We see this question come up often with various answers. What is the max boost S55 stock twin turbos can handle? How much PSI can they handle without completely killing longevity? These are valid questions but, unfortunately, there is no one size fits all answer. In this guide, we will lay it out as straight forward as possible and provide additional insight.
*S55 Doc Race Upgraded Twin Turbo Kit pictured above
S55 Stock Twin Turbo Max Boost/PSI
There are a few things to consider before we dive into the numbers. You likely do not want to completely max the S55 stock turbos if you're concerned about longevity. Also, pushing turbos too far outside their efficiency range provides diminishing gains. Furthermore, S55 turbos are capable of pushing mid-range torque to the upper limits of the engine. Nonetheless, the S55's max boost on stock turbos is as follows:
- ~30 PSI max boost
- 25-26 PSI "safe" limit
If you're set on taking the stock turbos to the limit they will hold about 30psi peak boost. It's likely that kind of boost is going to push the S55 into the ballpark of 675-700+wtq. Not only is that straying outside of the factory DCT limits, but it's also pushing the limits of the engine itself. There's a lot that goes into that previous comment, and we don't want to get too far off track. However, larger aftermarket turbos are generally safer than OEM turbos at 30psi. This is due to more efficient turbos and manifolds that assist in reducing back-pressure in the manifold. Stock turbos are well outside their efficiency range at 30psi and that back-pressure becomes a serious concern for turbo and engine longevity.
Ultimately, we believe 25-26psi is a relatively safe limit for S55 stock turbos. That limit also assumes you have appropriate supporting mods, such as downpipes. Yes, there is still risk. Not many S55's have been pushed at this kind of boost for 40,000+ miles so the long-term effects are still somewhat unknown.
*We only considered max peak PSI for the purpose of this post. Boost tapers off towards the top-end and will bring torque down along with it.
S55 Turbo Longevity at 25+ PSI
As with any longevity questions - it's tough to put exact numbers on this. In general, higher boost will have a negative impact on S55 turbo longevity. Even that's not a perfect statement. Quite frankly, there haven't been enough S55's running 25+psi for a long enough period to know. One set of S55 turbos may decide to let go at 23psi after 30,000 miles. Another set may hold 27psi for 80,000+ miles.
Then you have differences in driving style and how often the S55 turbos are called upon. For example, an S55 running 200 miles at the track for every 1,000 miles driven should expect a shorter turbo life than the S55 that's highway driven and only sees peak boost a few times every 1,000 miles.
We apologize for the vague information and we truly wish there were a way to put exact numbers on it. Most importantly, understand the further and harder you push the turbos the more risk you're accepting. They may hold up for 100,000 miles at 25+psi, or you may be dropping in new turbos in 25,000 miles.
S55 Stock Turbos at Elevation
We won't dive too far into this, but air becomes thinner at higher elevations. This means the turbos are working harder as they are tasked with compressing thinner air. As a quick example, standard air pressure at sea level is 14.7psi as compared to 12.2psi at 5,000 feet. Now let's assume two S55's are both running 25psi at each elevation:
- 5,000 feet = 25 psi / 12.2 psi = 2.05x
- Sea level = 25 psi / 14.7 psi = 1.70x
The S55 at elevation is compressing air 2.05x relative to ambient air pressure of 12.2psi. At sea level, the S55 is only compressing air 1.7x denser that ambient pressure. Point is - the turbos at elevation are working quite a bit harder. As such, we recommend backing out a few PSI at higher elevations.
Fortunately, for those of us at higher elevations, we often have cooler year round temperatures. PSI isn't everything. Colder temperatures result in denser air. Additionally, colder air may allow you to advance timing and rely on that as a means to picking up a few extra horsepower - instead of simply pushing the turbos to their limits.
S55 Stock Turbo Max Boost Summary
Hopefully this post was helpful despite it's vague nature. Unfortunately, these things are just tough to put exact numbers on. We do know the S55 stock turbos will make roughly 30psi absolutely max boost. Though, it's really not recommended as it begins to push the turbos, engine, and DCT to their limits. A "safe" upper limit for the S55 stock turbos lies in the 25-26psi ballpark. Push too far beyond 26psi and the turbos quickly exceed their efficiency range.
Longevity is another concern when pushing turbos too far. In general, the stock turbos should hold up well anywhere under 25-26psi. Though the long-term effects of driving aggressively for 50,000+ miles at that PSI is still relatively unknown. For those at higher elevations - we recommend dialing back boost a few PSI to compensate for the thinner air. Nonetheless, the S55 is an excellent engine that is highly capable even with stock turbos, fueling, and transmissions. Play it safe (or accept the risk of pushing the limit), and most importantly have fun.
Check out our post about building a 600+whp S55 on stock turbos
Or our S55 upgraded twin turbo post for those who want more out of their S55
Please feel free to drop a comment and share your experiences with the S55 stock twin turbos