What is a "pressure relief valve"?

When a modified car sps by, we often hear an engine acceleration sound of "buzz..." followed by a "whoosh..." sound - which gives people a feeling of momentum. Where did this voice come from? Why is there no such sound on ordinary civilian cars, or on some high-performance sports cars (while some ordinary modified cars do)?

In fact, this unique "croak..." sound is the sound emitted by the pressure relief valve of the turbocharged engine when unloading pressure, it can be said that all turbocharged engines will produce this sound, but for everyday civilian vehicles, manufacturers will design this sound as noise to deal with it, reduce it as much as possible. This is done by venting pressure into the intake manifold, so there is very little noise - this type of pressure relief is called leakage.

For ordinary civilian vehicles equipped with turbocharged engines, it is generally impossible to detect the "croaking" sound when unloading pressure without listening carefully. This is like for ordinary civilian cars, engine noise and rumbling exhaust sound are negative parameters (while high-performance cars can sometimes emphasize this sound), and the original design is as good as possible to eliminate this sound. For drivers who like to have fun driving, this sound has become a catalyst for their driving passion (we often see that the exhaust pipes of many cars are changed to look like cannon barrels, and the sound is rumbling).

In addition to stimulating driving passion, "dazzling" is also one of the demands of modification, many people want to change their car to look like a super high-performance car, even if the car power is not particularly strong. In general, the size of the "croak, croak" sound made by a turbocharged engine is related to the intensity of the boost - a large turbocharger is more likely to make such a sound. Therefore, if this sound is relatively loud, it can appear that the supercharger of this car is larger, which makes people have a very dazzling feeling.

Many modification enthusiasts are also for this purpose, their own civilian car engines (such as Bora 1.8T, Passat 1.8T) leakage pressure relief to leakage type, and further increase the "croak" sound, the reason is very similar to the barrel exhaust pipe. So why do turbocharged engines make such sounds?

When we step on the accelerator pedal to accelerate, the throttle opens, the engine discharges high temperature and high pressure exhaust gas energy to push the exhaust gas turbine to rotate, when the turbocharger working speed (that is, the turbine rotation is more than 100,000 revolutions per minute), the turbocharger compresses the surrounding air, so that the engine intake volume increases, and the power of the engine is improved.