6 Reasons Why 4-Stroke Engines Are Quieter than 2-Strokes


As a rule of thumb, 4-stroke engines are always quieter than their 2-stroke counterparts.

Why? Simply put, the six primary reasons why 4-stroke engines are quieter are as follows:

  1. Fewer explosions at a given RPM
  2. They feature valves
  3. Different exhaust design
  4. Lower sound frequency
  5. Different power delivery
  6. They idle more smoothly

If you want to drill into the details, keep reading. We at PowerSportsGuide have compiled the key facts under one roof!

Are 4-Stroke Engines Quieter?

Yes, it’s safe to say that 4-stroke engines are always quieter than 2-strokes, especially when idling. At higher engine speeds, the difference is much less significant but still clearly audible.

Although modern fuel-injected 2-stroke engines are designed for quiet operation, they still don’t compare to the lower noise level of 4-strokes.

Surprisingly, even if 4-stroke engines are quieter, they have a deeper sound that travels much farther. This can be an issue around off-road and on snowmobile trails, especially if they are close to residential areas.

While the higher frequency sound of 2-strokes fades more quickly, 4-strokes can be annoying even if they’re far away!

6 Reasons Why 4-stroke Engines are Quieter

1. Fewer Explosions at a Given RPM

There’s no question that one of the main reasons why 4-stroke engines are quieter is the lower number of explosions. Due to their design, 4-stroke engines have half as many explosions per RPM cycle as a 2-stroke. And with fewer explosions you have a quieter operation!

In other words, 4-strokes engines are quieter because all of their cylinders fire once for every two rotations of the crankshaft, while 2-strokes fire once for every rotation.

Because of this, 2-stroke engines necessarily run at higher RPMs and produce significantly more power, which translates to higher noise levels.

                        

2. Valves

Another reason why 4-stroke engines produce less noise is that they have exhaust and intake valves.

These valves keep the cylinders completely closed during the explosions. The exhaust valves open after the explosion inside the cylinder is over in order to let the exhaust fumes out of the engine.  

Therefore, the exhaust valves “separate” the noise of the explosion and then open to let the gases leave through the exhaust system. And the noise of the engine mainly comes from these explosions!

In contrast, 2-stroke engines don’t have exhaust valves, and their power phases are not entirely separated from their exhaust phases. Since these phases overlap to a certain extent, the explosions of 2-stroke engines are more audible throughout the exhaust system.

3. Different Exhaust Design

4-stroke engines not only feature a different design, but they typically utilize a different exhaust system as well. In general, 4-stroke engines have a more restricted exhaust system, which also has lower noise emissions.

On 2-stroke engines, power is more of a priority, so their exhaust systems are designed to be less restrictive.

That’s why you can typically see a tuned pipe or “expansion chamber” on many 2-stroke engines. These tuned exhaust systems are engineered to improve volumetric efficiency, which ensures more power output.

As you might assume, these high-performance 2-stroke exhaust systems are much noisier as well!

4. Lower Sound Frequency

Thanks to their design and features, 4-stroke engines generate sound waves of much lower frequency. And these low-frequency sounds seem quieter to our ears.

This is because human ears can hear frequencies between 20 – 20,000 hertz, but they are most sensitive between 2,000 and 5,000 hertz. If you go below this range, the sensitivity drops off as well.

Since 4-stroke engines run at lower RPMs, they usually generate lower frequency sounds, which is more tolerable to our ears.

5. Different Power Delivery

4-stroke engines also run quieter as they offer a different power delivery. The general rule is that a 4-stroke engine always develops much more torque at lower engine RPMs.

This means 4-stroke engines require less throttle to produce the same amount of power. And the less throttle you apply, the quieter the engine runs!

In contrast, 2-stroke engines have a different torque curve, producing lower torque at lower RPMs. That’s why 2-stroke engines are typically run at higher RPMs, which results in the characteristic “two-stroke scream.”

6. Smoother Idling

Last but not least, don’t forget that 4-stroke engines idle more smoothly, so they are typically quieter and less annoying at idle RPMs as well.

Conclusion

Thanks to their design and features, 4-stroke engines run quieter than 2-strokes.

Their cylinders fire only once for every two rotations of the crank, while their exhaust valves effectively reduce explosion noise.

What’s more, 4-strokes engines run at lower RPMs, idle more smoothly, and utilize more restrictive exhaust systems, all of which contribute to a quieter operation.

Since 2-stroke engines are generally much noisier than 4-strokes, when it comes to powersport vehicles, they are typically used for racing purposes. However, specific recreational vehicles like snowmobiles and dirt bikes are still available with 2-stroke power sources!

References:

Widex.com, Universal Technical Institute

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