Capable of crazy power, here’s everything you need to know about BMW’s twin- and single-turbo straight-sixes. Words: Ben Koflach. Photos: BMW/John Colley. BMW proved it could build successful turbocharged petrol engines with the 1973 2002 Turbo, though after that there were very few uses of forced induction for its road cars. That was until the N54 was launched in 2006, which was the start of BMW’s turbocharged revolution. We look into what made it and its successor (the N55) great and how you can get the most out of each.
The N54 engine first appeared in the 2006 E92 335i and was introduced across the BMW range over the following three years. The engine can be thought of as essentially an M54 (3.0-litre straight-six) with a pair of small turbos on the side – the bore, stroke, capacity and compression ratio are identical between the two. However, there are several big differences – the M54 is a closed-deck single-piece block where as the N54 is open deck and consists of two pieces. The water pump is also electric on the N54 whereas on the M54 it’s cast into the front of the block. As well as this, the M54 used a more traditional fuel injection method, whereas the N54 features state of the art direct injection.
Designed to drive like a normally-aspirated engine, the N54 has a relatively high compression ratio for a turbocharged engine at 10.2:1. Two turbos were used to keep lag to a minimum and give just 8.8psi of boost. BMW were successful in their aim – the N54 drives more like an enhanced normally aspirated engine than an out-and-out force-induced motor.
The N54 saw a small revision in 2008, upping power from 306hp to 326hp and torque from 295lb ft to 332lb ft. Its ultimate guise was found only in the 2010 E82 1M Coupé – labeled the TO version. It pushed out 335hp and 332lb ft with an overboost function for even more performance for short stints. While the 1M used an N54, most models had been revised to use the N55 by then.
The main difference between the N54 and N55 is the method of forced induction. Where the former used two small turbos, the updated engine used one larger one, which features a twin-scroll compressor housing. This means that the exhaust-side of the turbo is divided into two parts, each fed by three exhaust primaries each. The result is a big reduction in lag compared to a traditional turbo, giving the N55 very similar driving characteristics to the N54. In fact it’s been said to have even less lag. The reason for using a single turbo is purely for efficiency – 15% better fuel economy and lower emissions, which are helped by the introduction of Valvetronic variable valve lift technology (not fitted to the N54). The N55 produces 302hp and 330lb ft of torque in standard form or 315hp and 330lb ft in its High Performance (HP) variant.
The N54 and N55 share their internal dimensions with the engine they’re based on: the M54. This means that they have an identical bore and stroke (84x89.6mm), and the compression ratio is the same too at 10.2:1, which goes a long way towards explaining why it drives so much like a normally-aspirated engine.
Interestingly, the N54/N55’s normally-aspirated cousin, the N52, uses an aluminium-magnesium engine block, whereas the N54 and N55 do not. Though the magnesium-aluminium material is great for weight saving it was deemed to be too weak so BMW opted for the more traditional approach of using all-aluminium with cast iron cylinder liners. As a result the N54 weighs 195kg versus the N52’s 161kg. Many independent tests find that BMW underrates these engines quite significantly, possibly in some cases to distance ‘35i’ badged models from M3s and the like. In our own test back in PBMW 05/13 we found the M135i, a car claimed to have 315hp from its N55HP, to actually produce an impressive 341hp on Evolve’s Dyno Dynamics rolling road.
Despite an impressive output as standard, the N54 and N55 engines are extremely easy to get significant gains from. Being so similar, the process is the same in essence: by getting gases out of the engine faster and increasing the boost big improvements in output are achieved. The restrictive point of both the N54 and N55 engines are the downpipes. On the N54 you have one of these from each turbo, and on the N55 there is just one. The reason for them being so restrictive is that they have a catalytic converter in them, despite the fact there is also one further down the exhaust. By fitting decatted downpipes (available from the likes of AR Design or aFe – prices start at around £380 for the N54 or £410 for the N55) a decrease in back pressure is achieved. Even alone these can help the turbos spool up quicker and a gain in power will be seen.
It’s when coupled with a remap or piggyback ECU (JB Tuning and Vishnu are very popular choices for these, with packages from around £185-£520 depending on what features you need) to increase the boost with fuelling and timing to match, the results are pretty incredible. Up to 400hp can be expected on an N54, though it’s worth noting that the OE plastic diverter valves aren’t really up to the job. Upgrades from Forge and Turbosmart resolve this issue for around £220. Going further with bolt-on modifications such as a free-flowing intake, exhaust system and uprated intercooler will see around 450hp with relative ease. Unfortunately the N55’s single turbo isn’t as capable as the N54’s twins and so around 420hp can be expected by this point.
Beyond bolt-on steps, you’ll be looking at swapping out the standard turbos. As the N54 and N55 have a high compression ratio you have to be careful about how much boost you apply – too much can result in detonation and melted pistons. Therefore you need to look at reducing your intake temperatures. The tried and tested route is water and methanol injection. Bolt-on kits are available from the likes of Vishnu and Burger Tuning, both of whom offer an intake elbow pre-fitted with injection nozzle mounts, meaning this too is a bolt-on upgrade, although a slightly more involved one. Effectively increasing the octane rating of your fuel while reducing combustion temperatures, it opens a whole new level of safe power without the need to open your N54 or N55.
This power will come in the form of turbo upgrades. We’ve been seeing hybrid turbo upgrades for the N54 for a while and these can give you an improvement in power. Vargas Turbo produces a few different options, from simply improving the standard turbo reliability to its Stage 2 offering which has completely different internals. Prices range from £720 to over £1600. There are plenty of competitor products to look out for, too, including AWD Motorsports and RB.
Beyond hybrids, you’re looking at the world of big turbos. This is where things get hardcore. For the N54, Vargas actually produces kits that use two Garrett GTX turbos (from around £5k) although another very popular route is converting to run one big turbo. Because this only requires one turbo, which is more efficient and cheaper, it’s a good option – the pay off being turbo lag, though this is still relatively minor. You’ll be looking at 650hp+ with these (based on a FFTec Motorsports kit using a 58mm Precision Turbo, which comes in at around £4600). With a bigger turbo you can go all the way up to around 800hp although you again start falling foul of detonation worries. Meth is good enough for around 700hp but beyond that you’ll need to run race fuel or E85.
Turbo options for the N55 are still limited and as it has a weaker cast crankshaft (vs the N54’s forged item) it’s tricky to know just how far it can be taken. However, knowing the BMW tuning industry as we do, it’ll only be a matter of time before there are similar tuning packages available for the N54’s replacement.
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