have a very limited stretch, much the
same as a chain as the internal fibres are non extendable.  If the belt starts
to stretch the belt is not long for this world!

Incidentally I remember reading somewhere about the first car to use a toothed
belt for the cam drive, I think it was the Pontiac?  The manufacturers of the
belts strongly advised against it as the belts were not designed for an
oscillating load, the sort that a cam shaft with all its wobbly bits would
impart.  Anyway Pontiac went ahead and used it and it became the de facto
standard that everyone else had to copy to keep up with the game.

Keith Duckworth, the head of Cosworth Engineering, looked at the use of timing
belts for his F1 engines and rejected them as the slight elasticity inherent
in them upset the valve timing events.  Instead he opted for a complicated
train of gears to drive the 4 camshafts.  Noise was not an issue in the F1
engine.  Jaguar similarly started off using a multiple gear system for the Le
Mans quad-cam engine and then a three chain layout, it was replaced with a
single Triplex chain on the final version.

John
Durban
South Africa
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided