Mercedes CDI Engine Guide – OM611 OM612 OM613 vs OM646 OM647 OM648 Compared

Mercedes CDI engine selection is surrounded by hype and misconceptions. Today we cut through it – covering the old versus new CDI engines, high pressure pump upgrades, ECU tuning and why the OM648 obsession is largely a Facebook phenomenon.

How Common Rail Diesel Works

A common rail diesel engine operates like a modern petrol engine with one fuel rail feeding all injectors – except the operating pressure is dramatically different. Petrol common rail systems operate at three to four bar. Mercedes CDI engines operate at 1600 to 2100 bar. This requires a dedicated high pressure pump rather than a standard electric fuel pump.

Mercedes uses Bosch CP series pumps across their CDI range. The CP1 and CP3 are the most relevant. The CP3 is the standard performance upgrade and comes in different displacement versions – the L90 and L110 being the most common. Mercedes engines use left rotation pumps while BMW uses right rotation – L versus R in the designation.

The Two CDI Engine Generations

Older CDI engines – OM611, OM612, OM613 Four cylinder, five cylinder and six cylinder respectively. Commonly known as 220 CDI, 270 CDI and 320 CDI. Displacements of 2.2, 2.7 and 3.2 litres. These engines use the EDC15 engine management system and solenoid injectors.

Newer CDI engines – OM646, OM647, OM648 Also four cylinder, five cylinder and six cylinder. Also marketed as 220 CDI, 270 CDI and 320 CDI which creates confusion. Found in post-2000 vehicles. These use EDC16 engine management and piezo injectors.

The OM648 Hype Problem

Everyone wants an OM648. Prices are rising constantly because Facebook decided it is the engine to have. The reality is more nuanced.

The OM613 and OM648 are functionally identical engines. Straight six, four valve, common rail diesel. The differences are:

OM648 advantages: comes with CP3 L90 pump as standard, rear sump which suits some installations.

OM613 advantages: significantly cheaper, uses EDC15 which is much simpler to tune and make standalone, solenoid injectors which in some applications are preferred over piezo units.

The CP3 pump upgrade on an OM613 is straightforward – not rocket science. Fit the CP3 and you have the same fuel delivery capability as the OM648 at a fraction of the purchase price.

A knowledgeable friend converted his OM648 back to the older solenoid injector specification because they suited his application better. This suggests the OM613 injector specification is not inherently inferior.

ECU Tuning – EDC15 vs EDC16

This is where the older engines have a clear advantage for standalone and high performance applications.

EDC15 on OM611, OM612, OM613 is simple. Easy to read, easy to write, relatively few maps. Straightforward to tune and to make standalone. DSL1 compatible interface exists providing excellent ECU capability for these engines.

EDC16 on OM646, OM647, OM648 is significantly more complex – Mercedes own implementation adds further complexity. Making EDC16 standalone without expert knowledge nearby is challenging. For street tuning with a professional tuner it is manageable but for DIY standalone builds the older EDC15 engines are considerably easier to work with.

The V6 and V8 CDI Engines – Underrated

The OM642 V6 CDI gets little attention and deserves far more.

It is physically tiny – approximately three and a half cylinders in length, 72 degree vee angle meaning it fits in chassis where nothing else will. Aluminium construction so it weighs very little. Common rail with a good high pressure pump, turbocharger and a tune produces 400 horsepower reliably. The reputation for unreliability is almost entirely based on sensor failures – lambda sensors, coolant temperature sensors. The engine itself is extremely robust.

The OM628 V8 – the older 400 CDI – does have some reported reliability concerns. Its successor the OM629 in the 420 CDI is widely praised as an excellent engine. Approximately four and a half cylinders in length, aluminium construction, shorter than an OM605. Exceptional for builds requiring compact high displacement diesel power.

Stroke and Bore – One More Time

For anyone still claiming that the OM60x engines rev high because of short stroke consider this:

All CDI inline engines – OM611 through OM648 – use 88mm bore and 88.3mm stroke. They are essentially square engines. The supposedly torquey low-revving OM648 has the same stroke-to-bore relationship as the supposedly high-revving OM606.

The OM642 V6 uses 83mm bore and 92mm stroke – longer stroke than any of the 2.3, 2.9 or 3.5 litre stroker builds of the OM604, OM605 and OM606.

The OM628 and OM629 V8 are exactly square at 86mm bore and 86mm stroke.

Stroke does not determine power delivery character. Camshafts do. This point is now proven across every engine family Mercedes has produced.

Summary

• Mercedes CDI engine choice: OM613 offers equal performance to OM648 at significantly lower cost
• CP3 pump upgrade on OM613 is simple and brings fuel delivery to OM648 specification
• EDC15 on older CDI engines is simpler to tune and make standalone than EDC16 on newer engines
• OM648 hype is Facebook-driven – the price premium is not justified by the performance difference
• OM642 V6 CDI is compact, light, powerful and underrated
• OM629 V8 CDI is an excellent high displacement diesel option
• All CDI inline engines are essentially square – 88mm bore, 88.3mm stroke
• Stroke does not determine engine character – camshafts do

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