BMW M51 Tuning

"All you need to know about tuning the BMW M51 engine!"

The BMW M51 engine, produced from 1991 to 2000, is a notable diesel powerplant known for its robust performance and efficiency.

The M51 engine features a six-cylinder inline configuration, offering a balance between power and fuel economy.

During its production years, this engine found its place in a variety of BMW models, including the 5 Series and 7 Series, providing a compelling option for those seeking diesel-powered luxury and performance.

The M51 engine introduced direct injection technology, which optimized fuel combustion and improved fuel efficiency while meeting emissions standards.

Furthermore, BMW incorporated an intercooler in some variants, enhancing the engine's performance by cooling the compressed air before entering the cylinders, resulting in more power.

The M51 engine is celebrated for its durability and longevity, making it a reliable choice for diesel enthusiasts who value long-term performance.

Enthusiasts have also explored tuning options for the M51 engine to further enhance power and torque, making it a versatile choice for those looking to improve their vehicle's performance.

In summary, the BMW M51 engine, produced during the 1990s, is a noteworthy diesel powerplant celebrated for its balance of power and efficiency, direct injection technology, and reputation for reliability and longevity. It remains a popular choice among diesel enthusiasts and those seeking performance and fuel economy in their BMW vehicles.

The BMW M51 are good project engines and with the optimum performance upgrades like a remap, turbo kits and camshafts you will dramatically maximize your driving experience.

TorqueCars will consider M51 tuning and summarise the ultimate mods that work. The M51 is a lovely torquey diesel engine fitted to a wide range of BMW's and Landrovers, and we even saw it used in the Opel Omega.

Different ECUs were used and the primary differences are the way the wastegate operates, other than that the engines were similar in all applications, save the ECU mapping where either more mid range power or low end torque was provided to suit.

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History, Power & Specs of the Engine

The M51 is a turbocharged diesel, and most have an intercooler. The chain driven OHC camshaft worked well, and had a simple 2 valve per cylinder setup.

  • 1991-1996 E36 325td M51D25 UL
  • 1996-1998 E36 325td M51D25TU UL
  • 1993-1996 E36 325tds M51D25 OL
  • 1996-1998 E36 325tds M51D25TU OL
  • 1992-1996 E34 525td M51D25 UL
  • 1991-1996 E34 525tds M51D25 OL
  • 1996-2000 E39 525tds M51D25TU OL
  • 1996-2000 E39 525td M51D25TU UL
  • 1996-2000 E38 725tds M51D25TU OL
  • 1995-2001 Land Rover Range Rover 2.5 D/DSE
  • 1995-2001 Opel Omega

Tuning the BMW M51 and best M51 performance parts we have a diesel tuning video tutorial. We will be providing a more specific video for the M51 in the future, so please subscribe to our channel to avoid missing out.

Please Watch my Video on "beginners' guide to Turbo Diesel Tuning mods".

Best M51 mods

When talking about the best top for your M51 engine, we are going to concentrate on the parts that give the biggest return for your cash. One very popular upgrade on these is an engine swap, the M57TUD30 or M57D29, but this article is about improving the M51 so we are going to look at mods that will work on it.

On automatic boxes a tune M51 will suffer from losses through the torque convertor which is setup for standard engine power levels.

As diesel engines operate at lower RPMS than petrols, there are only small gains to be made with a fast road cam profile, it will add a little more peak power though.

Choosing a higher performance cam profile raises the power band accordingly, on a diesel engine like the M51 you won't gain much but it can help you wring out every last bit of performance.

Fast road camshafts normally increase the power throughout the rpm band, you could drop a little low end power but the high end rpm power will be higher you might even get a slightly quicker pickup.

Different M51 engines respond better to mild camshaft durations check your engine on a rolling road.

The map and fuel pump and injectors also will say much on the bhp gains you'll get.

A longer valve duration can alter the bhp band and on most engines the exhaust and intake durations do not need to match, although most cams and tuners use matched pairs there are some advantages to extending the intake or exhaust durations.

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Best mods for your M51

  1. Tunes - A tune/remap ensures the most power compared to your outlay, aftermarket ECU upgrades, and Tuning boxes are all alternatives.
  2. Upgrades to turbochargers and superchargers - forced induction is the most dramatic method to increase air supply, allowing you to use more fuel and make power gains. It is one of the most costly modifications it offers big gains.
  3. Head work - The goals of porting and flowing the head are to get air flowing into the engine while removing flow restrictions and turbulence.
  4. Fast road cams are one of the most significant mechanical changes, but they must be fitted by someone who knows what they are doing and you might struggle to find one but there might be a local firm to regrind a stock camshaft for you.
  5. Air Induction and Exhaust Upgrades - Please note that on their own these mods won't ADD POWER in most cases, but they can help enhance power after other mods by removing the restriction.

Tunes: Engine tuning or remapping is a highly effective method to extract more power and torque from the M51 engine. Chips are generally the route to go on the M51.

It involves adjusting the engine control unit (ECU) to optimize various parameters, including air-fuel mixture and ignition timing.

This modification is known for providing the most significant power gains compared to the investment.

Additionally, aftermarket ECU upgrades and tuning boxes are alternatives to consider for tuning the M51 engine to your desired specifications.

Turbochargers: Upgrading to forced induction, whether through turbocharger (Swaps or hybrid upgrades), is one of the most dramatic ways to increase the air supply to the engine.

Forced induction allows the engine to ingest more air, which, in turn, permits the use of more fuel, resulting in substantial power gains.

While this modification is among the costliest, it offers substantial increases in horsepower and torque, transforming the M51 engine's performance.

Head Work: Porting and flowing the cylinder head is a critical performance enhancement for the M51 engine.

The primary objective is to optimize the airflow into the engine while eliminating flow restrictions and turbulence.

This modification plays a pivotal role in improving the engine's breathing and, consequently, power output.

Air Induction and Exhaust Upgrades: Enhancements to the air intake and exhaust systems are essential components of maximizing the M51 engine's potential.

While these modifications might not directly add power on their own, they reduce restrictions and allow the engine to breathe more freely.

This helps to improve overall engine efficiency and performance, particularly when combined with other modifications like engine tuning, turbocharging, or head work.

M51 Tuning Stages

Typical stage 1 mods often include: drilled & smoothed airbox, Fast road camshaft, Intake manifolds, Sports exhaust header/manifold, Remaps/piggy back ECU, Panel air filters.

Typical stage 2 mods often include: induction kit, Sports catalyst & performance exhaust, fuel pump upgrades, high flow fuel injectors, Fast road cam, Ported and polished head.

Typical stage 3 mods often include: Crank and Piston upgrades to alter compression, Internal engine upgrades (head flowing porting/bigger valves), Twin charging conversions, Engine balancing & blueprinting, Competition cam, Adding or Upgrading forced induction (turbo/supercharger).

The M51 engines are fantastic to work on and we're pleased to see that there is a lot of upgrades and performance parts out there.

ECU flashing allows a tuner to establish the full potential of all the mods you've fitted to your M51 and this is probably the single best mod you can do on a turbocharged M51 engine. Just 2-3 psi of extra boost will net around 30bhp more power.

The Bosch ECU's usually throw a fault code and limp home mode around 1.5 bars depending on the map sensor.

It will usually give you around 30% more power on turbocharged vehicles and on diesels you usually get better fuel economy, but power output often depends much on the mods you've applied and the condition of your engine.

Pushing air and fuel into your M51 is the whole point to any engine tuning job.

Intake manifold take the air from the filter and allow it to be drawn into the engine and mixed with fuel.

The size of bore and shape and flow characteristics of the Intake manifold can make a big difference to fuel atomisation and engine efficiency on the M51.

We often see plenum chambers are improved through motorsport parts, although a few OEM provide well optimised plenum chambers.

Increasing the M51 valve size, doing some port matching and head flowing will also increase power, and importantly will permit raising the power increase on other parts.

M51 Turbo upgrades

The more air you can get into an engine, the more fuel it can burn and uprating the induction with a turbocharger upgrade makes massive power gains. The stock turbo can usually boost happily to around 200bhp.

However, the factory failsafes will limit fuel or boost if too much boost is detected, resulting in a large flat spot if you don't use a boost controller to deceive the wastegate into letting more exhaust flow through the turbo. The greatest way to solve this issue is remapping.

If your M51 doesn't have a good sized efficient intercooler, you should install one to keep the air intake charge cold. If you have a big intercooler, you will be able to stay in high RPM ranges for a longer period of time since the heat soak will not occur as quickly.

A turbocharged engine makes it easier to add modifications, because turbo engines are manufactured with stronger components.

Higher power restrictions need better components for most engines, though.

Identify these restrictions, and then modify the engine with improved pistons and crankshafts.

There are many individuals who spend a lot of money on turbocharger enhancements for the M51, only to have the engine go up in smoke on its first tour.

Bigger upgraded turbos tend to experience a bottom end lag, and low capacity turbos spool up really quickly but do not have the high rpm torque gains.

Because turbocharger technology is always evolving, we now have variable vane turbochargers, which adjust the vane angle based on speed to reduce latency and boost top-end power.

A common turbocharger upgrade for the M57 is the Garrett GT2256 VNT turbocharger, which is as close to a direct exchange as you're going to get.

The M51 has been upgraded with VNT turbos, which work well and provide enough of boost at low RPMs. In order to utilise positive manifold pressure, you'll have to swap out the actuator. The ECU-controlled ones are more difficult to set up since they were used in Landrovers and BMWs, and their operation differed significantly.

The exhaust gases are diverted into two channels by dual scroll turbo units, which then force them through the turbocharger using vanes with varying angles. They also help the engine's scavenging capabilities.

If you're pumping in a lot of air, it's normal for the air flow sensor MAF/MAP on these engines to have a limiter.

While the OEM air sensor was limiting torque at much lower levels, the 4 bar air sensors were able to handle quite a bit more power.

Adding a supercharger or additional turbo will make large torque gains, although more difficult to setup. We have this in depth look at twinchargers if you want to read more.

M51 Fuelling Upgrades

When you increase the bhp and torque you will need to increase to the fuel system.

More bhp and torque needs more fuel. We would recommend you to be generous with your flow rate on the injectors. The OEM M51 fuel pump will usually run out of steam around the 250bhp mark at which point, you'll need to upgrade it.

The accepted safe increase is to add 20% when fitting an injector, this allows for injector deterioration and allows you some spare capacity should the engine require more fuel.

We think this one is common sense, but you'll need to match your fuel injector to the type of fuel your car uses as well.

M51 Performance Exhausts

You should look to increase your exhaust if your current exhaust is actually creating a restriction in flow.

IF YOU DON"T HAVE A RESTRICTION YOU ARE WASTING YOUR TIME and the OEM M51 Flows really well on most models.

The exhaust flow rate should be enough on most factory exhausts even with minor power improvements, but once you start increasing the power, you'll need a more efficient exhaust system.

The optimum exhausts for power improvements typically range from 1.5 to 2.5 inches in diameter, so don't get the biggest one you can find. This will limit the exhaust flow rate. The geometry and composition of the bores bends and curves are more important than the diameter.

Adding a freer-flowing performance catalyst eliminates the limitation often seen in emissions filters. These performance cats, like decats, can keep your automobile legal on the road and perform similarly to decats in terms of horsepower and torque gains.

Weak spots Issues & problem areas on the M51

The M51 engines are generally reliable and solid units, as long as you follow the manufacturers service schedules, and use a good quality oil to ensure longevity. Few problems should happen as long as they are regularly serviced and maintained.

Carbon build up in the head, particularly around the valves which will sap power or create flat spots, this is a larger issue on direct injection engines but should be looked out for on all engines. We have tips on removing carbon build up.

Some of our members have had issues with flat spots or glitches after applying mods and upgrades or tuning, this is not usually related to this engines design, so instead see our article on diagnosing flat spots and problems after tuning which should help you get the bottom of this issue.

Watch out for wastegate creep on tuned M51's if engine tuning/remapping or doing a turbo swap it is a false economy to keep the standard wastegate system in place when there are much better uprated units around.

Regular oil changes are vital on the M51, especially when tuned and will help extend the life and reliability of the engine.

If you would like to know more, or just get some friendly advice on Tuning your M51 engine please join us in our friendly forum where you can discuss tuning options in more detail with our tuning articles to get a full grasp of the benefits and drawbacks of each modification.

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