Which is better, the N54 or N55?

A common debate in the BMW world, the N54 is the predecessor of the N55, found in the 335i from 2007 to 2009 where the N55 began to be phased into production.

Depending on what you want from each engine will dictate the best choice for you. Below we will analyse the difference between each engine, and what impact this has to the car.

 

Key Differences

Twin Turbo vs Single Twin-Scroll

The earlier N54 features two turbochargers whilst the N55 boasts a single twin-scroll turbocharger. A twin-scroll is a single turbo where each bank of cylinders feeds into a separate scroll of the turbo. Its design ensures each 3 cylinders on the same firing cycle feed into the same scroll aiming to reduce exhaust reversion. Reversion is exhaust gases entering the combustion chamber as the gases react with each other, which causes higher temperatures resulting in less power and higher emissions.

Therefore the twin-scroll design is more efficient, resulting in reduced spool time, increased power and lower cylinder temperatures. Despite the advantages of the twin-scroll design, a true twin turbo setup will also reduce reversion as each bank of cylinders on the same firing pattern will feed into each turbo.

A minor advantage of the twin-scroll vs twin turbo is the amount of oil caught up in the turbos. As the sump distributes oil to the engine as well as turbos, by having one larger turbo results in (marginally) less oil in the turbochargers, resulting in reduced oil temperatures.

 

Forged vs Cast Iron Internals

From factory the N54 has a forged crankshaft and connecting rods, and cast iron pistons. The N55 used cast iron crankshaft, rods and pistons. Both internals are durable and can take plenty of abuse from the factory. Unless you’re looking to push a lot of PSI through either of these engines, the stock internals will hold up fine.

On paper the forged internals are stronger, and therefore can take more abuse. This would mean you have more potential to add power without worrying about core engine components, and can spend money elsewhere.

Interestingly the N55 crankshaft weighs 20.3KG, approximately 3KG less than the crankshaft in the N54 engine.

 

Piezoelectric vs Solenoid Injectors

Piezoelectric injectors were featured in the N54 which were replaced by traditional solenoid injectors when the N55 was released. Many owners reported problems with the piezoelectric injectors, from reliability to carbon build up due to their design.

The injectors are piezoelectric, compared to the improved N55 design of solenoid injectors. Solenoid injectors which are found in most vehicles are more reliable, however their downside is less precision and response time. This becomes a bottleneck when running higher boost and more fuel to achieve increased WHP.

Bosch state piezoelectric injectors can react up to 5 times faster than solenoid injectors. This translates into precise injection of fuel, and enables multiple injections within a single combustion.

The piezoelectric injectors have been an issue compared to solenoid injectors due to unreliability, more noise and higher expense when they need to be replaced.

 

Crankcase Ventilation

With any engine a crankcase breather or PCV system is necessary. This system is put in place to ventilate blow-by gases and oil vapour which are a by-product of combustion. The system will route these gases into the intake tract in an effort to improve emissions.

Due to the upgraded design introduced in the N55, less carbon build up within the intake manifold occurs within the same time period. It’s not uncommon for N54 engines to require walnut shell blasting to remove the build up around 50k miles. Compared to the N55’s design which has a much longer life, with some owners still running on original components (with no symptoms of failure) at 100k miles!

 

Exhaust

The N54 has two main catalytic converters immediately after the tubos, with two smaller catalytic converters in the mid-section of the exhaust. The N55 uses one larger catalytic converter immediately after the turbocharger with no secondary catalytic converters.

Both engines are equipped with a resonator at the end of the dual pipes, and have slight variations of muffler design. In the N54’s muffler, the Y pipe is perforated resulting in the exhaust flap having a minor change to the exhaust note. The N55’s exhaust pipe is solid which means the exhaust flap has a larger effect on the exhaust note.

Subject to opinion the N55 sounds more aggressive and raw from stock, partly helped by the fewer catalytic converters to muffle sound and solid Y pipe in the muffler. Here is a great video that demonstrates the difference:

https://youtu.be/SaPGNReWdR8

Tuning Potential

Twin vs Single Twin-Scroll

The earlier N54 responds better to tuning, due to its twin turbocharged design which moves more air and does so more efficiently. From factory both engines produce the same power, with the N54’s advantage to add more power comes from the twin turbo setup. For example a simple tune can add up to 100hp (Think JB4, COBB, Dinan) to the N54, but the N55 gains are a little less at 60hp on a stock motor.

A key difference between both engines is the turbochargers attached. The N54 uses a twin turbocharged design, whereas the N55 uses a single twin-scroll turbocharger. The twin-scroll is able to produce peak torque 100RPM sooner, but doesn’t have the tuneability the twin turbocharged design offers. Despite the lack of horsepower gains the N55 boasts reliability, as it’s an improved version over its predecessor.

 

Downpipes

Both engines with the addition of a catless or better flowing downpipe will see performance gains, especially if paired with a tune increasing boost pressure from turbos. The drop in air pressure pre and post turbo aids in the turbocharger spool. A common misconception is a slight backpressure is best for turbocharged engines, where in reality the best flowing exhaust is better suited.

By eliminating the backpressure created by the catalytic converter you reduce the backpressure, enabling the turbos to spool faster. Your exhaust note will also be more aggressive, whether you like this is down to personal taste.

On the N54 engines expect to see 15-20WHP gains, compared to around 5 on the N55 due to higher flowing catalytic converters from stock.

Also remember your car will most likely not pass emissions test, and will throw a Check Engine Light. If you plan to perform this modification it’s best to abide by your areas emissions testing.

 

Intake

Another restrictive section of the engine are the intakes. Easy to install and generally a good HP vs $ gain in terms of power, depending on the intake purchased. Most commonly fitted to the N54 platform is a dual code intake, however this can cause heat soak as air is drawn from inside the engine bay where the air is a higher temperature.

It’s best to choose a sealed intake that draws cold air only, as heat soak from the engine bay will have a detrimental impact to performance as the air is less dense.

Assuming the intake is restricting power gains, expect 15-20WHP on the N54 platform, and 10-15WHP on the N55 platform. This can vary depending on which intake is fitted, and whether your engine can utilise the increased flow of air.

 

Intercooler

The downside of the stock intake systems is heat produced by turbochargers affecting air intake temperatures.  As the air intake temperature heats up from spirited driving, the DME will pull timing and boost due to the air being less dense.

To remedy this an intercooler will cool the hot air after leaving the turbocharger, resulting in consistent performance and a small gain. You will notice after a few WOT pulls that the car doesn’t feel as aggressive, and your butt dyno isn’t wrong. This is exaggerated if you run a tune that increases boost.

A larger intercooler has more surface area, therefore increasing the amount of air that can be cooled simultaneously. As colder air is denser you will have HP gains.

As with the intake, HP gains can vary depending on size of intercooler and ambient air temperature at the time of testing. On the N54 platform you can expect around 5-10WHP which is the same as the N55. The biggest advantage of an intercooler is consistent performance despite increased engine temperatures from hard driving.

 

Tune

Due to the turbochargers attached to these engines make them a viable platform for tuning. There are many different options for both platforms, each with their own advantages, disadvantages and gains.

On the N54 platform you have several options, JB4 being a common addon which we will use as a comparison on the N55 platform. The tune is a piggyback on the ECU’s base map, making it easy to turn on and off. It also has 9 maps, progressing in performance gained.

Running on high octane fuel you can expect around 30WHP using the JB4 tune with an N54 stock engine. This is assuming map 1 is used on a stock car. Compare this to a similar 30WHP gain on the N55 platform on a stock engine.

The advantages of the JB4 platform is the multiple maps supplied. As you upgrade your car to handle more boost, you can choose a more aggressive map to reap larger gains.

It’s also important to note that these tunes may supply more or less gains depending on the condition of your car, ambient air temp, fuel used…etc, and should be used as a guide only.

 

Reliability

There were many teething problems with the N54, many of which had generous warranties to remedy these issues. However the N55 is the redesigned engine based on the N54’s failures, therefore having less issues from the start.

 

High Pressure Fuel Pump

The HPFP was the most common problem found with the N54. The pump provides injectors with the fuel needed for combustion, failure of the pump results in a non-start. Despite this large issue BMW have issued a 10 year/120k mile warranty on the pump, if you’re on the market this is a must as the warranty may be near expiration.

Symptoms of HPFP Failure

Sometimes you can experience symptoms before the pump completely fails, some of which are commonly thrown codes:

  • P142E: low pressure in HPFP system – injection disables to protect catalytic converters
  • 2FBF: Fuel pressure at injection release
  • 2FBE: Fuel pressure after motor stop
  • 29DC: Cylinder injection switch off
  • 29E2: Fuel injection rail
  • P030(1-6): cylinder misfire (last digit of code is the cylinder responsible)

Or you may experience:

  • Long crank times when attempting to start the car
  • Check Engine Light (Will refer to one of the above codes)
  • Car will enter limp mode with reduced power, rev range and top speed to protect engine internals

If you have an original fuel pump it’s a good idea to get it changed. Interestingly there doesn’t appear to be a correlation between tuning your car and failed pumps, which points to a design/manufacturing fault.

If you’re out of your warranty period with a failed pump, you can find a fantastic DIY detailing the process here(http://www.e90post.com/forums/showthread.php?t=654820). Expect the spare part to cost around $400 not including installation, and is a fairly advanced DIY.

 

Injectors

Another problem is the N54 fuel injectors. Paired with fuel pump issues it’s not surprising that the injectors are next on the list. The purpose of fuel injectors is to produce a fine and controlled spray of fuel over the spark plugs and ignition coils ready to be combusted on an ignition stroke of the engine.

Clogged Injectors

As carbon naturally builds up within the engine as part of the combustion process, injectors are also prone to carbon buildup. Normally carbon is burned off due to the high temperatures within the combustion chamber, however when the engine cools it can cause excess buildup. The issue with this is it effects the spray pattern of the injectors, causing either too much or too little fuel contacting the spark plug, resulting in cold start issues or misfires.

Leaking Injectors

Leaking injectors is another issue with the N54, the angle of the injectors means any leaks will drip onto the spark plug. If there is a considerable leak this will soak the plugs inhibiting a normal spark needed to ignite the unburnt fuel. This will also produce misfires or difficulty starting the car.

 

Turbocharger Failure/Wastegate Rattle

The wastegates job is to regulate the flow of exhaust gases away from the turbochargers, this then controls how fast the turbos spool and therefore generate boost pressure. In simpler terms the wastegate controls the maximum boost pressure created by the turbos, to prevent overboosting and running your engines internals.

The common rattle is actually the wastegate actuator arm and rod improperly functioning, due to wear and tear from normal use. There is a recall available which covers the turbos for up to 10 years or 82,000 miles, and is recommended to be completed because the whole procedure takes around 15 hours, with the turbochargers costing upwards of $1,000.

 

Chargepipe Leak

If your car is fully stock it’s fairly unlikely you will run into this problem. When you introduce tunes which increase the amount of boost produced by the turbos, the chargepipe is one of the first components to break.

The function of the chargepipe is to hold boost until it’s released by the blowoff valve. As this is a plastic component it becomes brittle especially when running increased boost over stock. The increased pressure can cause cracks which leak air, or complete burst of the pipe.

If you’re running a methanol/ethanol setup an upgraded chargepipe is a mandatory upgrade. If you spend any time on forums you will dig up many posts of users chargepipes bursting while driving.

 

Valve Cover Leak

Another issue is a valve cover gasket leak, prone to degrading which results in oil leaks down the side of your engine block and lean running condition.

It’s also common for heat cycling of the engine to cause cracks in the gasket, which also result in oil leaks and a loss of vacuum within the crankcase.

You may also smell the burning oil, lose a lot more oil than expected (resulting in frequent top ups, or a low oil light). Your spark plugs can also be sitting in a pool of oil depending where the degradation has occurred.

Replacing this gasket on its own is around $35, or $450 for the cover and gasket. If you don’t want to attempt this yourself you will need to add labour on top, it’s not an extremely difficult DIY but certainly not recommended for a novice.