Tapered intakes, port velocities etc.

[Todd Knighton]

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Guys,
	Here's the article I had talked about, the thing wouldn't OCR because
it's a low grade fax, so I typed it in.  Sorry, no pictures, the few
that were in there weren't worth much.

Todd Knighton
Protomotive Engineering

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Cylinder Head Tech:

Motorcyclist June 1996



	At first, the task of clearing and recharging the cylinders in
a high-speed, four-stroke engine seems impossible.  Such
processes need time, and it's hard to believe there's enough
available for this one, which faces many impediments and is
crowded into the merest fragment of a clock's tick.

	The intake stroke lasts for 180 degrees of crank rotation,
which is only three-thousandths of a second at 10,000 rpm. 
Camera shutter openings are as brief., but light has no mass and
moves at 950 million feet per second.  Air's mass makes it lag,
and it hits a sonic wall about 1100 feet/second, with localized
shock waves further blocking the intake ports at much lower air
speeds.

	Yet cylinders get filled-with efficiencies sometimes exceeding
100 percent-without mechanical supercharging.  This is possible
because the intake process actually begins in the preceding
exhaust stroke and extends far into the following compression
stroke.  We've methodically learned to make the pesky effects of
inertia work for us; and minimized the bad effects of problems
that cannot yet entirely be solved.

	On a cylinder head's intake side you have only atmospheric
pressure, 14.7 pounds per square inch at sea level, working to
stuff air into the cylinder.  No matter how hard the descending
piston tries it can't pull air in behind it.  It can only create
a space for atmospheric pressure to fill.

	It's a different story over on the outlet side, where a
pressure close to six atmospheres exists when the exhaust valve
opens to begin the event called "blow down".  Further, after
blow-down, pistons mechanically force exhaust products from the
cylinders, and do so against the resistance of undersized
valves, badly designed headers or steel cork mufflers.

	The more important exhaust event is the high-velocity shove the
rising piston gives exhaust gases during the exhaust stroke. 
The shove peaks at maximum piston speed (in most engines
occurring a little less than 80 degrees of crank rotation before
the piston reaches top dead center), where it suddenly gets
yanked to a stop.  But the momentum of the gases in the exhaust
pipe continues, leaving behind a partial vacuum.  This starts
the air/fuel mix above the part-open intake valve moving into
the cylinder before the piston begins it's intake stroke.  

 	Engines benefit from exhaust-augmented intake flow in two
ways; an obvious advantage is that it gives the too-brief intake
period an early start.  The second effect, less obvious but also
important, is that combustion chamber cross-flow during valve
opening overlap (the period during which both intake and exhaust
valves are open) clears residual exhaust gases, which slow
combustion, depress power by displacing part of the fresh
charge, and can require some weird kinks in the ignition advance
curve. 

	Exhaust systems primarily aid intake flow by their manipulation
of the combustion "sound wave".  A sound wave creates a
disturbance ahead of it and leaves one behind;  such "positive"
waves bursting from the exhaust port are followed by negative
pressures.  When the strongly-positive exhaust wave emerges from
the end of a pipe, it leaves behind a negative-pressure tail,
which then reflects back toward the port.  If the length of the
pipe is right, the negative wave will arrive back at the exhaust
valve as the piston reaches TDC, thus further assisting in
clearing the combustion chamber.

	Sound waves are reflected by any cross-section change in the
duct in which they are traveling.  The sawed-off end of a pipe
is one such change;  the closed end of a pie is another.  The
difference is that increases in section invert the wave while
reflecting it, changing positive waves to negative and
vice-versa; section reductions reflect the wave with the same
sign.

	While speaking of sonic waves, I should caution you about
confusing their behavior with that of the media in which they
travel.  Like all sound-conducting media, air has mass and the
other properties of matter.  sonic waves are by contrast, purely
energy and thus follow an entirely different set of rules.  such
waves make zero-radius 180 degree turns and reversals without
delay or loss of strength.

`	Plain pipe ends do a poor job of returning the energy of an
emerging sound wave, which is why horns have flared open end-to
get better energy recovery and thus amplitude.  Megaphones, the
exhaust pipe horns known in engineering as diffusers, are vastly
more efficient in this regard.  Racing two-stroke engines
expansion chamber exhaust systems have elaborate blow-down
diffusers, because of their heavy reliance on this
vacuum-cleaner effect to pull air through the transfer ports.

	Four-stroke engines seem perfectly happy running with plain
parallel-wall pips, though engines developed for megaphones have
to be reworked to function well without them.  Harley-Davidson's
famous racing chief, Dick O'Brien, never was totally convinced
that the megaphones used on the "low Boy" KR's did anything but
make noise.  At the time I was sure he was missing something,
but now I believe his reservations were valid.  

	Oddly, the 45-degree cut-off at the end of KR straight pipes
did coax a tad more power out of H-D's cranky old side-valve
engine;  O'Brien was at a loss to explain this oddity.  I tried
a 90 degree cutoff once, and found the KR didn't like it.  No
coherent theory I've heard or conceived explains why that should
have been so.  

	It now appears exhaust pipe diameter, meaning gas velocity in
the exhaust system, is more important than sonic wave activity. 
actual gas velocities vary in ways tough to grasp and impossible
to calculate, but the nominal speed is easy to figure and
provides a useful rule-of-thumb: simply multiply piston speed by
the ratio of cylinder bore and pipe areas.

	Nominal gas speed were well below 200 feet/second in most
vintage bikes, but in the AJS 7R of the 50's it was up to 220
feet/second.  By 1972 the small diameter pipes on H-D's XR750
raised that engine's exhaust velocity to just above 300
feet/sec.  The Triumph 650 TT Special I used to set a Bonneville
record (and acquire an abiding dislike of Wendover, Utah) years
ago also had small pies and 300-plus exhaust gas speeds.  It had
1 3/8-inch pipes, which almost everyone thought too small.  My
slide rule said they were the right size, and the
larger-diameter pipes we tried slowed the bike.

	Gas velocity is even more important over the engines intake
side, where it packs air into the cylinder between the intake
stroke's ending and intake valve closing.  This is crucial,
since with high-speed engines there is a significant lag between
the piston beginning the intake stroke and the flow of air into
the cylinder.  Outflow in the exhaust can pull air across from
the intake to give the intake process a head start, but cylinder
pressure still precipitously falls through the first half of the
intake stroke.  Air simply can't keep up with the piston, which
at 9000 rpm in the XR750 goes from it's stop at TDC to 80 miles
per hour in 1.5 inches, reaching that speed in 0.0014 seconds.

	Fortunately, the air inertia that delays air/fuel inflow causes
it to crown in at the end of the intake stroke, and beyond.  The
XR750's intake ports are small enough to raise the nominal gas
speed to 370 feet/second, which gives it plenty of momentum. 
This is why intake valve closing is delayed for many degrees
after the piston has finished it's intake stroke and begun
compression. Closing the intake valve while air is still flowing
into the cylinder, or closing it after flow reverses, gives less
the best power.  You have to close the intake valve(s) just as
the inflow slows to a stop, thus trapping the greatest weight of
air/fuel mixture in the cylinder.

	Serious tuners need some means of shifting cam timing ( in
increments no coarser than 1.5 degrees) to let them experiment
their way to the optimum intake closing.  This is usually done
with multiple oversize bolt hoes in the driven cam sprockets and
offset bushings, although my old Aermacchi required woodruff
keys with a sideways-jog at the shaft  and timing gear join to
shift camshaft phasing.

	High-performance engines' intake valves close typically 60 to
80 degrees after the intake stroke ends and the compression
stroke begins, so you know gas inertia is playing a major role
in cylinder filling;  if it didn't there'd be no need to delay
intake closing, and no sensitivity to the timing of that event. 
None of the other valve actions-exhaust opening or closing, or
intake opening-are nearly as important.

	Flow benches can be used to blow a lot of smoke up your shop
coat when you're looking for horsepower.  You can always make
air flow numbers rise by increasing valve head diameter, or by
enlarging the passages leading from the atmosphere.  But higher
air flow numbers do not necessarily translate into more power,
as many in the engine development field (including yours truly)
have discovered.

	Mercedes-Benz made the big-port mistake with the design of its
awesomely complex eight-cylinder M196 GP car, which had desmo
valve actuation and intake ports the size of drains.  They found
themselves being out-horsepowered by the British Vanwall, with
an engine that was virtually four Norton 30M Manx Cylinders and
heads bolted to an aluminum Rolls Royce armored car crankcase.

	Ford's 1960's four-cam V-8 also had huge intake ports, and
while it turned more revs than the Offy four-banger engines then
dominant at Indianapolis, it was no better than a match for
them.  When given an early peek at the Indy Ford's cylinder-head
castings, I expressed the thought that its ports might be too
big.  Ford's engineers were too polite to tell me how absurd
they considered my remark to be, but their expressions made it
plain.  I was too polite to send them an "I told you  so" note
after Dan Gurney sent one of the engines to Weslake Engineering
in England, where it's intake ports were made smaller and its
output got bigger.

	Ford's engineers were then vastly ignorant of the world beyond
Michigan's borders.  They had no idea Harry Weslake and Wally
Hassan (who created the very successful Coventry-Climax racing
engines) had learned years before not to take too literally what
the flow bench said.  They were narrowing intake ports to
provide nominal gas speeds in the range of 350 to 400
feet-second, making good use of the fact that kinetic energy
packing air into the cylinders increases with the square of it's
velocity.

	Harley-Davidson's experience with the highly successful XR750
should have kept it from making the big-port error in the
CR1000. Yet, that's exactly what it did:  the VR's intake ports
were made so big, nominal intake velocity was down at 200
feet/second, which may explain why it's proved sadly inferior to
engines that do not test nearly as impressively on the flow
bench.

	Grand prix car engines represent the pinnacle of four-stroke
development.  Formula One's designers are spinning 3.0 liter
V-10 engines up to 15,000 rpm's and getting close to 800
horsepower. Ford's GP Zetec V-8 is doing the same with 375cc
cylinders, which implies that it's possible to build a 750cc
V-twin that will make nearly 200 horsepower.

	Cosworth Engineering's Keith Duckworth was the creator of the
modern high-output four-stroke.  Casting aside tradition,
Duckworth combined large-bore short-stroke cylinders with
narrow-angle valves and a compact combustion chamber.  He didn't
originate the use of high-intake port velocities to ram-charge
cylinders, but he and those he's influenced now design for
nominal intake speeds approaching 450 feet/second.

	Of course, there's a lot more to cylinder gas exchange than
port velocity.  But unless you've spent eons dragging air
through ports, manifolds, etc.,, at a flow bench, you probably
have no real understanding of what aids flow and what slows it. 
If there is any rule for the inexperienced to keep in mind.  it
is that everything a reasonable intelligent person should
intuitively believe to be right will probably be totally wrong.

	Take valve shape for example, these days typically an
unstreamlined disc on the end of a stick  Your eye will tell you
the shape is horrible, an example of how we've fallen into
decadence since the days of those British power plants with
beautiful, deeply tuliped intake valve.  Then you hit the flow
bench and find that the one with all the loveliness of an
overgrown nail better at all lifts.  And then you repeat the
experiment with another port and find it responds better to a
tuliped valve.  Some ports are like that, by virtue of slightly
different interior contours or different valve angles.

	Or you can try valve seating surfaces-maybe someday you can
tell me why sharp edges are better here than rounded ones.  The
worst valve I ever tested was one I made the mistaken belief my
eye could judge how air would behave between the valve and seat.
 I ground a valve head with a radius instead of a flat where it
seated, along with a similar-shaped grinding stone for the seat.
 Testing this idea required tons of work, yet my streamlined
valve and seat combination was worse at all lifts than the
typical series of abrupt, sharp-edged flats.	

	You'd think that getting the valve completely out of the way
while flow-testing ports would let the air really whistle on
through.  But peak flow almost always occurs with the valve in
place, at a lift equal to about 30 percent of valve diameter. 
And this is with a manifold and carburetor in place, and a
cylinder between head and flow bench receiver ( the cylinder's
adjacent walls can significantly influence flow around intake
valve heads).

	Multiple valves ( more than two per cylinder) actually offer
little or no real valve-area advantage.  You can prove this to
yourself by drawing circles representing valves inside a larger
circle signifying the cylinder bore, Unless you fudge the whole
thing with unrealistic provisions for valve seats, clearance
around the valves, etc., the total for valve head areas is about
the same for two, three or even five valve layouts.  The benefit
lies in the fact that total head area counts only at or near
full lift: at lesser lifts, flow is largely limited by the valve
seat ring area, really more a function of the total of valve
circumferences than area.  Viewed this way, multiple valve
layouts are better, though only Yamaha has found any gain with
more than four valves.

	Air flow in ports takes paths totally unlike those you would
normally envision, unless you happen to have an abundant
knowledge of compressible fluid dynamics.  In your imagination,
air may move in orderly lines of travel, with particles marching
along the roof of the port staying high, those on the floor
staying low, and all traveling in neat, linear streams.  The
reality is a very different matter.  

	When flow in a duct ( an intake port, for example) arrives at a
bend, it loses any semblance of orderly behavior.  Particles on
the inside of the bend travel the shortest distance (offering
the least resistance to flow), so they tend to maintain speed in
the downward turn to the valve seat.  But flow in the top of the
port slows relative to the floor, creating a large velocity
gradient.  Pressure in a moving fluid varies inversely with it's
speed, so the velocity gradient creates a lower pressure at the
port floor than at it's roof.  this differential causes air at
the sides to move upward and the midstream air to move down,
with the resulting flow stream made to divide into to
contrarotating vortices where the port bends.  Add to this the
invisible "smoke ring" vortex forming beneath the opening intake
valve and you have enough disorder to confound even the best of
minds (or computers).

	Port and valve configuration (both shapes and angles) can
profoundly influence combustion efficiency as well.  Jack
Williams AJS 7R made it's best power with an intake port shape
that compromised flow in favor of creating more combustion
chamber swirl and redirecting incoming fuel droplets away from
the cylinder walls. I am reliably informed that Keith Duckworth
has settled on the intake valves leaned 15 degrees from the
cylinder axis, and ports at 30 degrees from the valves in a
similar trade-off between flow and combustion.

	Intake flow influences combustion because both carburetors, and
fuel-injection nozzles deliver fuel in liquid form.  The best
you can hope for is a fog of droplets small enough to stay
suspended in the air while evaporating; big drops are
centrifuged out of the air stream, splatting against the intake
port and cylinder walls, which is bad for power, fuel efficiency
and emissions.  Fuel can't burn until it evaporates; if you have
raw fuel still trying to burn when the exhaust valve opens, it
goes out the pipe, wasting your money and polluting the air.

	My experience (not the final word on anything even for me) is
that the biggest improvement in flow from a change in port
shape- with the least port enlargement and resulting velocity
loss- is obtained by widening the port floor upstream from the
valve seat.  Air likes to take the most direct route, and the
more you ease that route the better flow becomes.  Shaving metal
out of the lower sides of the ports bend (making a D-shaped
cross-section, with the port floor on the flat side has in my
tests shown big flow improvements in sharply bent ports.

	Smoothing intake flow (thereby minimizing the turbulence of the
main flow stream) is best accomplished by making sure the port's
section area decreases all the way from the carb inlet to the
bend above the valve seat.  The small diameter, high-velocity
section of the port needs only a slight convergence of 1.5
degrees included angle, which doesn't sound like much.  But a 12
inch section of aluminum pipe taper-bored for a 1.5 inch inlet
and a 1.498 inch outlet flows better than a parallel-wall pipe,
and a lot better than air going from the cones' small end to
it's beg end.  Sound waves love a divergent duct, air flow does
not.

	I'm not convinced that polishing a port's interior surfaces to
a mirror finish does anything but look good.  The problem here
is that while we know there's a degree of roughness beyond which
flow suffers, we can't agree on the limit to which polishing
helps.  One those rare occasions when I do porting myself, I
settle for a smooth but not polished finish.  If I were in the
head porting business like my long-tie friend Jerry Branch, I'd
put a spit shine inside the ports and combustion chamber, just
as he does.  The way Jerry does it, his customers never have to
wonder if the ports are smooth enough.

	Jerry has discovered that some ports flow better if he cuts
tiny slots across the floor of the bend upstream from the valve.
 The slots apparently act as turbulence generators that energize
the air and make it stick to the port floor, following the bend
more closely.  That's the theory anyway, though like so much we
believe about port air flow, it's arguable because air hides is
secrets behind a cloak if invisibility.

	In time, we will know a lot more about the details of flow in
and out of cylinder heads.  For decades, researchers have used
smoke, pinwheels, dye droplets, etc. in their attempts to see
what air is doing.  The water-anaolgy method, where water
substitutes for air and flow is made visible with fine bubbles
or aluminum particles, is still used in many labs.  But the
growth of mystery-dispelling technologies has recently brought
doppler-laser metering and computer imaging to the field.  Maybe
one day soon we'll learn why the things a century of experience
has taught us actually do work, and why others do not.




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