Mercedes M139 Tuning

"All you need to know about tuning and performance parts on the Mercedes M139 engine!"

The M139 replaced the capable M133 and was hand built, with an engineer taking full responsibility for each engine. This guarantees a higher build quality than you would get with assembly line production units.

This was an important engine for Mercedes, and one of the most powerful 4 cylinder production engines around.

In this article we review M139 tuning and highlight the best modifications. Mercedes M139 are good project engines and with carefully chosen modifications like ECU maps, turbo upgrades and camshafts you will enhance your driving experience.

The M139 is a phenomenal engine, getting a four cylinder engine to push 400hp is no small feat. This was primarily down to the clever twin scroll turbo setup giving over 30psi of boost.

Purely from an engineering point of view this is one of the best engines we've come across for power and efficiency in this configuration. But the old adage is that you can never have too much power so we'll look at how to get even more from the M139 block.

History, Power & Specs of the M139 Engine

  • 382 hp at 6,500 rpm 480Nm (354lbft) at 4,750-5,000 rpm
  • 416 hp at 6,750 rpm 500Nm (369lbft) at 5,000-5,250 rpm



M139 (382 hp version)

  • 2019- Mercedes CLA (C118) 45 AMG 4MATIC+
  • 2020- Mercedes A-Class (W177) 45 AMG 4MATIC+
  • 2021- Mercedes GLA 45 AMG 4MATIC+

M139 (416 hp version)

  • 2019- Mercedes CLA (C118) 45 S AMG 4MATIC+
  • 2020- Mercedes A-Class (W177) 45 S AMG 4MATIC+
  • 2021- Mercedes GLA 45 S AMG 4MATIC+

Tuning the Mercedes M139 and best M139 performance parts.

Best M139 tuning parts

The best M139 upgrades on an engine are in our opinion the ones that give the biggest return for your cash.

We won't be swayed by popular M139 upgrades, they need to be cost effective.

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

Fast road cams commonly raise the performance throughout the rev range, you may sacrifice a little low end bhp but top end will improve.

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

On a typical daily driver must carefully try to match your engines power to your usage of the car.

You will never find a M139 Race camshaft is a pleasure to live with when in heavy traffic because low end power will be very lumpy. Competition cams are designed for maximum power at the top end of the RPM range, a place that most daily commutes will not permit!

Different M139 engines respond better to less aggressive camshaft durations so set your engine up on a rolling road.

The ECU mapping and fuelling also have an effect on the power gains you'll hit.

Longer valve durations can alter the power 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 Engine Mods for your car

  1. Engine Tunes - engine tuning/remapping provides the most advantage in terms of cost savings,  aftermarket ECUs, and piggyback ECUs are all alternatives.
  2. Fast road cams are one of the most significant mechanical changes, but they must be installed by someone who knows what they're doing and they are not always easy to source but you might find a local firm to regrind a stock camshaft.
  3. Intake and Exhaust - Note that on their own these mods will NOT ADD POWER in most cases, but they can help enhance power after other mods by removing the restriction.
  4. Upgrades to turbochargers and superchargers - forced induction is the most efficient approach to increase air supply, allowing you to burn more fuel and make more power. It is one of the most costly upgrades but provides the best gains.
  5. Head work - The goals of porting and flowing the head are to get air flowing into the engine while removing flow restrictions and turbulence.

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

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

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


The M139 units are fantastic to work on and we see that there is an increase of mods and tuning parts about.


ECU flashing should help to unlock the full potential of all the modifications you've done to your M139.

(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 end result usually differs on the modifications you've done and the condition of your engine.

It is the aim to any tuning project to pull fuel and air into the M139 engine

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

The shape and rate of flow of the Air Intake manifolds can make a big difference to fuel mixing and power on the M139.

Most intake are crying out for performance upgrades, although some OEM provide decently flowing intake.

Adding a M139 larger valve kit, getting port matching and head flowing will also improve performance, the fantastic side effect is it will raise potential for raising the performance increase on other modifications.

Turbo upgrades

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 M139

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.

If the engine has a turbo already fitted mods are giving better power gains and you will discover turbocharged engines are built using uprated components.

However you'll find engines will have power limits

Discover these restrictions and install higher quality crank and pistons to handle the power.

There are many people spending a loads of money on turbo upgrades on the M139 only to experience the engine block go up in smoke just after it's used in anger.

Larger capacity turbos often suffer no power at low rpm, and smaller turbos spool up more quickly but do not have the peak rpm engines power gains.

In recent times the market of turbos is always moving on and we are seeing variable vane turbos, permitting the vane profile is altered according to speed to lower lag and increase top end bhp and torque.

Twin scroll turbos divert the exhaust gases into 2 channels and flow these at differently designed vanes in the turbocharger. They also increase the scavenging effect of the engine.

You'll commonly see there is a limit in the air flow sensor MAF/MAP on the M139 when loads 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 power at a much lower level.

Adding a supercharger or additional turbo will make large bhp and torque gains, although more difficult to setup. We have this feature on twinchargers if you want to read more.


Don't dismiss the need look at the fuel delivery when you are increasing the bhp - it makes the car more thirsty. We strongly recommend you to be generous with your injector capacity.

The rule of thumb is to add 20% to the flow rate when fitting an injector, helps cope with injector deterioration and allows a bit of 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.

All the following flywheel power targets will assume an injector duty cycle of 80% and a base of 58psi of fuel pressure at idle.

4 Cylinder turbocharged engines

  • 58 PSI 682cc/min 400hp
  • 58 PSI 1022cc/min 600hp

4 Cylinder NA (naturally aspirated) engines

  • 58 PSI 285cc/min 200hp
  • 58 PSI 426cc/min 300hp

4 Cylinder supercharged engines

  • 58 PSI 625cc/min 400hp
  • 58 PSI 937cc/min 600hp


You only need to replace your exhaust if the current exhaust is creating a restriction in flow.

On most factory exhausts you'll see the flow rate is good even on modest power gains, but when you start pushing up the power levels you will need to get a better flowing exhaust.

Don't go with the biggest exhaust you can find this will reduce the exhaust flow rate - the best 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 come around the catalysts installed, so adding a faster flowing high performance aftermarket one will improve air flow, and rather than doing an illegal decat, will keep the car road legal.

Weak spots, Issues & problem areas on the M139

The M139 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 M139, 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 M139 engine please join us in our friendly forum where you can discuss tuning options in more detail with our M139 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 mods 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 M139 tuning guides which get regular updates and revisions.

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One Response to “M139 Tuning”

  1. yadiel mayo says:

    Hi I have a 2020 cla amg 45 stage2 downpipe is tuned by weistec and I wounder how much horsepower this engine could hold, I know this block could hold 2360 psi that means, I think this engine could hold a lot horse power but theres not much infortmation about this engines this car is amazing I am doing 0 to 60 3.3 seconds and 11.3 1/4 miles to be A 4banger is impressive.

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