comnoz said:
I have asked a couple experienced people that same thing and got the impression that cylinder pressure has very little affect on the valve action except when a blower is involved.
That would make sense to me as the only time there is any real pressure involved is for the exhaust valve opening where there is often still 100 psi in the cylinder. Jim
This may change your mind:
A reduction in crankshaft inertia can also cause a loss of horsepower, and sometimes
not a small loss. Same thing goes for the inertia of camwheels.
This is worth a read:
"TIOC May 2005
Eaten Alive by Parasitic Oscillations
by Kevin Cameron
As a new hi-fi amplifier is designed by an electronic engineer,
particular care must be taken to be sure that unforeseen combinations
of resistance, inductance, and capacitance do not permit so-called parasitic
oscillations; to build up, destroying the music.
Each property of electrical circuits has a mechanical analog, and one
mechanical system that is often rife with its own parasitic
oscillations is the valve train. Rocker arms bend, pushrods compress
and expand, camshafts deflect between their bearings, and valve stems
flop from side to side while valve heads deflect like trampolines or
floppy disks. All of this is invisible to us. Its symptoms are valve
seat recession and valve spring and valve breakage. In many cases,
after tests with a new cam that gives really good power, we have to
reluctantly back up to a previous, less powerful set-up because we
can't afford the DNFs and breakages that the hot set-up
produces. Valve train failures are hit-and-miss, trial-and-error, a
mystery. Lighter valve train parts and stronger springs sometimes just
seem to make everything worse. Is there any truth?
Back in the 1920s Percy Goodman decided it was time to put aside
clattering pushrods and adopt trendy OHC valve drive on the TT
Velocettes he manufactured. Soon he was driven crazy by erratic valve
motion and sensibly resorted to the use of a strobe light to reveal
what was happening. But naming the illness is not the same as a cure.
When I was recently at the NHRA drag nationals in Houston, I had the
opportunity of conversation with Byron Hines, whose purpose-designed
giant 160-cubic-inch V-twin engine has recently begun to win Pro Stock
races. I asked what had made the difference after some uncompetitive
seasons.
The Spintron, was his answer. It showed us that our valve
motion was nothing like what was in the cam profiles.
When I was recently at the NHRA drag nationals in Houston, I had the
opportunity of conversation with Byron Hines, whose purpose-designed
giant 160-cubic-inch V-twin engine has recently begun to win Pro Stock
races. I asked what had made the difference after some uncompetitive
seasons.
The Spintron, was his answer. It showed us that our valve
motion was nothing like what was in the cam profiles.
Spintron use a big electric motor to spin your engine while a variety
of instrumentation is used to measure the actual trajectories of the
parts you are interested in. This is different from using a
strobelight in that the information you get is detailed enough to
allow the flexing of each part to be isolated and understood.
Byron went on to say that valve train dynamics are particularly
difficult to control in big twins because of the large variations in
crank speed as each cylinder fires. The cam profiles were originally
developed to work at a particular maximum rpm, provided that the
camshafts turn smoothly. They do not turn smoothly because the crank
does not turn smoothly; it advances in a series of fairly violent
jerks. 80 percent of the recoverable energy in the hot combustion gas
does its work between 10-deg ATDC and 80 ATDC. Out of the 720 crank
degrees in the engine cycle, power is given during only 10%. This
means that the instantaneous speed of the cam can often be much higher
than its average speed. This also means that as a cylinder fires and
the crank accelerates suddenly, an open valve in the other cylinder
may be tossed right off its cam profile, or dropped prematurely onto
its seat.
This reveals why tuners of singles and twins found that their bikes
top-ended better and faster the heavier their cranks were made. A
younger generation of tuners has rejected this idea as turn-of-the-century
dirt-track nonsense, reasoning that physics requires lighter flywheels to
result in faster acceleration.
Vintage racer Todd Henning learned the heavy crank truth
in back-to-back testing of his highly tuned Honda twins, as did Rob
Muzzy in 1981-83 with 1025-cc Kawasaki in-line fours. The heavier the
crank, the smoother its rotation becomes, and the less power stroke
disturbance, or is transmitted to the cams. Where
does the lost power go when a light crank is used? Erratic valve
motion is one answer, and big valve bounce after closing is another.
Power pulsing is not the only disturbance to the valve train. Consider
a parallel twin, a flat twin, or an in-line four. In all of these, all
the pistons stop simultaneously. This means that as pistons decelerate
to a stop, the crank must accelerate rapidly because there is nowhere
else for the pistons' energy to go ; its conservation of
energy. Peak piston speed is about 1.5 times mean piston speed. This
means that all the kinetic energy in the pistons, moving at near 100
feet per second, is suddenly dumped into the crank. This is especially
bad for in-line fours, which have small-diameter cranks and little in
the way of flywheel mass. This sudden crank deceleration/acceleration
cycle is performed twice per revolution. No wonder new engine
development usually involves coping with cam drive breakage. No wonder
there are mystery failures of valve train parts.
Allan Lockheed is the man behind the engine design software Engine
Expert, and he talks to engine people all over the world. He has
tales of engines whose valves were stable with a chain or belt cam
drive, but which mysteriously began to break valve springs as soon as
a much more rigid gear drive was put in its place. The slight
flexibility of chain or belt took the sharp edge off the sudden crank
speed variations, preventing high frequency motions from reaching the
valve train. This may be why Yamaha retain a chain cam drive in their
M1 in-line four MotoGP race engine. The oil film between each of the
cam chain's pins, bushings, and rollers can be thought of as a kind
of viscous damper. Such fluid film damping is one of the motivations
inclining the designers of rocket engine turbopumps to give up rolling
element shaft bearings in favor of plain journal bearings. A gear cam
drive has fewer than 10 oil films between the crank and cam, but a
chain has a great many more.
Phil Irving, designer of Vincent motorcycles, suggested construction
of parallel twins with crankpins not at the traditional 360 degrees,
but separated by 76 degrees. With usual rod ratios, the piston reaches
maximum velocity at about 76-deg ATDC. At this point, the crank arm is
at right angles to the con-rod; the condition for maximum piston
velocity. This crankpin angle would therefore cause one piston to be
stopped when the other was at maximum speed. The result would be that
the two pistons would exchange their kinetic energy only with each
other, and not with the crankshaft, eliminating one important source
of cam drive disturbance.
Wide-angle Vee engines are better in this respect than parallel twins
but even they have their problems. The classic Cosworth DFV V8 GP car
engine of 1967 was estimated during design to have no more than a 35
pounds-foot torque peak in its cam drive, but actual testing revealed
peaks ten times greater; leading to drive failure. As the engine
was already near production when this was discovered, designer Keith
Duckworth had to scurry around and design a compact spring drive
(analogous to what is found in clutch cush hubs) that could be
incorporated into one of the gears in the drive.
Another approach is that seen on certain WW II German V-12 aircraft
engines, and on late race versions of Honda's RC30. In these and
other cases, small flywheels have been attached to the cams
themselves.
It is interesting to note that both Velocette and Ducati have found
that changing the stiffness of cam drive towershafts can be used as a
tuning measure to adapt an engine to a given race track a stiff
towershaft on short courses, and a more limber one for longer
tracks. The parts seem stiff and strong only in our imaginations and
in our not-very-stiff protein hands. In fact, at speed, everything in
engines is flexible because the amounts of energy moving from part to
part can be so large.
Byron Hines noted that as useful as Spintron's instrumentation is,
even more so is the experience and advice of Spintron personnel. One
of the first things they suggested was that he replace his aluminum
rocker arms with steel, for accurate motion depends more on stiffness
than on weight. He also said that full benefit from Spintron requires
making several laps through their process. A first lap involves re-configuring
the cam profiles and valve train to settle the motion and eliminate float
and excessive bounce. A second lap becomes necessary when it is realized
that after lap one, the engine's power curve has sagged in some places.
This is because before, cam timing and lift had been unwittingly chosen to
at least partly compensate for the uncontrolled valve motion. Once the valve
motion is settled, timing and lift are wrong. Now lap two consists of finding
new optimum cam timings and profiles to again maximize power. That, in turn,
brings to view new dynamic problems to be solved in lap three, and so on.
The people with the greatest sophistication in all this are, naturally, NASCAR engine
builders.
As an extreme example of what can happen, airflow
pioneer Jerry Branch was once called upon to dyno a special V-twin
that was conceptually a slice off a small-block Chevy, crank and all.
Being intended as a motorcycle engine, it had no flywheel other than
its little piece of V8 crankshaft. Branch said that although the engine
had plenty of displacement and hot tuning parts that suggested an
easy 100-hp, it never made over 35 horsepower. Each time it fired,
with almost no flywheel mass to smooth the pulse, it launched its
valves into orbit.
Another aspect of unplanned valve motion is the vibratory modes of
valves themselves. Particularly with rocker-arms (which exert some
side-thrust), a valve can be excited laterally as it lifts, the head
of the valve whipping from side-to-side on the stem; the stem
possibly made more flexible by undercutting to squeeze out that last
CFM of airflow. When this flopping valve approaches the seat, one edge
can hit first, causing a motion not unlike the final stages of a spun
coin's motion. Or, approaching its seat squarely, the rim of the
valve stops but the stem and center of the valve head keep right on
going; the trampoline mode of valve flex. When the motion
at the center finally stops, the valve head is quite deformed and it
now snaps back, tossing itself back up off the seat in a cycle that
can make several hops; with considerable lift being reached in the
process. This is a cautionary tale for those who wish to carve away
valve head mass in search of ultimate lightness. Or for those who wish
to replace existing valve shapes with something quite different. Think
about it if things don't go right. You may have made transformed
those elegant chunks of metal from valves into springs.
Spintron is not cheap but all of us can afford imagination. If you
have valve or spring problems, think about what has worked in your
experience with your particular engine, and what has not. Careful
mental sorting here can reveal a lot. Besides, what else is a body to
think about while waiting for a dental appointment? Sit with the
engine and rotate it through its cycle, thinking about what is
happening at each point. Over time you can develop a mental picture of
what may be happening and what might correct the problems. There is
more to valve trains than light parts and heavy springs."
In 1956, Johnny Allen set a land speed record over 214 mph on a 650 Triumph. The crank in that engine weighed 10 lbs more than a stock crank.
