Here is how should looks the curves under every of your BOV samples it's the way the game triggers the sounds only when the engine isn't onload anymore, it's not depending on throttle level but on the load the engine, if you are letting throttle off about 40% the bov might triggers if the boost level get under on of the sample trigger setting we saw in First Page

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I can't really explain how the games decide to trigger this fonction but I know how to use it the way I understood it

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I decided to share a blank project with the full Turbo fmod event shown in the screens in this explanation, if you need more help to understand things just CONTACT me :