[T.Maas]

Generally turbo cars have some type of bov or wastegate, I can hear my turbo when I let off the throttle but unlike the previous gen 335i there are no bov or wastegate aftermarket parts. Why is this?

[S4NoMore]

See diagram below, it's from the VW 1.8t but the same concepts apply and the BMW N20 diagram is a little more complicated.



The deal is, generally turbo cars have a wastegate and a diverter valve, not a blow-off valve.

BOVs are the common aftermarket solution made popular in the tuner scene where, in legitimately necessary applications, the BOV is required because the buildup up boost is so large a diverter valve wouldn't be able to relieve the intake charge when you close the throttle. This is rarely the case anymore.

From the factory we have a DV to recirculate air back into the low-pressure intake path, because this air is metered by the HFM/MAF and when you blow it off to atmosphere the loss of air can't be taken into account. The DV is not audible with a factory airbox (aka intake silencer). In modern cars a BOV will cause a bad air/fuel mixture.

Aftermarket DV solutions will only be necessary when/if we see factory DV failures, if we boost our engines that high. If you want a "woosh" noise similar to a BOV, look into intakes. In my opinion it's not worth it, as any intake that makes noise will likely do so because it's less efficient (pulling air from the engine bay).

On the exhaust side we find the wastegate. Its function is to regulate the speed of the compressor, which is to say it limits boost to protect the engine and turbo. It does that by opening a valve which allows a proportion of exhaust flow to bypass the turbine.