BMW M51 Tuning

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

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.

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.

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.

Motorsport and race camshafts, increase the high end rpm power band but as a result the car will not idle smoothly and low end power nearly always suffers.

A Motorsport and race camshaft won't do well if driving around busy urban areas. This is because a competition cam causes a very lumpy idle, and makes the car more prone to stall or jerk along in stop start traffic, sadly though many ignore this and end up ruining a perfectly good car and having to revert back to a fast road, or OEM cam profile.

You should ideally optimize your bhp range to your preferences so for a road car stick with a shorter duration M51 camshaft

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.

Typical stage 1 mods often include: drilled & smoothed airbox, Fast road camshaft, Intake headers, Sports exhaust 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.

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.

Using a boost controller you can trick the wastegate into allowing more exhaust flow through the turbo giving you a fairly simple power boost, but beware the factory fail safes, if too much boost is detected it will cut fuel or boost and you'll have a massive flat spot. Remapping is the best way around this issue.

Intercoolers keep the air intake charge cool, if your M51 doesn't have one then this is a good upgrade to do. A larger intercooler will prevent heat soak from occurring so suddenly so although it won't add power, you'll get to sit in high RPM ranges and enjoy the power for longer.

When the engine is fitted with a turbo mods are simpler to install and turbo engines are built with more solid components.

However most engines will need better parts at higher power limits

Discover these limitations and install better pistons and crank to utilize the power.

We see many people spending a loads on turbocharger upgrades on the M51 only to have the engine go up in smoke on it's first outing after it's completed.

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.

Thankfully the choice of turbo units is always improving and we now see variable vane turbo units, where the vane angle is altered according to speed to lower lag and increase top end power.

The Garrett GT2256 VNT turbocharger found on the M57 is a popular upgrade on this engine block as it is as close to a straight swap as you can get.

VNT turbos have also been fitted to good effect on the M51 and they give plenty of boost at low RPM ranges. You'll need to change the actuator to use positive manifold pressure, the ECU controlled ones are tricky to setup, but much depends on which vehicle your car is fitted to as these were put in Landrovers and BMW's and there were a few fundamental differences in the way they work.

Twin scroll turbo units divert the exhaust gases into 2 channels and push these at differently angled vanes in the turbo charger. They also boost the scavenging effect of the engine.

It is common that there's a limit in the air flow sensor MAF/MAP on these engines when a lot more air is being pulled into the engine.

We note 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor was restricting torque at a much lower level.

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.

Fuelling

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.

Exhaust

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

On most factory exhausts you should find that the exhaust flow rate is ok even on modest power gains, but when you start pushing up the power levels you will need to get a better flowing exhaust.

Please don't run with the largest exhaust you can buy this will reduce the exhaust flow rate - the best exhausts for power gains are usually between 1.5 to 2.5 inches. It is the shape and material more than the bore size.

Typically exhaust restrictions are traced to the emissions filters installed, so adding a freer flowing performance catalyst removes the restriction. We note that performance cats perform similarly to decats and have the added benefit of keeping your car street legal, as decats or catalyst removal is illegal in most territories for road going cars.

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 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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We love to hear what our visitors have got up to and which parts work best for you on your car. Which helps us keep our guides and tips up to date helping others with their modified car projects. Your feedback and comments are used to keep this page up to date, and help improve the accuracy of these M51 articles which are continually updated.

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