Engine noise under decelration

[cob427sc]

Took the 818 out for it's first short street drive and I noticed a strange sound. I accelerate hard and glance at the boost gauge showing 15+ pounds of boost, then let off the gas and I hear a noise that sounds like an air hose blown off a fitting. I'm assuming this may be a pressure release from the turbo? I've had numerous turbo equipped vehicles (all factory built) and never heard this sound. Only lasts for a second or two. Anything to worry about?

[TBull]

More than likely when you come off the accelerator, the vanes on the throttle body close and that pressure has to go somewhere. That's where the blowoff or wastegate come in. More than likely you're venting to atmosphere. Most OEM cars vent into the exhaust for a quieter experience or maybe into mufflers to deaden the sound.

[Neeraj Ahluwalia]

This is more than likely your blow off valve or bypass valve (likely bypass for ej). It could also be compressor surge, but if this happens after throttle liftoff, it's not really an issue. If you're valve is really a blow-off (vent to atmosphere) I recommend switching to a bypass recirculating which sends the air back into the intake. Subarus are MAF based systems which are not set up to handle non recirc.

[Neeraj Ahluwalia]

Also in the 818 the engine is right behind your ears. You can really hear the sounds of the turbo which would usually be deadened by the OEM car sound muffling and material. If you submit a sound clip I might be able to tell if it is the turbo bypass valve.

[cob427sc]

Thanks guys. I suspect that it is the turbo blow off due to the way it reacts. It must be vented directly to the atmosphere as it sounds like an air hose being disconnected from an impact wrench. I didn't do any of the engine install but the previous owner told me he had upgraded the turbo and the relief valve. I'll check and see if there is an outlet port or pipe that would allow the excess pressure to vent out.