What causes..?
Answers:
Engineering clearances - all the moving bits rattling against each other.
The purpose of a blow off valve is to reduce the amount of back pressure applied to your engine. When you rev the engine and let up off the gas, like what is said above, you create excess pressure that tries to make its way through the cycle and out the exhaust but can't. This is due to the inertia required to speed up the turbine inside the turbo. So you create back pressure. This back pressure limits the amount of fresh, cold air allowed into the engine during the intake stroke. This chokes the engine and temporarily, very temporarily, reduces power. This can cause carbon scoring inside the cylinder and on the piston over time (that is bad). Does it boost horse power? No. Will it provide excellent preventative maintenance on your forced induction engine? Yes. So it is up to you whether or not is worth the added protection. On almost all professional turbocharged drag cars you will find blow off valves. Although, they aren't the kind that make the popular high pitched sound so you really can't tell that they are there. Remember, cool sounds cost money! I bet you can find silent valves that cost less. I am not that good at explaining things sometimes so if you need a diagram go to www.howstuffworks.com. This is a great site, lots of easy to understand animation
when the full egnight it produces many differnt thing, sound being one of them, the differnt sounds depend on what it has to pass though, ie, the bodyword, engin block etc.
The sucking sound of the air going in and the explosive sound of the air going out
it is the sound of the exploding air/gasoline mixture in the cylinders
Its actually a bit of a fallacy to say its all from
1.The sucking of air (induction noise).
2. The explosion of air and fuel
3. The pulse of the exhaust expelling the used air fuel mix.
A large %age (variable by engine) is actual due to mechanical movement, all the valves moving etc. If you ever get the chance to listen to an engine being dry driven then do so. This means the engine is driven by an external motor to replicate normal running conditions. It certainly isn't silent.
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