BMW M21D24 Tuning

"All you need to know about performance tuning the BMW M21D24 engine!"

The BMW M21D24 have loads of potential and with carefully picked parts like remaps, turbo upgrades and camshafts you will positively maximize your driving enjoyment.

In this article we detail the best approach to M21D24 tuning and summarise the ultimate upgrades.

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History, Power & Specs of the Engine

  • Turbo diesel 114 hp 155 lbft @ 2400 rpm
  • NASP 84 hp 112 lbft @ 2500 rpm

The M21 was fitted to the following cars, and was made in quite large numbers. In it's day it was one of the best and most powerful diesel engines around. The NASP version was released later in countries that had higher vehicle taxation costs.

  • 1983 - 1988 E28 524td
  • 1985 - 1988 E28 524d (naturally aspirated)
  • 1985 - 1993 E30 324d (naturally aspirated)
  • 1985 - 1993 E30 324td
  • 1988 – 1991 E34 524td
  • 1984 - 1985 Continental Mark VII
  • 1984 - 1985 Lincoln Continental

Tuning the BMW M21D24 and best M21D24 performance parts see our diesel tuning video tutorial.

Please Watch my Video on "beginners' guide to TDi Tuning".

Best M21D24 modifications

Just because particular modifications are appear in lots of M21D24 projects it doesn't mean it is good, instead we'll concentrate on the top modifications that will give your M21D24 the best power gain for you money.

Instead of us falling into the common trap of the "if it's shiny and makes more noise it must be good" mentality of many car sites and tuning magazines.

Sadly the M21 isn't really a tuners dream, it can be tuned but costs a lot for very little return, especially the NASP version.

For a car used daily must carefully try to match your torque band to your cars usage.

Each engine responds better to more aggressive cam durations check your engine on a rolling road.

The engine timing and fuel pump and injectors also have a large bearing on the bhp gains you'll achieve.

Extending exhaust or intake durations 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.

Please watch our video which covers the 5 principles of tuning your car. Be sure to keep up with our latest YouTube content and subscribe.

Best mods for your M21D24

  1. Lightened Flywheels - a lighter flywheel will improve the engines rev changes. In some cases flywheel lightening is not a great mod for all M21D24 engines.
  2. ECU Tunes - A tune/remap offers the biggest gains in terms of your investment, replacement ECUs, and inline Tuning boxes are all alternatives.
  3. Air Intake and Exhaust - Note that on their own these mods won't ADD HP on most cars, but they permit you to enhance power after other upgrades by removing the restriction.
  4. Fast road cams are are often the best upgrade for an engine, but should always be setup by someone qualified to set them up and they are not always easy to source but you might have a local firm to regrind a stock cam for you.
  5. Flowing and porting the Head - Head work will get air flowing into the engine while removing turbulence or restrictions.
  6. Upgrades to turbochargers and superchargers - Adding a turbocharger is the most efficient approach to increase air supply, ensuring you are able to combust more fuel and make power gains. It is one of the most expensive upgrades it offers big gains.

M21D24 Tuning Stages

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

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

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

Plan your options and then source your upgrades and set yourself a power target to avoid costly mistakes.

Mapping will help to establish the full potential of all the upgrades you've done to your M21D24.

(In some cases, as the factory ECU is locked flashing is not an option, so an aftermarket ECU is the route to take, and many of these will outperform factory ECU's but make sure it has knock protection and that you get it setup properly.)

It will usually give you around 30% more power on turbocharged vehicles and you can expect to see around 15% on NA (naturally aspirated) engines, but the outcome usually rely on the upgrades you've carried out and the condition of your engine.

It is vital to any car tuning job to feed fuel and air into each cylinder

The intake manifold transmits the air from the air filter and allow it to be fed into the engine and mixed with fuel.

The bore size, shape and rate of flow of the Plenum can make a noticeable change to fuel atomization on the M21D24.

We often see intake are ripe for performance upgrades, although a few makers provide reasonably well-designed intake.

Adding a M21D24 larger valve kit, getting port matching and head flowing will also increase bhp and torque, & importantly will allow you to get increasing the bhp and torque increase on other modifications.

Turbo upgrades for the M21D24

NA (naturally aspirated) 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 M21D24 it's not impossible to turbocharge the NASP version of this engine if you know what you are doing.

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.

If your motor is fitted with a turbocharger mods are going to make more power and you'll see that turbo charged engines are made using more solid components.

However every engine has weak spots so we recommend you find these limitations and fit forged components to survive the power.

We see many car owners spending a lot of money on turbocharger upgrades on the M21D24 only to have the car explode soon after it's first rolling road session.

Twin scroll turbo chargers divert the exhaust gases into two channels and push these at differently designed vanes in the turbo. They also increase the scavenging effect of the engine.

You'll commonly see there's a limit in the air flow sensor AFM/MAP on the M21D24 when loads more air is being drawn into the engine.

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

Adding a supercharger or additional turbo will make large power gains, although more difficult to install. We have this guide to twinchargers if you want to read more.

Fuelling Upgrades for the M21D24

When you raise the torque you will need to increase to the fuel system.

More torque needs more fuel. Don't forget to over specify your flow rate on the injectors.

The rule of thumb is to add 20% when specifying an injector, this accounts for injector deterioration and provides a little 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.

M21D24 Exhaust Mods

Only look to upgrade your exhaust if the existing exhaust is actually causing a restriction. To be frank a diesel doesn't need much flow, the RPM nature of the engine and massive OEM bore size makes it unnecessary, plus I find that the sound a diesel makes through a sports exhaust is unpleasant at best and irritating at worst.

So on most factory exhausts you'll see the exhaust flow rate is still ok even on modest power gains, but when you start pushing up the power levels you will need to get a better flowing exhaust - particularly the DPF filters/Catalysts and headers where you'll see a restriction forming.

Typically exhaust restrictions can be traced to the filters installed, so adding a better flowing race alternative such as a sports catalyst pretty much removes this restriction, thanks to its larger size and surface area, and will effectively raise the performance to levels you would expect without having a catalyst installed, but keeps the car road legal.

Don't go with the widest exhaust you can source you'll slow the exhaust 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.

Weak spots, Issues & problem areas on the M21D24

The M21D24 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.

Regular oil changes are vital on the M21D24, 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 M21D24 engine please join us in our friendly forum where you can discuss tuning options in more detail with our M21D24 owners. It would also be worth reading our unbiased 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 tuning 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 M21D24 tuning guides which get regular updates and revisions.

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One Response to “M21D24 Tuning”

  1. Dale Janssen says:

    very helpful reading. I agree with you on the items I understand. I have experienced setting a standing mile record with a class A motorhome powered by an M21TD. I have been led to believe the ECU can be excluded from the equation. I would like to know who can educate me on this element. During our attempts to break the world record we stretched head bolts and also found out the pump we were using stopped advancing at 4,000 rpm making EGTs skyrocket and power tapper off at 48 to 5000 rpm. we have had the pump worked over by Giles in Canada and it now advances to 6,000 rpm where we shift. EGTs are no longer as much of an issue. we hold our boost short of 30# thinking we can avoid stretching the new head bolts. we are unable to find studs that are perfect for this block. it will take 300 hp to get is to our goal of 130 mph. we are at 114.3 now with the old pump. Dale

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