OM606 Mods Guide – Pre-Chambers, Electric Fuel Pump, Clutch Kits and Adapter Plates

Which modifications are worth doing on OM604, OM605 and OM606 engines and which are a waste of money? Fred covers pre-chamber mods, injector nozzles, the cooling mod, electric fuel pumps with three bar fuel pressure, adapter plates versus welded gearboxes and clutch kit pilot bearing mistakes.

OM606 mods that are worth doing versus those that waste money – today we go through the full list covering pre-chambers, injector nozzles, cooling, electric fuel pumps, adapter plates and clutch kits.

Pre-Chamber Modifications – Still Unnecessary

Pre-chambers on OM604, OM605 and OM606 engines are made from an extremely hard material that essentially never fails. Stock pre-chambers from 25 year old engines are still structurally sound and flow whatever fuel you give them.

Drilling the holes larger – no power gain, no reliability improvement. Thinner wallet only.

Aftermarket balls – cool parts, new versus 25 year old originals, but no power or reliability advantage. The originals will outlast everything else in the engine.

Injector Nozzles – Still Unnecessary (Except OM604)

Stock injector nozzles flow anything you can throw at them. Aftermarket nozzles from India or anywhere else claiming performance gains on OM605 and OM606 engines deliver nothing except a smaller bank balance.

The one exception remains the OM604 with Lucas injectors. Lucas injectors look identical to Bosch, parts appear interchangeable, but they cannot flow properly. If you have an OM604 replace Lucas injectors with genuine Bosch units. For every other engine in this family – leave the injectors alone.

Heavy Flywheels – Stop

Twenty kilogram flywheels. Stop. Not discussing it further. Just stop.

Cooling Mod

The OM606 overheats. This is well known. The cooling mod is useful primarily for one specific reason – it provides a perfect inlet and outlet for a water cooled turbocharger. For Garrett turbo users with water cooling this modification makes perfect sense.

For general cooling improvement the mod itself does not provide dramatically better cooling capacity. There are other solutions including high flow head gaskets that address heating issues more effectively when the head is removed during a build. More on those another time.

Electric Fuel Pump – Do This

The electric fuel pump modification is genuinely worthwhile and here is why.

A stock lift pump handles surprisingly high power levels. A dyno session producing over 518 horsepower was completed with a stock lift pump running dual inlets to the injection pump. So it is not strictly necessary for power.

However the advantages of an electric fuel pump go beyond raw power:

Priming – turn the key and the pump primes the system before starting. Bleeding air from the diesel fuel system becomes almost automatic. Simple and reliable.

Fuel pressure – running three bar of fuel pressure to the injection pump provides real benefits. Testing at Diesel Mekano showed that higher fuel pressure makes the injection pump rack harder to move. On high power mechanical pumps the throttle linkage moves very freely – small bumps in the road can cause the engine to surge unexpectedly. Three bar fuel pressure adds resistance to the rack movement, improving idle quality, smoothing low RPM operation and making the car more predictable to drive.

How to Route the Electric Fuel Pump System Correctly

The stock fuel return components are too restrictive for an electric fuel pump system. The original return valve is small and restrictive – it cannot be used.

The correct setup:

• Full flow inlet line to the injection pump
• Full flow return line from the injection pump
• Fuel pressure regulator on the return line set to three bar
• Return line from regulator to tank or catch tank

Critical point – injector return lines must have their own dedicated return line directly to the tank. Injector returns cannot share any line that has pressure in it, including the line after the pressure regulator. Run a completely separate injector return line.

Adapter Plates vs Welded Gearboxes

Adapter plates are convenient but not ideal. When an engine is designed the measurements and tolerances between engine and gearbox are precise. An adapter plate moves everything ten to twenty millimetres. Tolerances shift, alignment changes and nothing sits exactly as the manufacturer intended.

Welded gearbox conversions are the correct approach. Cut the bell housing, weld it to the new gearbox and all original measurements are maintained. Any gearbox can be mated to any engine this way with correct alignment throughout.

Clutch Kits and the Pilot Bearing Problem

Mercedes six speed gearboxes used behind OM606 engines have a short input shaft. A short input shaft requires a pilot bearing in the flywheel to centre the shaft correctly.

Many people running these gearboxes buy single mass flywheels designed for long input shafts – flywheels without a pilot bearing location. The input shaft has nothing to centre it. It moves around, wears components and eventually causes failures.

Before buying any flywheel confirm whether your gearbox has a short or long input shaft. Mercedes six speed gearboxes after the OM606 era typically use short input shafts requiring a flywheel with a pilot bearing. Get this wrong and it works for a few days before things start breaking.

Summary

• OM606 mods worth doing: electric fuel pump with three bar pressure, cooling mod for water cooled turbos, welded gearbox conversions
• Not worth doing: pre-chamber modifications, aftermarket injector nozzles, heavy flywheels
• OM604 specific: replace Lucas injectors with Bosch – this is necessary
• Electric fuel pump routing requires full flow lines, a pressure regulator and separate injector return lines
• Adapter plates work but compromise precision – welded conversions are better
• Always verify input shaft length before buying a flywheel – short shaft needs pilot bearing in flywheel

List done. Fourteen minutes. See you another day!