Mercedes OM651 Tuning

"All you need to know about performance tuning the Mercedes OM651 engine!"

The Mercedes OM651 are good project engines and with carefully picked uprated enhancements like remapping, turbo kits and camshafts you will improve your driving enjoyment.

In this article we examine OM651 tuning and report on the premier mods that work.

History, Power & Specs of the OM651 Engine

  • 136 PS (100 kW) (badged x200 on rear wheel drive models)
  • 170 PS (125 kW) (badged x220)
  • 204 PS (150 kW) (badged x250)

NB: 170 PS (125 kW) and 204 PS (150 kW) versions employ a 2 stage (bi-turbo)

2 shorter stroke "square" variants of this engine with 1.8 litre

  • 109 PS (80 kW) (badged x180)
  • 136 PS (100 kW)

Tuning the Mercedes OM651 and best OM651 performance parts.

Best OM651 parts

Just because particular mods are appear in lots of OM651 projects it doesn't mean it is good, we shall best mods that will give your OM651 the best power gain for you money.

Altering your OM651 cam will make a dramatic difference to the engine power band. Choosing a higher performance cam profile raises the power band accordingly.

Fast road cams tend to push up the bhp and torque through the rev range, you might lose a little low down torque but your high end rpm power will be better.

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

For a road car must carefully try to match your torque band to your driving style.

I'd be amazed if you have ever thought or claimed that a OM651 Race camshaft is a pleasure to live with when 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.

Each engine responds better to mild camshaft durations so set your engine up on a rolling road.

The map and fuelling also will say much on the torque gains you'll hit.

Longer valve durations 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.

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

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

    Typical stage 3 mods often include:
    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 OM651 engines respond well to upgrades and thanks to their popularity there is a growing number of parts and tuning parts around.

    Mapping helps unlock the full potential of all the parts you've done to your OM651.

    (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 your mileage will depend much on the parts you've done and the condition of your engine.

    Pulling more fuel and air into your OM651 is the whole point to any engine modification project.

    Intake manifolds flow the air during the suck phase from the air cleaner and allow it to be pulled into the engine cylinders with fuel for the squish phase.

    Design and flow characteristics of the Intake can make a large improvement to fuel atomisation on the OM651.

    On popular production engines air intake manifolds are in dire need of motorsport parts, although a few manufacturers provide reasonably well designed air intake manifolds.

    Big valve conversions on the OM651, getting port matching and head flowing will also lift performance, and as an added benefit will allow you to get a better performance increase on other tuning mods.

    OM651 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 OM651

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

    If the engine has a turbo already fitted parts are relatively easy and most turbo charged engines are built with uprated components.

    However you will find engines have weakspots.See where you'll find these restrictions and install better quality components to cope with the power.

    It's not unheard of car owners spending a a stack of money on turbo charger upgrades on the OM651 only to experience the whole thing explode when it's used on the roads.

    Large turbo chargers tend to suffer a bottom end lag, and little turbo chargers spool up much more quickly but do not have the peak end power band gains.

    Over the last 20 years the world of turbo units is always increasing and we are seeing variable vane turbo units, permitting the vane angle is altered according to speed to lower lag and increase top end bhp.

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

    You'll commonly see there is a limitation in the air flow sensor (AFM/MAF/MAP) on the OM651 when loads more air is being fed 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 get working. We have this guide to twinchargers if you want to read more.


    You will need to ensure that the engine is not starved of fuel so should increase the fuelling when you start going beyond 20% of a performance increase.It is important to be generous with your flow rate on the injectors.

    The accepted safe increase is to add another 20% when specifying an injector, helps cope with injector deterioration and affords 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.

    OM651 Performance Exhausts

    You only need to increase your exhaust if your current exhaust is actually causing a restriction.

    On most factory exhausts you'll see the exhaust 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 will certainly help air flow out of the engine but do not go too large or you could very well end up with a reduced flow rate. So generally speaking, keep to 1.5 to 2.5 inches as a rule of thumb.

    Common exhaust restrictions can be located the catalysts installed, so adding a better 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 OM651

    The OM651 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 OM651, 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 OM651 engine please join us in our car forums where you can discuss OM651 tuning options in more detail with our OM651 owners. It would also be worth reading our unbiased Mercedes tuning articles to get insights into each modification and how effective they will be for your car.

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    We love to hear what our visitors have got up to and which upgrades 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 OM651 tuning guides which get regular updates and revisions.

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    2 Responses to “OM651 Tuning”

    1. Paolo says:

      Hi who makes bigger regroups cams for the OM651

    2. TorqueCars says:

      My first port of call would be Piper Cams or Kent Cams in the UK. Where in the world are you?

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