Double valve springs

[Stephen Albrecht <albrecht@xxxxxxxxx>]

The entire purpose of double valve springs is to forestall valve float.
I recall my 1970 Honda 250 quad valve had double springs, so this is not
a new technique.  Yes the two springs are wound in opposite directions,
and they are concentric meaning one spring is nested inside the other. 

Any oscillating system will, at some specific frequency, reach a
standing wave condition. In a valve system at the standing wave
frequency (RPM), this means the valves remain fairly stationary, some
open, some closed. Power output obviously takes a nosedive when this
condition occurs. Each of the two springs has its own standing wave
frequency, and each spring is designed so that when one of the springs
is experiencing the standing wave, the other isn't. If we can assume
that the valve train designer has done his homework correctly, this
translates into a substantially higher RPM point before valve floating
(standing waves) occurs. This is one of the many factors that allows
motorcycle motors and Formula One motors reach such high revs. The cam
profile design is also an important factor in this equation. In order to
run at super high revs (>12K RPM), the cam must open and close the valve
very quickly. however, it mustn't allow the springs to slam the valve
against the seat. Also, very high valve acceleration rates require very
steep cam lobes, and that translates into high wear rates for the
lifters/followers. As in anything else, the design of a valve system is
a compromise of many tradeoffs between power and longevity. Newer
materials in the contemporary engines help with the longevity side of
the equation which is why BMW can reliably pull 282 HP from a 240 cu.in.
motor. That's well over the 1 HP/cu.in. figure of the 60's and 70's. 
Then there is piston velocity to consider, but that's a subject for
another time.

I hope my long winded post wasn't too boring.

Steve Albrecht
Golden Gate Chapter
'95 540

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End of bmw-digest V9 #595
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