Tapered intakes, port velocities etc. Part II
Todd Knighton
knighton at net-quest.com
Mon Dec 16 00:02:33 GMT 1996
Todd Knighton wrote:
Continued:>
> 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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