Whhhhhhhhh–Pssssshhhhhhhhhh! (that’s my attempt of a spooling turbo followed by the blow-off valve)
BMW's boost pressure release system is controlled by a diverter valve instead of a traditional blow off valve. A diverter valve serves the same purpose as a blow off valve, although they function somewhat differently. N54's come stock with diverter valves, mounted onto a chargepipe, which is responsible for routing air to the engine. As we'll discuss below, N54 chargepipes are notorious for breaking.
We're going to walk through the importance of chargepipes and discuss the benefits and function of a BOV vs. Diverter Valves. From there we will talk about why the N54 chargepipe fails so easily and what you can do to prevent this mechanical breakdown.
What Does the N54 Diverter Valve Do?
The N54 comes stock with a chargepipe and a diverter valve. To explain a diverter valve with a simple explanation, let’s think of a Y-pipe. Under normal driving, the air is being sent to the left. But, as you begin accelerating, the turbochargers build up boost and begin to push against a diaphragm in the Y-pipe, which starts bleeding air into the right side of the Y-pipe. As you let your foot off the gas, the diverter valve will open fully and release the air to the right of the Y-pipe, which releases all of the boost pressure.
Understanding the Chargepipe
The N54 chargepipe routes the pressurized air from the:
Turbo > Intercooler > Chargepipe > Engine
It essentially carries the compressed air (boost) into the engine for combustion. To explain it simply: the chargepipe is responsible for holding boost and routing the "charged" air to the engine.
N54 BOV vs. Diverter Valves
A BOV releases the pressurized air into the atmosphere (engine bay, essentially), aka it releases all of the boost pressure. The diverter valves, rather than releasing air into the atmosphere, recirculate the air back into the engine system.
The big difference: recirculating the air back into the turbo doesn't create that "whoosh" sound. Diverter valves are quieter.
As your diverter valve ages, it is subject to normal wear and tear which can result in "leaking" boost. The leak is usually minimal, but typically results in an increase in wastegate duty cycles which puts more wear and heat on the turbochargers which ultimately reduces efficiency and longevity of the turbo.
If you are concerned of leaking boost, or excess turbo wear, here is the definitive answer: if you like the "whoosh" sound, buy a BOV. If you think that noise is rice, then upgrade your diverter valves.
N54 Chargepipes Fail Extremely Easily: Why?
As the air passes through the chargepipe prior to entering the engine, it is highly compressed from the turbocharger. The more boost (psi) you are running, the more compressed the air is. This creates a ton of pressure and puts a lot of stress on the chargepipe to hold that pressure.
BMW made a big mistake by making the chargepipe plastic. Due to all the pressure from the compressed air, the chargepipe is extremely prone to cracking, leaking boost, or just exploding from the pressure.
Leaking boost or completely breaking your chargepipe is highly probably if you run above stock boost. If your chargepipe breaks while your driving or on the highway, you will need to tow it home – so we highly recommend upgrading it prior to this happening if you are going to tune your car. Upgraded chargepipes are cheap, so it’s a quality preventative maintenance/safety upgrade. You might notice some power increase if you are currently leaking boost, but otherwise, this upgrade is purely preventative and will not significantly impact performance.
Preventing N54 Chargepipe Failure
I didn't want my title to be deceiving, so I'm going to let you in on the secret to preventing chargepipe failure:
UPGRADE YOUR CHARGEPIPE!
Upgrading your Chargepipe + BOV
Chargepipes and blow-off valves (BOV) go hand in hand as the BOV is mounted onto the chargepipe. When it comes to upgrading, you have the option to just replace the chargepipe, or replace the chargepipe and add a BOV. By simply replacing the chargepipe, your N54 will still use the OEM diverter valve system.
Adding a blow-off valve will provide a little performance benefit, and zero reliability benefit to your N54. However, a BOV will hold higher levels of boost than the stock diverter valves will, since the diverter valves begin to slowly bleed (a minimal amount of) boost at high psi levels. The performance benefit isn't huge, but it's enough to point it out. The higher the boost levels you are running, the more noticeable the benefits of a BOV (or upgraded diverter valves) will be.
If you are going to upgrade your turbos and run very high boost levels, then I'd recommend adding the BOV, or upgraded diverter valves.
Recommended Chargepipes and BOV’s
Masata N54 Chargepipe - $225, best for people International readers
Turbosmart Kompact Dual Port BOV - $475 - (this is claimed to be the loudest direct replacement on the market)
It is worth noting that Turbosmart and Forge Motorsports make upgraded diverter valves. The price of these is comparable to a chargepipe and blow of valve combo. I have no preference
Chargepipes are pretty much made equal, the ER chargepipe is only more expensive because it is anodized and polished. Same goes for BOV’s, there is no real difference between the two other than sound. The HKS blow-off valve is known to be louder, so go for that one if you want a more aggressive, louder sound.
DIY’ing the chargepipe is easy for beginners. I’d recommend doing it yourself and not paying a shop, here is a good guide: ECS Chargepipe DIY Guide