Tuning the BMW M51

"Comprehensive guide to performance parts and tuning the BMW M51 engine!"

Our aim here is to detail the best approach to M51 tuning and outline the ultimate mods that work. BMW M51 offer good returns when tuned and with a few sensible motorsport tuning mods like remapping, turbo kits and camshafts you will really maximise your driving experience.

The greatest M51 tuning parts on an engine are sensibly the ones that give the biggest return for your cash.

We won't be swayed by popular M51 tuning parts, they need to be cost effective.

The cam profile plays a big part in the engines power output so cam upgrades make quite a large difference. The intake & exhaust durations will alter depending on the chosen cam profile, so large power band gains are on offer for cam upgrades.

NB: Fast road cams normally push up the bhp and torque across the rpm band, you might lose a little low end power but the top end will be better.

Competition cams, push up the top end band but as a result the car will not idle smoothly and low end power nearly always suffers.

For a road car should ideally to optimize your power band to your preferences.

I'd be surprised if you find a M51 Motorsport and race cam is a pleasure to live with when driving in heavy traffic.

Different M51 engines respond better to less aggressive cam durations so view each engine as unique.

The engine timing and fuelling also will say much on the torque gains you'll get.

A longer valve duration can alter the torque 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.

Stage 1 modifications: Sports exhaust manifold, Panel air filters, Drilled & smoothed airbox, Remaps/piggy back ECU, Intake headers, Fast road camshaft.

Stage 2 modifications: Fast road cam, high flow fuel injectors, Ported and polished head, fuel pump upgrades, Sports catalyst & performance exhaust, induction kit.

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

The M51 power trains are fantastic to work on and we're finding that there is an increase of modifications and performance parts about.

Mapping will help fully realize the full potential of all the parts you've fitted to your M51.

It will usually give around 30% more power on turbocharged vehicles and you can expect to see around 15% on NASP engines, but the end result will depend much on the parts you've applied and the condition of your engine.

Shoving more fuel and air into your M51 is the aim to any engine modification job.

The intake plenum carry the air during the suck phase from the air filter and allow it to be pulled into the engine cylinders with fuel for the squish phase.

Structure and flow characteristics of the Intake manifold can make a noticeable effect on to fuel atomisation and engine efficiency on the M51.

Many mass produced engine plenum chambers are begging for aftermarket tuning parts, although a few car makers provide decently flowing plenum chambers.

Adding a M51 larger valve kit, doing a bit of port work and head flowing will also lift bhp, & more importantly will raise potential for an improved bhp increase on other tuning mods.

Turbo upgrades

NASP engines need quite a lot of work when you add a turbo, so we have a separate guide to help you take into account the pros and cons of going this route on your M51

The more air you can get into an engine, the more fuel it can burn and uprating the induction with a turbocharger upgrade makes superb power gains.

When your car has a turbo already fitted modifications are relatively easy and you will discover turbo charged engines are built with more solid components.

There are practical limits for every engine, with some being over specified and some just sufficiently able to handle stock power

Research these limits and install better quality crank and pistons to utilize the power.

We've seen mechanics spending a loads of money on turbo charger upgrades on the M51 only to suffer the humiliation of seeing the whole thing literally blow up on it's first outing after it's been finished.

Larger turbo units tend to experience a bottom end lag, and low capacity turbo units spool up more quickly but do not have the high rpm torque gains.

Thanks to progress the world of turbochargers is always moving on and we commonly find variable vane turbochargers, permitting the vane profile is altered according to speed to lower lag and increase top end power.

Twin scroll turbochargers divert the exhaust flow into two channels and direct these at differently designed vanes in the turbo. They also boost the scavenging effect of the engine.

It is not unusual that there is a restriction in the air flow sensor (AFM/MAF/MAP) on the M51 when loads more air is being drawn into the engine.

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

Adding a supercharger or additional turbo will make large bhp gains, although harder to get working. We have this article covering twinchargers if you want to read more.

Fuelling upgrades and mods

When you lift the bhp you will need to look at to the fuel system.

More bhp needs more fuel. It is important to over specify your injector capacity.

As a rule of thumb add 20% when specifying an injector, this allows for injector deterioration and allows a little spare capacity should the engine need 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 upgrades and mods

You only need to to boost your exhaust if the existing exhaust is creating a restriction in flow.

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

Sports exhausts generally help improve air flow through the engine but do not go too large or you could will reduce the flow rate. Stick to 1.5 to 2.5 inches as a rule of thumb.

Common exhaust restrictions can be traced to the filters installed, so adding a freer flowing sports alternative will help avoid this restriction.

History of the Engine

Some are turbocharged and 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

Weak spots Issues & problem areas on the M51

The M51 engines, if regularly serviced and maintained, are generally very reliable and have few issues.

Regular oil changes are vital on the M51, particularly when the engine has been modified and is putting down more power than the manufacturer intended.

For more information 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.

We need your help improving this article, so please send us your feedback in the comments box below and pass on any tips, points or facts we have wrong or have not covered.

We really like hearing from our readers, and hearing about which parts were the most effective for them, it helps us improve our recommendations and articles to reflect current trends in modifications and ensures that our M51 guides and tips are kept up to date.

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