Re: alfa-digest V8 #187

[James Montebello <jamesm@domain.elided>]

On Tue, 30 Oct 2001, alfa-digest wrote:
> It sounds like you may be describing a desmodromic valve.  If so, they're 
> hardly new technology.  Back in the late 70's I had a Benelli 750sei and my 
> friend had a Ducati 900 SS 'Desmo'.  The 'Desmo' designation referred to 
> its desmodromic engine valves, and I recall as we were writing our checks, 
> the salesman mentioned that the desmo valvetrain was a Ducati invention 
> (Dr. Fabrizio something, I think).  Anyway, I was too anxious to fire up 
> all six cylinders of the new Benelli to argue at the time, but I beleive 
> Mercedes-Benz had previously used a desmo valvetrain in an F1 car of the 
> 50's.

Actually, Eric was describing something else completely.  The valvetrain
he mentioned isn't reciprocating, but involves semi-spherical valves
which rotate to open and close the ports.  Similar idea to the sleeve
valves employed as early as the 1920s, while trying to solve some of
the longetivity and sealing issues of those valves.  The jury is out on
how much better they really are to conventional poppet valves.  Note,
btw, that the Wankel rotary was itself originally designed as a valve,
later a pump, then finally an internal-combustion engine.

As for desmo valvetrains, Dr. Ing. Fabio Taglioni is the name you're
grasping for.  He designed the two-valve desmo valvetrains used on some
Ducatis starting in the mid-50s (initially racers only, but quickly
applied to their sporting production machines).  The Mercedes design
was different in detail, but the same basic idea.  For the uninitiated,
a desmodromic valve system has no valve springs, but instead uses two
cams per valve, one conventional one to open the valve, and another to
close the valve.  Typically, both cams operate through rockers.  In the
Ducati design, the closing rocker is forked on the valve stem end, to
engage a washer-like hat on the valve.  In the Mercedes design, it's a
pin that engages a hole in the valve stem.  The first known desmo-type
system appeared before WWI.  I don't know that it ever saw production.
In their heyday (roughly prior to 1975 or so), a desmo system allowed
much higher revs without valve float or spring breakage than contemporary
spring designs.  However, they're very picky about tolerance, requiring
frequent checking and careful adjustment.

These days, metallurgy has improved to the point where a conventional
spring design can perform at least as well as a desmo system, with a
far less complicated (and much more compact) mechanical arrangement.
For hyper-revving engines, like F1 engines, pneumatic systems are better
than both.  Ducati still uses the desmo system exclusively on all of
it's bikes, including production street machines, but it's basically a
marketing exercise, not for any technical superiority.  Mercedes used
the system only once, on the straight-8 found in both the 300SLR sports
racer, and the W196 F1 car, in 1955.  

james montebello
84 gtv6
74 berlina
(once had an 87 Ducati 750F1, with desmo valves)

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