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.

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

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.

Altering your M51 cam will make a dramatic difference to the engine power band. Choosing a higher performance cam profile raises the power band accordingly.

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.

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.

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.

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

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

Stage 3 mods: 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 to establish the full potential of all the mods 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 power output often depend 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 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

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 massive power gains.

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.

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.


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 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.


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 dont 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 alternative will help avoid this restriction.

Weak spots Issues & problem areas on the M51

The M51 engines are generally reliable and solid as long as they are regularly serviced and maintained.

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

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.

Please help us improve these tips by sending us your feedback in the comments box below.

We love to hear what our visitors have got up to and which parts work best for them on each model of car. Comments are used to improve the accuracy of these M51 articles which are continually updated.

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