How much power can the stock N55 engine handle? What are the limits on the BMW N55 block, rods, pistons, crank, etc? These are common questions in the aftermarket world, but there is never a perfect answer. However, there are some general safe limits for the N55 engine. In this article, we lay out the N55 engine limits in a straightforward way. We also dive into a deeper analysis of BMW N55 strength and limits.
If you are looking for more N55 content, check out our Ultimate BMW N55 Engine Guide.
BMW N55 Limits - Quick Answer
The N55 can handle around 550whp and 550wtq on the stock engine block and internals. Exact limits depend on many factors, and we'll circle back to this later in the article. For now, we're sticking with the simple answer. However, this next paragraph is very important.
Any power limits we discuss in this article are estimates. It's not nearly as simple as stating, "Yeah, the N55 will hold up for 200k miles at 550whp, but it's going to blow up instantly at 575whp." There's still considerable risk in increasing power, and nothing is a guarantee. Nonetheless, the general rule applies. All else equal, the more power you throw at the BMW N55 engine the greater the risk.
Stock N55 Limits - Rods, Bearings, Pistons (550-600whp)
These are often the first parts to let go on the N55. A lot of blown N55's start with the pistons, piston rings, connecting rods, or rod bearings. The N55 took a small step down in strength compared to the older N54. It's still a fairly strong motor, but seems a little less tolerant to abuse. A bad pre-detonation at 500+whp can quickly bend a rod.
Those same pre-detonations may also crack a piston. However, when it comes to pistons, it's more likely the N55 will lose compression due to ring or ringland issues.
Rod bearing problems are a hot topic for N55 limits. Keep in mind - the internet can quickly blow things out of proportion. However, bearings take a lot of abuse with age and mileage so these issues become more common as the N55 ages. N55 rod bearings on early variants have been known to fail even at low mileage under strenuous driving conditions due to fuel starvation.
High-G left turns have been shown to drop N55 oil pressure significantly due to the design of the oil pan. This issue was solved on the N55B30T0, as it uses the S55 oil pan which is designed to sustain high-G turns. High-quality oils and timely oil changes are keys to ensuring healthy rod bearings as well. Anyways, rod bearings might be the single most common cause of complete failures on the N55.
N55 Stock Engine Block Limits (600-700whp)
When it comes to N55 limits, engine blocks are an interesting discussion. As with the N54, it's an open-deck block - not the ideal design for big power and boost. Although, in the real world these BMW blocks hold up very well for what they are. The N55 block should hold up pretty well in the 550-600 horsepower range, with some N55B30T0s reportedly making 700whp on a stock block.
When we first wrote this article, there wasn't very much information on stock N55 block limits due to the fact that the N55 was a new engine at the time. Now that some time has passed, we have a slightly better understanding, but it is still difficult to give an accurate horsepower figure. Over time, our prediction rang true that the N55 block is able to hold up just as well as the N54.
*Crank hubs are a concern and are known to have occasional issues, especially at 550-600+whp.
N55 Transmission Limits (332 lb-ft ZF, 345 lb-ft 6MT Clutch)
There are other important factors to consider outside of engine construction as well. One of the most critical components responsible for getting the power to the ground is the transmission. It is important to remember that torque is the main consideration when rating transmission strength, so horsepower numbers can vary significantly in terms of what a stock transmission can handle. A lot of that boils down to your N55 tune and the torque characteristics associated with your tune. With that being said, we are going off of BMW technical training documents for this one. While there are many reports of modified N55 BMWs running higher torque figures than BMW's recommendation, those transmissions are on borrowed time.
BMW N55 ZF Automatic Torque Limits
There were two primary automatic transmissions used in BMW N55-equipped cars, both with their own specs and drivability differences. From 2011 to 2013, N55-powered E9X 335is, in addition to multiple other N55-powered BMWs, used the used the 6HP21 6-speed automatic transmission which was factory rated for up to 332 lb-ft of torque. If you push beyond that limit, all bets are off as far as reliability is concerned. Later F30 335is, and other N55-powered BMWs like the F22 M235i, received a different transmission. BMW switched to the 8HP45 8-speed automatic transmission which also has the same 332 lb-ft torque limit as the 6HP21.
BMW N55 6MT Torque Limits
In BMW's technical documents, the 6MT actually has a slightly higher torque limit than the automatic N55 transmission options. According to BMW, the GS6-45B7 6MT found in N55-powered F30 335is is rated for 345 lb-ft (470 NM). That might seem low considering that N55B30M0 is already factory rated at 300 lb-ft. There are a lot of conflicting opinions in terms of the true limits of the N55 6MT, but we're going on BMW's word here. Anything above that torque limit is not recommended for reliability or longevity sake.
Stock N55 Limits - Crankshaft (700+whp)
N55 cranks are a tough one. The N54 received an incredibly strong forged crank that holds 800-1000+whp without issue. However, N55 engines receive a standard cast crank. Some claim to have already run into crankshaft issues on the N55. Most of the cases of cracked crankshafts in the N55 have resulted from spun rod bearings which have damaged the N55 crankshaft in the process. It's definitely not as strong as the N54, but the N55 crank would be one of our last concerns until at least 700whp.
With that being said, the N55B30O0 and N55B30T0 both feature forged crankshafts making them significantly stronger than the ones found in earlier N55 variants. There are very few reports of N55 forged cranks having any issues. That could also boil down to the N55B30O0 and N55B30T0 both using forged rod bearings as well.
N55 Limits - Cylinder Head (700+whp)
We're lumping all of the valvetrain in the cylinder head discussion, including valves, springs, cams, etc. These are highly capable parts on the N55 that will support 700+whp. Random failures can and do happen, but it's not common even at high power.
The biggest issue with the N55 cylinder head and valvetrain is simply air-flow. Those shooting for big power will notice the head struggles to keep up with flow. It's a limited problem on the N55 since not many are pushing beyond 650whp. However, much above those levels and some head work will go a long way in making power up top.
Deeper Review of N55 Power Limits
Finally, we're circling back to one of our early comments. There are tons of factors that determine an N55 limits. What are those factors? How can you reduce the risk of blowing up the N55? We will break this discussion into the following topics:
- Power & torque
- Turbo Choice
- Tuning & supporting mods
It's not an exhaustive list. However, the above are a few of the factors we can control. Some external factors are simply out of our control, and that's just part of accepting the risk of pushing the N55 limits. Luck of the draw can play a role. Flukey failures happen. The list goes on.
Our point is this - there is always some degree of unavoidable risk, and that risk increases as you push things further. Regardless, it's important to reduce the risk with a proper setup. Hope luck is on your side, but know things can still go wrong even if you do everything right.
N55 Limits - Horsepower vs Torque
When discussing the upper "safe" N55 limits, torque is actually the better number to look at. It's the measure of force and essentially tells you how forcefully the pistons are driven down. Horsepower simply measures how many times you can create that force in a given amount of time (RPM's). The equation is as follows:
HP = Torque x RPM / 5252
Shifting the power curve to the right allows you to make more power with less torque (less force). Here's are some quick examples:
- 550wtq x 5000 RPMs / 5252 = 523whp
- 550wtq x 6000 RPMs / 5252 = 628whp
Both are subjecting the internals to the same force, but shifting the curve right allows an increase of over 100whp. It's a lot easier to make a given amount of power at higher RPM's.
N55 Turbo Setup
The turbo setup ties into the above discussion by shifting the power curve right. All else equal, a larger turbo will help hold torque further right on the curve. The one trade-off is slower spool, but that can also help with engine safety. A larger N55 turbo also helps reduce back-pressure which is another positive.
We'll avoid getting too technical and writing on and on about this. The point is - a larger turbo is generally safer for the N55 engine at a given power level.
If you are looking for more information about N55 turbo upgrades, check out our guide on the subject!
BMW N55 Maintenance
Alright, this one is pretty obvious. Stay on top of N55 maintenance. Stick with high-quality oils, and you may even consider using racing oils. An oil change interval of 3,000 to 5,000 miles is also a good idea.
When problems occur then fix them in a timely manner. Turn boost down until your N55 is fixed and running well. Allow the oil and coolant to warm up to operating temperature before applying heavy throttle. If things get too hot then give the N55 a gentle cool-down cruise.
It's basic stuff that's a good idea regardless of your N55's power. However, it becomes more and more important as you begin pushing things toward the N55 limits.
N55 Tuning & Mods
Tuning and supporting mods are one of the most important topics. Stick to the conservative side of things. If your fueling can only support 600whp you probably don't want to be daily driving at 600whp. Build in some headroom. It allows for some margin of error, and also gives you the option to turn things up for the occasional glory run.
On the tuning side - go conservative with that too. Ensure you're running the proper fueling. Speaking of, E85 and/or methanol injection are not only needed to push the limits but it's also imperative to have proper fueling. Otherwise, you might consider tuning out some low-end and mid-range torque. Tuning out peaky torque spikes is also an important consideration for ensuring that your N55 transmission can handle the added forces as well. We could go on and on. In fact, we have. Take a look at our N55 BM3 Tuning Guide for more info. Ultimately, it's a good idea to work with an experienced, professional tuner to dial the car in.
Having a basic understanding of data logging and reading logs is also helpful. You want to keep an eye on the engine parameters often. When pushing the N55 limits, a small issue can turn into a major one. You should know everything that's going on with your N55.
Look for lean AFR's, maxed fuel trims, timing pulls, etc. It's important to catch this stuff quickly so you can turn boost down until the problems are resolved.
BMW N55 Engine Strength Summary
The N55 is a solid all-around engine that offers respectable strength from the factory. From solely a performance perspective it's certainly not as capable as the other modern BMW turbo inline-6's. However, the N55 can still handle about 550-600whp with a proper setup.
Rods, pistons, and bearings are a few areas that will first cause trouble. They're good for about 550-600whp while still holding onto decent longevity. The block, cylinder head, and crank should all hold a bit north of 700whp. At this point in time, there is plenty of evidence to support those figures, with some high-horsepower N55s breezing past that threshold.
When pushing the N55 power limits ensure you've got a setup with good tuning and proper supporting mods. Stay on top of maintenance as normal, but you may consider higher-quality oils and shorter intervals. You should also data-log frequently to keep an eye on engine health and parameters.
What's your experience with high-horsepower N55 engines? Drop a comment and let us know!