Like many other little boys, I tried to magically participate in adult affairs by learning to make engine noises. One of my favorites was the series of explosive belches put forth by the dark green John Deere horizontal-twin tractor engines. The cylinders were huge, and they “spoke” with authority. My younger cousin Stewart found a way to make the instantly recognizable irregular firing sound of a Volkswagen flat four.

Sounds became more important to me when I went to the races starting in 1964. The two-stroke era was just beginning—Yamaha would really get it right with its 1967–'68 250cc TD1-C—and I had just completed a "learning experience" trying to make a racing engine out of a Honda 305cc Super Hawk. Finding myself on the wrong side of history, I decided to learn what I could about the two-stroke technology then so cheaply available.

Most racers at the time dismissed two-strokes because of 1) their reputation for mystery and seizure; and 2) their irregular sound at idle, causing folks to call them "two-poppers" or "ring-dings." The late Gordon Jennings, in his little booklet Two-Stroke Tuner's Handbook, dispelled some of the mystery, but the seizures and the sound remained.

I didn’t care about sounds made at idle. I loved the music on the track—the ascending rpm, punctuated by the rev drop at each upshift. In private moments, I would sing this to myself for my own enjoyment.

Honda CBR1000RR-R clutch
Hush, little baby, don't say a word… Clutches on modern sportbikes, like that used on Honda’s new CBR1000RR-R, are hidden behind cast aluminum covers that include sound-deadening panels.Honda

I bought those foolish records—“You are there!”—of the sounds of racebikes at the Isle of Man TT. I went to Mosport, near Toronto, to hear Giacomo Agostini’s MV Agusta triple and Mike Hailwood’s 18,000-rpm Honda 250cc six. It was the era of megaphones.

My first dry-clutch engine was the 1970 Kawasaki 500cc H1R. After being push-started, the rider would stop, holding the clutch while searching for neutral. During that moment, the many plates of the dry clutch—free to jingle because they were for the moment not compressed by the clutch springs—could be heard. At very low revs, you could also hear the backlash in the clutch teeth loudly chattering as firing impulses kicked the crank around.

When you pull the clutch and toe the transmission into gear, the "clack" you hear is the clutch inner hub and the steel plates splined to it, suddenly stopping as the first-gear dog set connects them to the rear wheel. On BMW's flat twins, which have fewer and larger-diameter plates, that clack can be particularly loud.

Clutches on big sportbikes today are noiseless, enclosed by a cast aluminum cover that includes a sound-deadening panel. The PR writers at Harley-Davidson said it best in the 1980s: "We're killing the noise so we can keep the music." When the law specifies a decibel limit, which would you rather have?

How does all that acoustic drama disappear if the exhaust is routed into mufflers? They are full of sonic interference devices called quarter-wave tubes, each tuned to attenuate a range of frequencies. Sound enters such a tube and is reflected at the far end with length chosen to put the reflected wave 180 degrees out of step with the original. This puts the peaks and valleys on top of each other, adding up to zero. By choosing which frequencies to suppress and which to let through, the tall foreheads in marketing can emphasize whatever the focus groups respond to, a deep rumble, a crispness, or chosen musical tones. Sounds appeal directly to the emotions.

2019 Yamaha MT-07 engine
Firing interval, not engine architecture, determines sound. Moving the second crankpin 270 degrees from the first transforms the sound emitted by Yamaha’s parallel twins into that of a 90-degree Ducati.Yamaha

But at the track you can have it all: the exhaust sound, the metallics of a dry clutch, the whine of straight-cut gears.

Standing by the track, we hear the changes in sound frequency resulting from the Doppler effect. As the bike comes toward us, its sounds take less and less time to reach us, giving each successive pulse a head start over the previous, making the pitch rise. As the bike passes, its pitch drops. The rider hears no such thing, and sound and vibration join into a single experience. Once on a straight and near top speed, the engine sound holds almost constant, like a test on an absorption dynamometer. That is two major forces in balance: the force of aerodynamic drag against the force of the engine, multiplied through the primary gears, the transmission, and the sprockets and chain, then through the spokes and rim to where the tire pushes back against the whole earth. And there is sound.

"Why do internal-combustion engines make any sound? They release heat by burning a compressed mixture of fuel and air, and that heat generates pressure."

During races in 1971, we listened anxiously for the single “pop” of a misfire, which would indicate that the rectifier diodes on our Kröber ignition were giving up.

Why do internal-combustion engines make any sound? They release heat by burning a compressed mixture of fuel and air, and that heat generates pressure. But we can’t hear any of that because spark-ignition combustion is smooth and progressive. And if the piston continued to expand that hot combustion gas until its pressure had fallen to the pressure of the atmosphere around us, there would be no exhaust flow and no sound.

But in real-world engines, every crank degree of rotation must perform in the sequence of necessary acts: exhaust, intake, compression, power. Like a busy office worker who can’t dally over lunch, the engine must begin the next job.

Therefore expansion of the combustion gas has to end as soon as its pressure has fallen too low to do useful work. Guns work the very same way: The hot combustion gas does its work of accelerating the bullet, the bullet leaves the barrel, and the residual pressure in the gun expands into the atmosphere with a loud bang. A gun is a one-stroke engine.

As an engine’s piston nears the end of its power stroke, when there is still a pressure of 50 to 100 psi in the cylinder, the exhaust port or valve(s) opens. We can clearly hear the sound of that gas expanding, especially if it’s happening 10,000 times a minute.

Honda’s ultra-high-revving NR500
On the racetrack, Honda’s ultra-high-revving NR500 sounded just like a two-stroke. But at warm-up speeds, the oval-piston engine boomed as if it were any large-displacement Superbike.Honda

Singles have their unique sound because there is such a long quiet time—the full 720 degrees—between firings. In England, suffering the German V-1 cruise-missile campaign of late World War II, people often mistook the “buzz” bomb’s deep sound for that of a sporting 500cc motorcycle.

Four-stroke twins like Edward Turner’s Triumph Speed Twin of the late 1930s fired every 360 degrees, and my two-stroke TD1-B fired every 180. Honda’s sporty 250cc and 305cc four-stroke Hawk and Super Hawk twins of 1964, having their crankpins at 180 degrees to each other rather than side by side as in a Triumph, made their syncopated “dut-dut…dut-dut…dut-dut” exhaust sound as irregular as their firing angle.

That irregularity gives a V-twin's exhaust sound a V-8-like syncopation; production auto V-8 engines have 90-degree crank-pin spacing. Imagine a parallel twin whose second cylinder can be magically rotated around the crank with respect to the first cylinder. When the cylinders are parallel, they fire every 360 degrees to give the drone of a British twin, or they can be fired simultaneously as a dirt-track "twingle," sounding and shaking just like a single. Rotate that second cylinder by 42 degrees and you have the original Indian "Powerplus" V-twin of 1916 to 1923. Another three degrees and you arrive at Harley-Davidson's 45 degrees, then swinging to 50 takes us to the classic Vincent. Modern V-twins are often at 60 degrees for compactness, but 90 degrees brings us to Ducati and Moto Guzzi, which can then cancel all primary shaking force without the complexity of a balance shaft or the weakness of staggered crankpins. Continue rotating that second cylinder to 180 degrees and we have reproduced the original Douglass flat twin. With its crankpins at 180 degrees, this is also BMW's "boxer" engine, self-balancing and making the same flat drone as the British twin and for the same reason: equal firing intervals.

As we know from Yamaha parallel twins of the present moment, it’s not engine architecture but firing interval that determines the sound. By moving the second crankpin 270 degrees from the first, its sound is transformed into that of a 90-degree Ducati. And by twisting the “flat” 180-degree crank of the traditional inline-four to a 90-degree crank-pin spacing in Yamaha’s YZF-R1, a deeper, authoritative V-8 sound is superimposed on the four’s normal high-rev scream.

For a brief moment in the 1990s, before it was banned, World Superbike inline-fours warming up in the paddock could be heard to whir in a new way: gears! That was the teams, trying to eliminate cam-chain stretch and power loss from altered valve timing by putting straight-cut gears in their place. Each had its own sound. The money saved by banning gear-driven cams helped the teams afford to replace stretched stock chains every weekend. Remember: To fit an endless cam chain, you must remove the crankshaft!

When Kel Carruthers and Kenny Roberts arrived at Daytona in 1979 for the Goodyear tire test, their engine made a similar whine on startup but louder. When the whine didn’t go away even once the engine had warmed up, it was clear that one or both of the crank gears was too tight, so without full-service tools and supplies on hand, they went back to California. Mike Baldwin rode the test on his personal production TZ750, which he’d bought used in Texas.

Suzuki RG500s also had separate cranks and made their own characteristic gear whine. Most revealing of all was Honda’s four-stroke oval-piston NR500. Because it peaked at 19,000 rpm and each cylinder’s four exhaust valves gave it a fast rate of exhaust port exposure close to the two-stroke range, on the track it sounded just like a two-stroke. But at warm-up rpm, it boomed just like any big authoritative Superbike.

Although change is slow at the moment, we are assured by futurists that soon the silence of electric powertrains will replace all this racket. Then we can fix playing cards or balloons near our wheel spokes to make such bikes sound like motorcycles.