While the phrase that something is "an art, not science" is used frequently, it does not hold true with turbo boost and tuning. Turbochargers, engine internals, and ancillarily engine support systems can only hold so much boost before they give in. We're here to answer some frequently asked questions on B58 boost levels:
How much boost can the stock B58 turbocharger handle? How much boost (psi) can I run without hurting reliability?
B58 Stock Turbo Max Boost
On the stock ECU tune, the B58 will produce a maximum boost of approx. 8-11psi, which is slightly up from the 7-10psi found in the N54 and N55. However, as with most BMW engines, the B58 is significantly underrated from the factory, and the engine is built to hold significantly more power and more boost than you'll find on the stock tune.
With that being said, there is a fine line between maximum boost, and maximum reliable boost. Running the turbos at max boost for a prolonged period of time will have significant wear on the turbos and overall engine, and therefore reduce longevity and reliability.
- 23-24 psi is the max boost we have seen so far
- 20-21 psi is what we consider the safe max boost, but have heard of a few folks running 21-22psi reliably.
To break the 20+psi mark, you're going to want full supporting mods (intake, intercooler, chargepipe, DP's, etc) and should be running meth/PI and an ethanol fuel mix. 20psi or more of boost is usually hit around the 500-1,000rpm range, and then tapers off until redline, with max redline boost around 11-13psi.
While the max boost on an N55 is in the 24-26psi range, the B58 seems to be capped due to fueling. Given the engine is a few years younger, there might be a few more psi to come in the future once the fueling piece gets figured out.
B58 Reliability and Longevity at 20+psi
It's hard to say exactly how long the stock turbo will last at 20psi or more. The one thing that is certain is that the engine internals are certainly capable of holding these boost levels. We've heard reports from people running 20-21psi for 20,000+ miles without any issues, with one report from a forum member saying he has been running 20psi for 50,00 miles.
From what we've been able to gather, it appears the turbocharger on the B58 is very strong. With that being said, there is obviously a significantly elevated risk of blowing a turbo at these boost levels. Just because one individual has made it 50k miles, doesn't mean yours won't die at 5,000 miles.
Running above the 20psi mark will create a flattened benefit curve, as turbo efficiency will decrease. Ultimately, you are pushing the turbo a lot harder for a smaller amount of performance gain.
The effect of altitude on boost pressure
For those of you who, like us, live at altitude levels materially above sea level, running 20psi+ of boost is more "risky" than at sea level. The air is thinner at higher altitudes which means the engine has to work harder to compress the same amount of air as it would at sea level. Therefore, we recommended turning the boost down slightly depending on your elevation.
On the flip side, the benefit of altitude is faster turbo spool and ability to run more aggressive timing due to the usually colder air temps. If you live in a high altitude area, we recommend looking for additional power gains by improving fueling (meth, port injection, etc.) instead of maxing out the turbo.
B58 Max Boost Summary
The B58 has a highly capable and reliable turbocharger. The maximum we've seen to stock turbo run is 23-24psi at the time of this writing. There appears to be a fuel cap that is preventing the turbo from being pushed further at this time. The reliability threshold is going to be around 20-21psi, although you are likely still deteriorating your longevity a meaningful amount at these levels.
Above ~21psi, you are taking more risk than reward as turbo efficiency decreases and stress increases. Additionally, if you live at higher elevation, tone back your boost by a few psi to account for the thin air.
What boost levels do you run on your B58? Let us know if you've seen the 24psi threshold be broken or if you have experience running high boost for a prolonged period of time!