Date: Wed, 02 Jul 1997 00:25:55 -0600
From: Will Wood <wkwood@burgoyne.com>
Subject: Re: Singleport vs. Dualport (Sunday! Sunday!)

All valve springs suffer from harmonics, as the spring is compressed the
metallurgy wants to push back, this causes the spring to be a spring.  Each
spring for a given size load and compression and rate of compression will
have a certain harmonic vibration associated with it.  Typically, the larger
the spring wire, the stronger the spring the higher rate of compression and
more compression it will take before excessive harmonics come into play. 

When excessive harmonics come into play, the spring starts oscillating this
oscillation results in a reduced amount of pressure the spring exerts.  My
boss used to work on software that was for ship engines, ya know massive
springs with literally hundreds of pounds of valve spring pressure.  The
cams for these ship engines had to be designed very carefully, otherwise 
the harmonics involved for the spring would cause them to break.  The
same thing holds true for a car engine, for a given spring load, lift
and rate of compression (cam ramp and RPM) will cause the harmonics and
oscillation to come into play.  When this happens, the valve doesn't fully
get the seating pressure it needs since one force is expanding the spring
and the oscillation is causing the spring to want to vibrate.  This causes
the valve to float or stay partially open.  This happens with cams with
fast ramps or high RPM.

Stronger springs resist the oscillation factor and allow you to run
more RPMs and or a faster ramped cam.  The Ramp is the rate at which
the valve goes from closed to open to closed and obviously has a direct
effect on the valve spring.

Why is this important in a bus?  Heat.  Heat reduces the effectiveness of
the spring in such a way that the oscillation or floating factor happens
and a lower RPM or with a comparably slower ramped cam that would be used
in a bug.  A bus does run hotter than a bug, no question about it.  The
engine has to produce more power for a given speed, weight and air and
everything else comes into play.  

St. Berg has numerous articles about it, but all high RPM racers and
every street bus I build now has at least high rev singles and in 
some cases duals.  The 2110 I have in my 64 has dual high rev springs, the
single high revs I originally installed wouldn't allow me to get up over
5500RPM easily, now I can rev to my hearts content.

Valve float causes a lot of problems from broken valves and keepers to
busted retainers and springs.  This also contributes to early seat failure
as well.  The trick is balancing the needs of the engine and it's design
with the amount of spring required.  If you never will see over 5000RPM
use single high rev springs.  I say this because the stock springs in the
busses have suffered from float at as low as 2500RPM.  That's slower than
freeway speed for most of them.

Naysayers will tell you "heavier springs require more horsepower", which
has some truth to it, but it is mostly bunk.  When you eliminiate float,
you provide better combustion, better valve life and your engine will work
better and produce more power.

Another naysaying is "Heavier springs cause the valves to break" again bunk.
Think about it, your forcing the valve to follow the cam precisely, the
cam is what closes the valve and it doesn't let the valve slam onto the 
seat!  This promotes better wear on the cam and lifter surfaces as well
since the lifter isn't flying away from the cam, they're maintaining
good contact.  As always, adjust your valves regularly.

You also can use singles or duals with stock aluminum timing
gears.  Using straight cut gears does reduce horsepower demands and cam
thrust bearing wear, but proper clearance checking of the thrust bearing
will allow full life of the cam bearings using stock gears.  Triple Springs
are available from GBE, but do require straightcut gears.

So, if you have a bus or use a non stock cam or want to run at higher RPM,
you should change your springs.  Single High Rev springs are about $35 bucks
and can be installed in under 1/2 hour.  You can rent the tool from a local
rental place and DIY.  High Rev duals require machining of the bosses
in the heads and isn't for the average DIY.

Will Wood

67 Standard Microbus
63 23 Window Deluxe
64 Crew Cab - Frankenstein
61 Single Cab - Annette (You guys figure it out)
61 Convertible
57 Oval Window Deluxe Sedan
56 Kombi
67 Westy (Parting out Smashed), 67 Deluxe rolled, 63 Kombi Taco-shelled
59 Panel Van
80 Rabbit X 2
93 Eurovan MV (Wifes)

Hey, what the heck, I've got to keep the weeds down somehow!