10 or 15 years ago the only cool inline-6 turbo engines people seemed to care about were the RB26 and 2JZ, but since then BMW has come onto the scene and shaken things up. It all started the N54 engine, which was their first mass-produced turbo engine, and then came later engines like the N55, S55, B58, and soon there will be other engines available too.
In previous articles we’ve taken an in-depth look at the N55 and we even compared the N54 to the N55, but today we’re going to do something a little bit different and we’re going to compare the N55 to the S55 and find out all the differences and which one is better. Let’s get into it.
Introduction to Both Engines
Not too long after the N54 was released in 2006, BMW began implementing the N55 in 2010 and compared to the N54, the N55 had a significant amount of changes that were designed to make it more reliable because the N54 did have a substantial amount of problems that BMW learned from. While the N54 was BMW’s first mass-produced turbocharged engine, the N55 was their first engine to use a twin-scroll turbocharger.
The S55 came onto the scene four years after the introduction of the N55 in the 2014 F80 M3 and F82 M4. The introduction of the S55 was a really interesting move by BMW because the E90/E92 M3 was the first M3 to use a V8 engine which was a far cry from the high-revving naturally aspirated inline-6 engines they had used in the E36 and E46 M3s. For them to go back to an inline-6 engine from a V8 was a sign of them acknowledging what a lot of their customers wanted.
Before we get into a lot of the details we should cover some basic info in case you’re not familiar with BMW engines. Starting with the N55, we’re dealing with an all-aluminum 3.0L straight-six engine with dual overhead cams, variable valve timing, and variable valve lift. Bore measures at 84mm and stroke at 89.6mm. This brings the total displacement up to 2,979cc. This is the same displacement, bore, and stroke as the BMW N54 before it, and it also features the same 10.2:1 compression ratio.
The S55 is just a high-performance version of the N55, meaning it carries a lot of similar parts and design features from the N55. According to BMW, the N55 and S55 share 75% of their engine components and 25% of the engine components are new developments. Both and stroke are left the same at 84mm and 89.6mm. It also maintains the same 10.2:1 compression ratio.
As far as applications go the N55 was used in the 135i, 335i, 535i, X1, x3, x5, 640i, 740i, M235i, and more. It was used in pretty much everything BMW offered, from small coupes up to large SUVs, which is a testament to how versatile this engine is. Depending on the specific model you’re looking at, power output ranges from 302hp and 295lb-ft ll the way up to 365hp and 369lb-ft.
For the S55 we’re looking at exclusively high-performance applications since that’s what it was specifically developed for. You’ll find the S55 in the F80 M3, F82 M4, F87 M2 Comp. Within these applications, there are a few different power ratings ranging from 359 horsepower up the 493 horsepower, which is a significant increase compared to the N55 it’s based on.
Working from the top down, let’s take a closer look at the cylinder heads of each engine. Both engines use an all-aluminum design, which means the cylinder head is constructed from cast aluminum. Compared to cast iron, cast aluminum heads are much lighter and offer superior thermal performance which is important because the cylinder head is a big determining factor in how much power your engine makes.
Both engines are also dual overhead cam with four-valves per cylinder.
In terms of design, the S55 head and the N55 are very similar, but BMW did make a few changes which allow the S55 to flow a bit more air and ultimately increased performance. VANOS remained pretty much the same on both engines but slightly improved with minor revisions, most of which are designed to help improve reliability since the N55’s VANOS system can become faulty if it gets slightly dirty.
One of the more interesting changes in the cylinder head is the S55’s addition of coolant passages around the injectors for indirect cooling. This isn’t something you’ll typically find on a direct injection engine so it’s cool that they added that in there.
For cam specs, the N55 and S55 are actually identical. They share the same adjustment range, duration, and lift.
- 32mm intake valve
- 28mm exhaust valve
- max valve lift int/exh: 9.9/9.7mm
- Vanos adjustment ranle int/exh: 70/55 degrees
Duration int/exh: 255/261
Another random little change you’ll find is the S55’s head which has different water through-passages for higher coolant flow rate and improved cooling efficiency. An important thing to note is that S55 intake valves are the same and the N55’s intake valves, but S55 exhaust valve stems are bigger and they’re sodium filled for improved heat transfer.
Where things get a lot different is is with the turbo systems bolted to the cylinder head of each engine. On the N55 you’ll find the Twin Scroll single turbocharger manufactured by Borg-Warner. Not too surprisingly, the OEM turbo is pretty small but provides great throttle response and a ton of low-end power, which is something you really want for a street car.
Without getting into too much detail, a twin-scroll turbo can provide better throttle response and increased boost pressure compared to a single scroll turbo. The reason BMW ditched the twin-turbo system on the N54 for the single turbo system on the N55 isn’t totally clear, which is ironic because the reverted back to a twin-turbo system on the S55.
The S55 uses two mono-scroll turbos, each of which have cast-in exhaust manifolds. With this system, each turbo has its own bank, meaning each turbo is powered by three cylinders. With this twin-turbo system came the need for a new intake system, exhaust system, and intercooling system. The S55 has indirect charge air cooling with two heat exchangers and a separate cooling circuit with an electric pump.
What’s really impressive about the S55’s twin-turbo system is that it actually weighs the same as the N55’s single turbo system according to BMW. It’s really weird how they went from the N54 being twin-turbo to the N55 being single turbo, back the S55 being twin-turbo. It’s almost like they can’t make up their mind on which one is actually superior.
Moving on to the fueling system, we find similarities but again the S55 basically just has upgraded components to support the increased power output compared to the N55. The biggest change in the fueling system is the S55’s upgraded double-piston high-pressure fuel pump which capable of flowing quite a bit more fuel than the N55’s single-piston high-pressure fuel pump.
Cylinder Blocks and Bottom End
Moving down to the block and bottom end of each engine, we again find the S55 to have some key upgrades over the N55. Most notably, the S55 uses a closed deck block for improved rigidity while the N55 uses an open deck block. We’ve covered open deck vs closed deck in another video, but to put it simply a closed deck block provides more cylinder rigidity at the top of the cylinder where it’s most likely to fail. Both blocks are constructed from cast aluminum.
The S55 does not have molded cylinder liners made from cast-iron like the N55, instead, the S55 uses LDS-coated aluminum cylinder liners. Since the S55 uses a mechanical water pump instead of an electric pump, you’ll also find different cooling ducts in the crankcase.
Interestingly enough, a lot of people claim the S55 has a lightweight crankshaft, but it’s only lightweight compared to the steel crankshaft found in the N5530B0 (m235i) engine, but it’s actually heavier by 2.2lbs compared to the cast-iron crankshaft found in the standard N55B30M0 engine. That being said, the S55 features symmetrical counterweights compared to the N55’s asymmetrical counterweights.
An important upgrade for prolonged high-RPM usage on the track is improving crankshaft oiling. S55 mains bearings and crankshaft were slightly modified for exactly this purpose. The pistons on the S55 were also improved and slightly stronger and they also feature Grafal-coating on the piston skirt which is necessary due to the LDS coated cylinder liners.
All in all, these changes made the S55 3% lighter than the S65 it replaced, which is pretty impressive considering it has two turbos strapped to the side of it.
The last few changes that I think are worth mentioning are the S55’s improved cooling system with a mechanical water pump instead of the electronic pump found in the N54 and N55, and the S55’s additional electric water pump for the turbos. The S55 also features a magnesium oil pan with is 1000 grams or 2.2lbs lighter than the N55’s aluminum oil pan.
I know we covered a lot of info in this video, but the important changes between the N55 and S55 are the S55’s closed deck block, stronger pistons, sodium-filled exhaust valves, double-piston high-pressure fuel pump, twin-turbo system compared to single turbo system, and revised intercooling system. As stated at the start of the video, the S55 uses 75% of the same parts as the N55, so they’re very very similar.
The S55 basically just has a handful of modifications that allow it to reliably produce more power and be prepped for track use. As far as which one is better, I think it’s pretty obvious what the answer to that question is. The S55 is just an upgraded N55, there are really no downsides to it.