If you’re a fan of MotoGP, you’ve no doubt noticed the peculiar sound of the bikes. The speeds these bikes travel and the skills of the riders are both super impressive, but you might still be wondering why MotoGP bikes sound so different from normal motorcycles.
MotoGP bikes sound so different because their engines run at up to 19,000 RPM or more, with a distinct engine ignition timing sequence when compared to street bikes. Their exhaust pipes are also shorter without any baffles, and they are variable too, which can also affect their sound.
Below, we cover in detail the source of the sound of a MotoGP bike and go over the specifics of why they sound the way that they do. But first, we tackle the question of why MotoGP bikes are so loud.
Why Are MotoGP Bikes So Loud?
MotoGP bikes are so loud because of the high RPM the engines run at, reaching upwards of 19,000 RPM and idling at about 4,000 RPM. In addition, the baffle-free exhaust pipe is rather short, which prevents the sound from dissipating, leading to a much louder sound.
MotoGP bikes are louder than normal motorcycles because their engines run at very high RPM. Many of these engines idle at 4,000 RPM, which is extremely high by street bike standards. All motorcycle engines racing in MotoGP are optimized for high power first, and other considerations, such as noise levels come later.
In the quest for higher engine power, the gases from the burnt fuel need to be removed from the cylinder as fast as possible so that they exit from the exhaust. Back pressure, or gases trying to get back into the cylinder, affects fuel ignition and engine performance. The easiest solution to getting rid of this problem is to use a short exhaust pipe so that the gases have no time to build up.
The short exhaust pipe means that the ignition sounds have a short distance to travel before they can exit the exhaust pipe into the air outside. The short distance gives the sound less time to dissipate before it exits the exhaust, so it comes out loud.
Why Do MotoGP Bikes Sound Different To Normal Bikes?
MotoGP bikes sound different from normal bikes for a variety of reasons. The bikes have 4-cylinders, which fire every 90-degrees rotation of the crankshaft, leading to a distinct sound. The length of the exhaust pipe also affects the sound of them, but the 19,000+ RPM also makes a difference.
There are multiple reasons why MotoGP bikes sound very different so let’s take a good look at each one of them. Compared to city bikes that usually have one or two cylinders, the MotoGP bikes have 4-cylinders. A cylinder fires at every 90-degrees rotation of the crankshaft, so there are thousands of mini-explosions every minute.
Unique Ignition Sequence
The sound of a MotoGP motorcycle is created by its unique ignition sequence and the timing interval between spark plugs firing. The sound of an engine whose ignition fires once every 360 degrees of crankshaft rotation is different compared to firing every 180 degrees or every 90 degrees, which will never be heard from street bikes that use a standard ignition.
All the cylinders fire in unison, creating the low-frequency thump or boom as mini-explosions take place inside each cylinder head to generate a huge amount of gas under high pressure that is rushing to escape the confines of the exhaust. As it comes from the exhaust pipe, it expands into the surrounding air making a low-frequency sound.
MotoGP Engines Explained
The design of a MotoGP engine involves a lot of compromise and balance, where tradeoffs are commonplace. An increase in power that comes at the cost of increased sound is acceptable, but a similar increase in power that comes from sacrificing fuel efficiency isn’t. This is because each competitor is given a limited quantity of fuel before the start of the race.
MotoGP engines are different when compared to the bikes in Moto2 and Moto3. Their engines are based solely on power and performance, which is everything in MotoGP. Many of the engines use shorter strokes which lead to much higher RPM and sound. The shorter stroke also contributes to the high-frequency sounds from the exhaust.
Why The V4 And The Inline Engine Sound Different
MotoGP motorcycles can use two different yet popular types of engines: the V4 and the inline-four. Both of them emit their own distinctive sound due to their internal construction. The V4 is so named because its four cylinders are arranged in a V configuration. This allows it to use a shorter, rigid crankshaft that means less weight and fewer bearings but is a step ahead in power output.
The inline-four has all four cylinders in a line with each other, connected to a common crankshaft. This, of course, means more weight and more bearings to support the crankshaft and makes a different type of sound as the engine is running. It also makes it more stable and less demanding for the rider on cornering, but it comes at the cost of a loss of power.
The Importance Of Releasing Burnt Gases To Avoid Back Pressure
When fuel is ignited, it creates gases at high pressure that drives the piston down. After the gas has done its job, it needs to be released from the cylinder through the exhaust valve. At this point, there is still pressure in excess of 100 psi (7 bar) rushing through the exhaust pipe until it is able to get out from the exhaust port, expanding as it exits.
The spent gases will travel towards the low-pressure area, which is the cylinder, causing back pressure that is going to degrade the engine performance unless the exhaust is efficient at expelling the gas already there. Any obstacle that blocks or slows down the gas from coming out of the exhaust port has to be removed.
The gas coming out of the cylinder contains minute particles of carbon and other burnt compounds that tend to get stuck inside the exhaust pipe and need to be cleaned regularly so that they do not accumulate and block the exit of gas. In city bikes, it is normal to have the exhaust pipes cleaned every few months and notice how the sound changes before and after the cleaning.
Since weight is not a consideration, normal bikes have long exhaust pipes that collect more residue from burnt gas. The loudness of a sound decreases with distance, so the longer exhaust pipes are quieter, but at the expense of weight. The result is a tradeoff between loudness and weight with the outcome in between that tries to satisfy both criteria to some extent.
Exhaust Pipe Length And Baffles
Each cylinder has an exhaust that is either connected to an exhaust pipe or an exhaust manifold. The exhaust pipe length is a major factor in reducing sound. Normal bikes have long exhaust pipes with exhaust baffles placed inside the exhaust pipe to further dampen the sound. The exhaust baffle is just a short pipe with a number of holes all over it and does the job of reducing the sound level.
Sound moves in waves, so as the sound waves move through the exhaust pipe, they move through the baffle, which is designed to reflect the sound waves against each other, leading to sound cancellation. Ordinary baffles made of metal can remove a lot of the high frequencies, but the lower frequencies pass through, giving the exhaust more of a deeper sound.
Expensive high-quality baffles that are encased in fiberglass are used on city bikes to make their sound more acceptable. Normal bikes used inside city limits have to comply with rules that limit their sound level, but motorcycles on the racetrack don’t. MotoGP bikes have their maximum noise level at 115 dBA, whereas a city bike is at 80 dBA.
Baffles For City Bikes
A wide range of baffles are available for most bikes to change the sound of the exhaust or to reduce the noise level. The most expensive baffles are the ones that are covered in fiberglass, while the cheaper baffles are all metal pipes with holes all around to reflect sound passing through it.
Cylinder Firing Order
The distinctive sound of a MotoGP engine is also due to the order in which the cylinders ignite. Racing teams use different cylinder ignition sequences in their effort to get maximum power from the engine. Some have adopted the big-bang method, where all the cylinders fire at the same time. Others use the screamer method, where one cylinder ignites for every 90-degree rotation of the crankshaft.
The ignition timing configuration makes a big difference in the engine output power, and other popular ignition timing sequences exist as well, such as igniting two cylinders together or in quick succession. Most of the research done by racing teams on ignition sequence configurations is proprietary and accessible only to their company.
The use of tuned exhaust pipe lengths changes the sound as the exhaust gases come out of the exhaust pipe. Using a longer exhaust pipe can result in a small drop in engine power. In the competitive arena of MotoGP, every bit of power counts, so the exhaust loudness is irrelevant when engine power is at stake.
KEY POINTS• MotoGP bikes rev very high compared to normal motorcycles
• The engines are also designed for sheer performance, which affects their power and sound output
• MotoGP engines come in two formations: the V4 and the inline-four
• These engines are connected to exhaust systems that contain no baffles and are very short
• All of this leads to a very unique sound from the bikes
MotoGP bikes sound so different for a variety of reasons, including the size of their exhaust pipe, the high engine RPM (19,000+), the specific types of engines used, and the fact that the engines have 4 cylinders. All of this works in conjunction to create their famously distinctive sound.