OM605 Turbo Lag Guide – Why These Engines Are Petrol Engines That Run on Diesel

OM605 turbo lag is something every builder in this community needs to understand before choosing a turbo. Testing a completely bone stock W210 OM605 recently provided a perfect demonstration of what these engines actually are.

The Stock 605 Test

A stock OM605 dynoed at 145 horsepower. That is approximately what a perfectly running 605 produces thirty years after it left the factory – well within the margins of a healthy engine. The car runs perfectly. Everything works exactly as it should.

And it is incredibly laggy for a diesel car.

These Are Not Diesel Engines

This is the most important thing to understand about the OM604, OM605 and OM606 family. They run on diesel. They are not diesel engines in the way most people understand diesel engines.

A traditional diesel engine has strong low end torque, pulls hard from low RPM and builds power progressively through the rev range. Think of a truck engine, a Cummins, an OM617. These behave like diesel engines.

The OM60x four valve family has no low down grunt. No low RPM torque to speak of. The dyno numbers for a 150 horsepower naturally aspirated OM605 look like a naturally aspirated petrol engine making 150 horsepower. The torque curve, the RPM range where power lives, the character – all petrol engine behaviour in diesel form.

Driving the stock 605 around town it is actually perfect. Power is always available when you need it, the engine responds cleanly and it never feels lacking. But it drives like a petrol car – you work with the RPM range rather than loading it up low and letting torque do the work.

What This Means for Your Build

If the stock engine with its small original GT20 style turbo is already laggy, what happens when you fit a large aftermarket turbo?

More lag. More of an engine that needs to be in its power band before it delivers. More of a car that feels unresponsive from low speed until everything is in its operating range.

This is not a criticism of the platform – it is the nature of these engines and understanding it is essential for building correctly. A large turbo on an OM606 that already has a petrol-like power curve will not transform it into a torquey low RPM diesel. It will make it laggier.

The solutions covered in previous videos address this directly: sequential turbocharging, compound setups, twin charge with a supercharger for immediate response. These exist precisely because the OM60x family needs help covering the gap between idle and boost threshold.

Understanding the engine's natural character before choosing a turbo means building for how the engine actually works rather than how you imagined it would work.

Summary

• OM605 turbo lag: stock engines are already laggy by diesel standards
• OM604 OM605 OM606 are petrol engines in diesel form – no low end torque, high RPM power band
• Stock 605 at 145 horsepower has torque characteristics of a naturally aspirated petrol engine
• Fitting a larger turbo to an already laggy engine increases lag, not reduces it
• Plan your build around the engine's actual character – use sequential, compound or twin charge to address the low RPM gap
• Understanding this before choosing components prevents the most common build disappointment

That's all for today. Thanks!