OM606 Crankcase Ventilation, Flywheel & Oil Cavitation Guide

OM606 crankcase ventilation, flywheel selection and oil pump cavitation are three topics that come up constantly in diesel tuning communities. Fred covers what works, what doesn't and one little-known oil pump modification for engines that rev above 7000 RPM.

OM606 flywheel selection, oil cavitation prevention and crankcase ventilation are three topics that come up constantly. Let's go through all of them.

Flywheel – Never Use a Heavy Flywheel

The 20 kilogram flywheel modification needs to stop. When building a performance engine you want rotating mass to spin as easily as possible. That's why performance builders use lightweight flywheels and underdrive pulleys – to reduce rotational inertia and free up power.

You have a four, five or six cylinder diesel engine with 22:1 compression ratio running three bar of boost. You have more than enough torque. You do not need a heavy flywheel to add more.

Use a single mass flywheel in the three to four kilogram range. That's what performance engine building requires.

Adapter Plate Builds and Flywheel Options

For builders running adapter plates between the OM engine and a non-original gearbox the flywheel options are more limited. There are Polish companies offering adapter plate kits with aluminium flywheels featuring interchangeable friction surfaces. This is a decent solution.

The advantage is that when the friction surface wears – and it will wear, especially in drift applications where clutches can be destroyed in a few competition runs – you simply unbolt the friction surface and replace it. No flywheel resurfacing, no full replacement.

I prefer welded gearbox conversions where everything sits as the manufacturer intended with correct measurements and alignment. But adapter plates have improved significantly with CNC machining and proper dowel placement. Do what works for your build.

Underdrive Pulleys – Why?

There are companies offering underdrive pulleys for diesel tuning applications. I genuinely don't understand the benefit in our world. If anyone has real data supporting underdrive pulleys on OM606 builds please share it in the comments. Until then I can't recommend them.

Oil Pump Cavitation at High RPM – The M104 Sprocket Modification

This is a modification most people don't know about and one that's genuinely useful for engines being revved above 7000 RPM consistently.

At very high RPM the oil pump can cavitate – meaning it spins fast enough that it can no longer draw oil effectively. The difference between cavitating and not cavitating is approximately one tooth on the oil pump drive sprocket.

The solution is the M104 crank hub sprocket modification:

1. Take an M104 crank hub sprocket – the unit that drives both the camshaft chain and oil pump chain
2. Cut it in half to separate the oil pump drive gear
3. Do the same with your OM606 unit
4. Match and weld them together – the tooth count difference means the oil pump now spins slightly slower
5. Fit a chain with one link less to compensate

The result is an oil pump that spins just below the cavitation threshold at high RPM. Safer oil pressure when revving hard for extended periods.

This is a specialized modification for serious high-RPM builds. Not necessary for most street engines but very worthwhile if you're consistently above 7000 RPM.

Crankcase Ventilation – Do It Properly

Crankcase ventilation on OM606 builds is often ignored or done badly. Here are the common approaches and why most of them are wrong:

Running a hose from the head directly to open air – works on a fresh built engine with good ring seal but on a worn engine you'll have oil everywhere in the engine bay.

Routing the hose directly to the turbo intake – don't do this. Running three bar of boost creates enormous suction at the turbo inlet. You'll suck the oil out of the engine.

Routing the hose to the turbo intake with a small hole drilled in it to reduce suction – a hack. Works partially but not a proper solution.

The Correct Solution – Catch Can

A proper catch can setup is the right answer:

1. Run a hose from the turbo intake to the catch can inlet
2. Run a hose from the catch can to the head
3. The catch can has an open port that breaks the full suction
4. Pack the catch can with steel wool to capture oil mist

This gives you just enough suction to aid crankcase ventilation without pulling oil through the system. The steel wool captures any oil mist before it can go anywhere. Clean engine bay, no oil contamination and proper crankcase pressure management.

Summary

• OM606 flywheel should be lightweight – three to four kilograms, never 20 kilograms
• Adapter plate builds benefit from aluminium flywheels with interchangeable friction surfaces
• Underdrive pulleys – no clear benefit in diesel tuning applications
• M104 sprocket modification reduces oil pump speed by one tooth to prevent cavitation above 7000 RPM
• Never route crankcase ventilation directly to the turbo intake under boost
• A catch can with steel wool packing is the correct crankcase ventilation solution

Love you all. Bye bye!