Valve Curtain Area

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

In response to my recent discussion of engines which use four valves per cylinder versus two, some readers have commented that I left out an important concept: curtain area. Let’s explore that.

Curtain area is the distance around a valve head, multiplied by how far the valve has lifted. That distance around—the circumference—is the valve’s diameter multiplied by the number 𝜋 (pi, approximately 3.1416).

As an example, let’s take a two-valve 500cc single with one 50mm intake valve. That 50mm x 3.1416 = 157mm of circumferential area. For each millimeter of lift, this valve will expose roughly 1.0 x 157 = 157 square millimeters of intake-flow area.

Now, for comparison, let’s do the same calculation for two intake valves whose head areas add up to the same as that of the single large intake valve. Arithmetic tells us that each of the paired valves will have a diameter equal to 50 ÷ 1.414mm = 35.4mm. And how much total circumference will the two valves have? That will be 2 x 35.4 x pi = 222 square millimeters.

More Valves Equal More Flow Area

That’s an impressive result. It means that, for each millimeter our two smaller intakes lift, they will generate a lot more curtain area than can a single intake.

And that, in turn, means that just by adopting two intake valves instead of one, equal valve lift will generate 222 ÷ 157 = 1.414 times greater flow area. This ability of two paired intakes to produce flow area faster than a single valve opening at the same lift rate is why four-valve engines need less valve-open timing than do two-valve designs.

But wait, as the infomercial pitchmen say, there’s more. Because the total weight of the two smaller intake valves is less than the single larger valve, our cam can safely open them more rapidly, giving the four-valve design a second advantage in speed of exposing curtain area.

Real-World Examples

Years ago I spoke with a real go-getter in the world of motorcycle hot-rodding who had specialized in speed work on two-valve Suzuki GS engines. He had built hundreds of “Midnight Specials” for impromptu (translation: illegal) three-in-the-morning street drags, and he knew his business. But when four-valve engines began to appear, the cam timings that had worked so well on the two-valve GS motors turned the new four-valvers into useless light switch jobs with nothing below 10,000. Soon he had studied the matter just as we have here, and began grinding cams with 20–25 percent less open timing. He not only recovered the tire-smoking torque his business depended upon, but was now able to deliver that high engine torque over a wider rpm range than before.

Valve Overlap

How did that work? Because 2V engines generate curtain area more slowly than can 4V designs, they must begin opening that single intake valve sooner and keep it open longer after bottom center to allow the cylinder-filling process more time to complete itself. While that works at higher rpm, when there is enough intake velocity to keep right on rushing into the cylinder after bottom center, at lower rpm the lower intake air velocity has less “ram effect.” After BDC, the now-rising piston soon stops the inflow and pushes some of it back out of the cylinder into the intake tract. This loss prevents complete cylinder filling, so it produces less torque at those lower revs.

Four-valve engines need less valve overlap because of the speed at which they produce curtain area. (Ducati/)

But the 4V engine, because of the speed with which each millimeter of valve lift produces curtain area, needs less valve-open timing. This means it has less need to leave the intake valves open past bottom center, and with that shorter timing it loses less torque to back-pumping at lower engine speeds. This produces a wider, flatter torque band.

Euro 5 emissions limits are forcing motorcycle manufacturers to use less valve overlap, the time when the exhaust valves haven’t yet closed but the intakes have already started to open, around top center at the end of the exhaust stroke. This is to prevent loss of unburned fuel out the exhaust pipes. Consequently, just about all production-bike engines with four valves per cylinder are being given radically wide, flat, easy-to-ride torque curves. We like it.

Tomorrow’s Bikes?

Right about now someone will object that soon electric bikes will deliver the widest, flattest torque of all, and that the above will interest only museum curators. But how soon? Look at MotoE, the electric class that Dorna runs as a halftime show at selected MotoGP events. The bikes are certainly powerful, but to complete just a few laps at speed they need batteries that raise their weight to nearly 600 pounds! Their lap times aren’t much quicker than those of the slowest GP class, Moto3—bikes propelled by 55 hp 250cc four-stroke singles.

While the electric revolution is getting itself together, there is still a useful role for the internal combustion engine. One might say the curtain hasn’t come down quite yet.

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