Dual (or triple) valve springs can allow for greater
valve lift, and/or more pressure, and/or less stress
to the spring.
Coil springs will 'bind' if compressed too far, this
is a function of the wire diameter and length of the
wire (shorter wire gives more space between coils for
a given uncompressed height).
Smaller wire diameter to accomodate more lift means
less pressure, so we use multiple springs. Two seems
like a practical limit, tho' some get away with
triple.
Shorter wire for more lift and greater pressure means
more stress in the spring wire, leading to shorter
life. Some drag race springs are only good for a few
runs before they lose pressure and must be replaced.
Carter
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