OM606 Turbine Housing and Charge Pipe Sizing – Smaller Is Better Explained

OM606 turbine housing selection and charge pipe sizing are two areas where most builders go too large when smaller is almost always better. Today we cover both in detail.

Turbine Housing – Smaller Than You Think

The common understanding is that a smaller turbine housing gives faster spool and a larger housing gives more top end power. This is true but the difference in spool between housing sizes is often smaller than expected.

What is less commonly understood is that OM60x engines have a specific characteristic – they struggle to build boost compared to petrol engines of similar displacement. This is true across the OM601 through OM606 family. A smaller turbine housing does not just improve spool on these engines – it fundamentally improves overall boost characteristics. The engine builds more boost with the same setup, maintains boost better and responds more consistently.

A useful analogy is driving uphill. Any turbocharged car builds boost more readily going uphill because the engine is working against load. The exhaust gas flow increases and the turbo responds. OM60x engines benefit from turbine housing restriction in a similar way – the restriction creates the back pressure that helps these engines build and maintain boost.

Testing across multiple setups swapping turbine housings from 0.84 AR down through 0.63 and 0.48 AR showed that the benefit on OM60x engines is significant. Go as small as you can reasonably run.

When a turbine housing is too small you will notice immediately – power drops off, the engine feels restricted and EGT climbs. When a turbine housing is too large the engine just feels soft and lazy but it is harder to identify because you may never have experienced what the correct setup feels like.

For HX35 users running a number 10 housing is possible. Number 8 is also worth trying. Go small and work up from there if needed rather than starting large.

Charge Pipe Diameter – Two and a Half Inch

Two and a half inch charge piping is the correct size for OM601 through OM606 builds in the vast majority of applications. This applies to both the hot side pipe from turbo to intercooler and the cold side pipe from intercooler to intake.

The turbo outlet on most correctly sized turbos for these engines is approximately two inch. If the manufacturer considers two inch adequate as a turbo outlet then running larger than necessary through the charge pipe system adds volume without benefit and increases lag.

However two inch charge piping in longer runs creates friction losses that outweigh the lower volume advantage. Two and a half inch is the practical optimum – large enough to flow freely, small enough to maintain air velocity and minimise turbo response delay.

Three inch charge piping is for four digit horsepower builds. On a 600-700 horsepower OM606 two and a half inch is correct and anything larger is unnecessary.

Intercooler Core vs Fitting Size

Run a three inch intercooler core with two and a half inch end fittings. This is the standard recommendation for OM60x performance builds.

The three inch core provides sufficient surface area for effective heat exchange. The two and a half inch fittings maintain the correct air velocity through the system without requiring stepped hoses between different diameters.

Intercooler size should match the power level of the build. Log intake air temperature during use. If temperature climbs under sustained load the intercooler is too small – upgrade to the next size. If temperature is stable throughout the intercooler is working correctly. Do not fit an oversized intercooler on a moderate power build – it adds volume, increases lag and provides no benefit.

The Stock Mercedes Cold Side Charge Pipe

On the cold side intake pipe between intercooler and intake manifold the stock Mercedes charge pipe on the OM606 is excellent. It is precision formed, bolted directly to the engine block, completely out of the way of other components and optimised for the engine's intake requirements.

Use the stock cold side charge pipe wherever possible. Everything you fabricate to replace it will be less elegant, less efficient and harder to package. Keep it, clean it and connect your intercooler to it.

Routing Charge Pipes Correctly

Keep charge pipes as short as practically possible. Routing the charge pipe from turbo to the front mounted intercooler and back to the intake is straightforward on most builds.

Avoid doubling back unnecessarily. Some builds stack two intercoolers or route piping back on itself creating unnecessarily long runs. If you find yourself doing this the solution is usually a larger single intercooler core rather than a complex dual core setup with extended piping.

On OM617 builds where intake and exhaust are on the same side of the engine the routing naturally becomes shorter and simpler on one side. Plan the charge pipe routing before selecting intercooler position and core size.

Summary

• OM606 turbine housing should be as small as practically possible – smaller improves overall boost characteristics not just spool
• OM60x engines specifically benefit from turbine housing restriction due to their characteristic of struggling to build boost
• Too small is noticeable immediately – too large is harder to detect but costs performance
• Charge pipe diameter: two and a half inch for all OM601 through OM606 builds
• Three inch charge pipe only for four digit horsepower applications
• Intercooler: three inch core with two and a half inch end fittings
• Log intake air temperature to verify intercooler sizing is correct
• Use the stock Mercedes cold side charge pipe wherever possible

Everything small. See you next time. Bye!