DO YOU NEED A BOV FOR YOUR N54?
This is a question we receive almost daily here at ARM. An upgraded BOV, be it TiAL, HKS, or any other brand is a pricey addition to your N54 charge pipe purchase, compared to just upgrading the charge pipe alone and reusing the stock N54 diverter valves. Is it really worth it? A valid question we want to address.
N54 CHARGE PIPE STYLES DISCUSSED IN THIS POST
HOW A BOV WORKS: THE BASICS
For those of you who are already familiar with this feel free to skip down to the next section, but if you’re new to turbocharged platforms, here is a simple breakdown of how a blow off valve (BOV) works.
A BOV is much more than just a fancy whooshing mechanism, it is responsible for holding and releasing all of the charge air on your N54. You press the throttle and the turbos spool up pushing compressed air into the engine. Suddenly, you lift off the gas pedal to slow down, the throttle body plate closes, but where does all that compressed air go?
The BOV opens, relieving the excess pressure saving your N54 from damage. Once the pressure is reduced, the BOV closes back up on its own making a tight seal allowing it to build more boost when commanded. So how does the N54 handle this?
THE STOCK N54 CHARGE PIPE
The OEM N54 setup uses two diverter valves (DV's) to release the pressure. These diverter valves are of the recirculating type. The recirculating design is such that when you let off the gas pedal the throttle closes, and the diverter valves, redirect the pressure from the charge pipe into the turbo inlets.
The main concern is “Will the OEM diverter valves hold boost?”, and the definite answer is, maybe. When working properly, we’ve seen the OEM diverter valves hold 20psi. We’ve also seen cases where they failed on just a Stage 1 tune. This can happen at 50k miles, or 120k miles, every car is different and there’s no way to be certain of when it will happen.
Coming from the factory, the N54 engine’s peak boost is only about 9psi. The OEM diverter valves are more than adequate for this boost level, but even with a basic off the shelf stage 2 map you will be hitting almost double the stock boost. Not to mention that many custom maps are now running over 20psi. The N54’s OEM diverter valves simply weren't designed to hold that much boost pressure. On failing OEM DV's the turbos begin to build boost the OEM DV's will begin to struggle trying to keep a tight seal and before you can hit peak boost, try as they may, they just can’t keep it together, forced open by the newfound boost, leaking the precious power you’ve worked so hard to accumulate.
WHY AN UPGRADED BOV IS A GOOD IDEA
An upgraded BOV will hold up to anything you can throw at it. The two most common BOV's for the N54 are the TiAL Q and HKS SSQV. Both of these BOV's utilize the venting to atmosphere design. Like it sounds, the BOV releases the built up pressure into the open air, not redirecting it back into the N54 turbo inlets.
While we offer both TiAL and HKS charge pipes, our favorite version is the TiAL and this is for a couple of reasons. One is that the TiAL Q BOV locks onto the charge pipe using a v-band clamp. This allows for a more secure connection than the HKS which uses a spring clip. It's also available with different spring rates allowing you to select the correct spring rate for different cars. While the SSQV has one design for all cars. For the N54, an 11psi spring is best (more on this below), and this is what we use on our Project N54.
However, some N54 owners prefer the sound of the HKS and that is enough reason to make it worth going with the SSQV. The HKS has more of a high-pitched chirping sound compared to the TiAL Q.
BOV SPRING RATES
Every BOV or diverter valve has in it’s guts a spring which is designed to keep it closed when it’s supposed to (under throttle) but allow it to open when necessary (off throttle). These spring pressures are also measured in psi. This makes it a commonly confusing situation where many often wonder how their BOV with an 11psi spring is going to stay closed on their N54 engine making 20psi of boost. A reasonable inquiry.
Therein lies the fallacy that the BOV spring pressure directly relates to the boost pressure. It does not. Rather, the spring pressure that is appropriate for a vehicle is determined by its negative pressure, also known as it's "vacuum". This is simply the amount of pressure the intake manifold wants to pull in when your foot is completely off the accelerator pedal.
For the N54, the correct spring rate using the TiAL Q BOV is 11psi. This is the same spring rate for the N54 regardless of how much boost your running. If you’re running 13psi of boost, your BOV spring rate should be 11psi, if you’re running 22psi of boost, your BOV spring rate should still be 11psi.
SO DO YOU REALLY “NEED” A BOV?
If you're going to keep your N54 fairly stock, you can get away with reusing your stock diverter valves. However, if you plan to get the most out of your N54, even on the stock turbos, and especially for upgraded turbo builds, upgrading to a BOV is a must!
Otherwise, if you have your heart set on the look or sound of the HKS BOV, then the ARM N54 Charge Pipe with HKS flange is the best option for you.
However, if you're in a pinch and looking to replace your faulty OEM N54 charge pipe without breaking the bank to get the job done, the best option is the ARM N54 Charge Pipe with OEM DV connections.
Hopefully your found this information helpful, if you would like more support for tuning your BMW, contact us at firstname.lastname@example.org