Close to the question regarding “Why the difference between motorcycle and car engine sounds?” My question is, why is that less cylinders in a motorcycle (V-twin) make more noise than four cylinder engines in cylces yet in cars, V10’s (dodge viper) make a hell of a lot more noise then a 4 cylinder, V6, V8? More clyinders=less noise in motorcycles. More Cylinders=more noise in cars. Why is that?
Warning: WAG.
V-twins tend to use big pistons relative to the amount of metal in the cylinder and the rest of the engine. Less metal to absorb noise means that more noise escapes. Four-cylinder motorcycles use smaller pistons and are generally quieter.
But a big factor is the exhaust system. V-twins are often engineered to make a throaty growl. It’s part of the image. I remember hearing that Mazda spent a lot of time engineering the exhaust note on the Miata so that it would resemble that of an MGB or other British sports car. Image sells, as can be seen by all of the Harleys on the roads.
Exhaust systems can not only be tuned for image, but also for performance. I think a free-flowing exhaust system helps an engine breathe better, and results in more power. With less restriction of the exhaust gasses, there is more noise. For example, my YZF-R1 has Yamahas EXUP system. There is a valve that opens up at higher revs. This improves performance, and there is a noticeable increase in engine noise.
Speaking of motorcycle vs car engines, has anyone ever tried a wankel rotary engine on a motorcycle? Any reason why it wouldn’t be a good idea?
Some of the noise comes from the exhaust, and some comes from the engine itself. Car engines are inside engine compartments, which tend to muffle noise. Motorcycle engines are exposed. Also, most motorcycle engines are air-cooled. The jacketing on a water-cooled engine tends to muffle noise.
Motorcycle engines tend to rev higher than car engines (mainly because motorcycle engines are smaller). Higher revs lead to more noise.
Many motorcycles have tuned exhausts, which tend to be noisy. A tuned exhaust relies on pressure waves being reflected back to the cylinder (a low-pressure pulse can help suck exhaust out of the cylinder, while a high-pressure pulse can help prevent unburned fuel from escaping out the exhaust port). Mufflers reduce noise by evening out the pressure pulses. A tuned exhaust won’t work well if the pulses are evened out too much. A tuned exhaust leads to better performance, and motorcyclists are often interested in high performance.
Also, most people who drive cars don’t want them to be noisy, while motorcyclists usually don’t mind the noise. In fact, some riders like the noise. Many car ads use quietness as a selling point, and some rides modify their bikes to make them louder. If riders wanted quiet bikes, motorcycle companies would design quieter bikes.
This also explains why high-performance cars like the Viper tend to be louder than normal cars: they have tuned exhausts, and the people who drive them probably like the noise. The Viper appeals to drivers who want a noisy beast of a car. It’s not the number of cylinders. It’s possible to design a noisy or quiet car with any number of cylinders.
It’s the exhaust system that’s typically the primary source of noise for small air-cooled engines. You’ll notice that lawn mowers, chain saws, and weed trimmers are also quite loud when compared to automobiles. To make matters worse some motorcycle enthusiast enjoy making their rides as loud as possible. It’s a wonder more of them don’t suffer hearing loss.
Marc
Mufflers can either magnify or diminish the sound of the exhaust! 