| This is from a post I made a while ago: Here's an explanation of how BOV's work. I'm quoting it from the article I found it on: "Here's a common example of when the exhaust gases are not driving the turbo, and neither are the intake gases. When you accelerate at full throttle, then lift off the gas to change gears. Suddenly the exhaust gas speed falls away, but there's still a full load of compressed air in the intake. Combine that with the backpressure wave from the intake air hitting the closed throttle plate and you get nasty intake backpressure waves crashing into the compressor wheel. This isn't very healthy to the long term life of a turbo, so there is usually some kind of pressure relief valve on the intake just after the turbo designed to open up when there's a large pressure differential between the intake and the manifold pressure to let this air out and save your turbo. The term I'm used to for naming this device is the blow-off valve, (BOV), and this is the source of all the weird whistle/whoosh noises during shifts or when letting off the throttle" Now, for maximum performance, it would be best just to let that pressure stay backed up behind the throttle plate waiting for you to hit the throttle again at which point the throttle plate will open and the pressurised air which is still waiting there will rush into your engine and produce power. The downside is that your compressor wheel will take a beating due to the pressure waves backing up and hitting the blades. For maximum reliability, it would be best just to bleed off all that pressurised air after you let off the throttle so it does not back up into the compressor wheel. The downside to that is there will be no pressurised air waiting to be let into your throttle body, you will have to wait for your turbo to spool up and produce pressure and you will experience much more turbo lag. The middle ground is to bleed off some of the air, but not all of it. That's why you have an adjustment on your BOV's- you can choose how much pressure differential is required for them to vent. Please note that they do not let off at a certain pressure, but a pressure differential before and after your throttle plate.
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