forward for Jim Steck (having email problems) re: cambelt discussion

["John Hertzman" <johnhertzman@domain.elided>]

 I want to comment on Dave's analysis of the timing belt layout on the V-6
Alfa engines.  Most I agree with, especially the insight on underhood temps.
causing premature idler / tensioner bearing failure.  Up to this point, no one
has discussed the wild fluctuations in torque required to drive a cam with
only three lobes on it.  This problem is the real downfall of belt-driven
cams. A valve event (from fully closed to open to fully closed) takes about
2/3 of a revolution of the crank.  Since we're talking about a 4 stroke-cycle
engine, three consecutive events fit perfectly into two full revolutions of
the crank. The torque required to drive a cam with 3 lobes varies much more
than it does with the 6-lobe cam of the 12-valve engine . . . as there is no
overlap of valve events.  This isn't a problem in a pushrod V-8 or V-6 . . .
there is enough overlap of valve events that the drive torque is almost
constant.
 Now consider the difference in drive torque from cam to cam and you can see
that at any given moment, large tension variations exist along the entire
length of the belt.    Compared to a chain, the timing belts are more elastic
. . . and in the case of the V-6 Alfa, much longer.  The belt tooth is also
shorter, so a smaller change in length will allow the belt to skip timing.
Combine, the small tooth, long belt and serious drive torque variations . .
.it's no wonder the 4-cam Alfa engine is notorious for skipped cam timing. The
Toyota solution with two cams driven from one pulley, or the 12-valve Alfa
engine with 6 lobes per cam, will have significantly reduced variations in the
belt tension when compared to a direct-drive 4-cam V-6.

 Personally, I don't like the idea that such a critical item requires
replacement every 30,000 miles.  IMHO the cam belt arrangement on the V-6
Alfas is marginal . . . not in the spirit of Alfa Romeo . . . the rest of the
engine is nearly indestructable.  The timing chain / tensioner in the
4-cylinders is not elegant, but will last well past 100,000 miles.  That's
what I want when buy an Alfa Romeo.

 Regards,

Jim Steck

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