For those who don’t know what a diverter valve is, these are what allow your turbo to build up boost(pressure) and then release it. Building up pressure in the turbo is what gives these forced induction engines so much power for just a 2 litter, 4 cylinder engine.
The difference between a diverter valve (DV) and a blow-off valve (BOV) is that a diverter valve routes the released air back into the system (sometimes monitored) while a blow-off valve releases it to the atmosphere. The venting to the atmosphere is what gives that Pssssht noise you sometimes hear in movies or tricked-out cars.
BOV’s have been very bad for VW/Audi engines because the ECU would monitor the air that was routed back in. If the air was released to the atmosphere as with a BOV it won’t see anything re-entering the system and screw up calculations causing check engine lights and poor performance. Hybrid valves which routed some to the atmosphere and some to the system were available and helped but still could cause problems. So most people just stick with diverter valves but go third-party ones due to reliability and adjustability.
On earlier VW/Audi engines the diverter valve has been a weak point and extremely common to replace with a third party one. They commonly used diaphragm diverter valves which tended to tear and leak, this makes it so the turbo cant hold boost well or at all. They also couldn’t handle increased pressure well over stock like when you flashed the car with performance software. The third-party manufactures such a Forge made solid diverter valves that a lot of us used due to the inadequacies of the stock one.
But this has all changed. Introduced during the MK5/B7 era VW/Audi now uses a piston-style diverter valve that is ECU controlled, which was a great step in the right direction. It held boost better and rarely failed, and with each reversion kept getting even better. Now with the MK7/B9, there is very little reason to even consider a third-party diverter valve due to how solid it is. APR even recommends the stock valve on their STG3+ setups, which is saying a lot!
This made third-party DVs more expensive due to having an extra component to trick the ECU into thinking it’s doing its job. With these third-party DV’s instead of it releasing and building boost when the ECU wants it to, it’s now going back to old school the method of a spring holding boost until it lets go. So even though the ECU thinks things are working the way they expect them to, in reality, it’s not. Most of the time this doesn’t cause issues but sometimes it can manifest problems in odd ways. I personally never liked the spring method and rather have the ECU controlling my DV.
With these new EA888 Gen3 engines on the MK7/B9 the ECU is not monitoring that rerouted air anymore due to switching from a MAF to a MAP sensor. A BOV will not cause problems with the ECU due to the lack of air getting reintroduced. It will still need that piece to trick the ECU but you can run one without throwing codes, you will just be reverting to that old-school spring method again.
Another much cheaper option still uses the stock diverter valve that is ECU controlled and allows for releasing to the atmosphere giving that distinctive sound is the DV spacer. But don’t get excited just yet, because a new issue has arrived. Due to the ECU-controlled DV, it isn’t black and white anymore when the DV is holding or releasing boost pressure. It doesn’t just hold it to X psi every time like when a spring was used. The ECU is constantly monitoring and adjusting, releasing pressure even at times when the old spring system would still be building pressure. Due to this, you get that Pssssht noise at very odd times and frequently. Most who go this route do end up removing it after due to how annoying it can be.
I highly suggest sticking with the stock DV. It’s totally capable as shown by APR and very reliable now that they have the issues figured out.
The DV spacer is a nice cheap option but can be annoying with the ECU controlled DV
A third-party DV or BOV can work due but you are tricking the ECU and reverting your car to primitive technologies. Most of the time these are fine but can sometimes cause problems.
Update (08/15/18): Turbosmart recently came out with a third-party DV and BOV that is controlled by the ECU and not just a pressure spring-based one. You can read more about it here.