From: agwiii@fang.att.com (Grayson Walker (660-7949)(12/92))
Newsgroups: rec.autos.sport
Subject: Re: Desmo Debut Delayed
Date: 29 Sep 92 13:51:57 GMT
Organization: AT&T Tax Systems Development, Maitland FL

Chris Prael writes:
>: The sole reason for multivalve heads is to increase the cross section 
>: (area) of the valve opening, thus increasing the volume of air/fuel 
>: mixture that can be moved into and out of the cylinder.  Valve cross
>: section is also the sole reason for the twin plug head design that Alfa
>: Romeo did in the 60s 

Absolutely and fundamentally wrong! First, there is no single reason for
multivalve heads. Instead, there are multiple reasons why an engineer
would design a multivalve head. The motives of the designer can only be
understood in the context of the technological limitations of the day.

In the early days of 4-valve heads (perhaps 1900 - 1915), one of the
major limitations was the metalurgy of the day. Larger 2-valve heads had
hot-spot problems and warpage.

By the 1920's 4-valve was considered obsolete. The state of the are in
the 1920's was probably Jano's 6C-1750 and 8C-2300. These had the head and
crankcase as one unit -- to deal with the day's sealing problems. You 
can't blow a head gasket that does not exist. These were supercharged
2-valve designs. One of Jano's innovations was the "mushroom" design
follower.

In the 1930's, the Rudge 4-valve was considered anachronistic when it
was introduced. It took what was considered to be a retrograde step to
4-valve design. The problem that Rudge was trying to solve? Cooling the
head, particularly the exhaust port area. (The Rudge looks like a
verticle twin to the neophyte. It has an exhaust pipe for each exhaust
valve.)

By the 1950s, the world had changed. Motorcycle racing was dominated by
the great Italian marques, such as Gilera and MV Agusta. Honda entered
the world championships in the 50, 125, 250 and 500 cc classes. To get
more power from these small atmospheric engines, it is necessary to spin
them faster. One of their limitations was valve bounce and valve float.
(The little 50cc triple spun past 20,000 rpms.) It was the need for
accurate control at these very high rotational speeds that took Honda
to multi-valve heads. The smaller valves could be controlled with
greater accuracy. The smaller the valve, the lighter the weight and
the faster they could spin the engine. (Honda have published some
techincal papers in this area.)

This is exactly the problem desmodromic valve address. Same problem, but
a different solution to the problem of valve bounce and float.

> Ummm, maybe for Alfa, but it's not the only reason for twin plugs.

Right Conrad.

There are also concerns about flame propogation that can lead
a designer to multi-plug heads. However, one of the other reasons is
that multi-valve heads cause less than optimal placement of the spark
plugs. The designer compensates by fitting two plugs. The Hondas
faced this problem and often resorted to developing smaller than 
standard spark-plugs, sometimes 2 per cylinder.

>Refer to rec.motocycles for a (current) discussion of twin plug heads.
>
>: and valve cross section is the primary reason that
>: short stroke engines produce more horsepower that do long stroke engines
>: of the same displacement.
>Primary?  Only at the same RPM and ignoring power loss/ ring float due
>to excessive piston speeds.

Primary? Short stroke engines *can* produce more horsepower than a longer
stroke engine of the same displacement if you spin it faster. Given the
same rev. limit, you may actually see higher power from the longer stroke
engine because of lower pumping losses.
-- 
Grayson Walker                                           CHC, Maitland, Florida 
Forza!                                            Internet: agwiii@fang.att.com
Voice: 407 660 7949               ". . . big brown river . . ." Tuli Kupferberg