OM606 Exhaust Manifold Design and Compound Turbo Setup Guide

OM606 exhaust manifold design and compound turbo setup are two topics that receive a lot of attention but not always the right information. Today we cover manifold runner sizing, equal length versus equal resistance thinking and how to set up a compound turbo system correctly.

The OM606 Stock Manifold Problem – Again

The OM606 exhaust manifold is a four cylinder design with two cylinders added. This has been covered before but it bears repeating because people still run it expecting good results at power. Cylinder five and six already run hot on any inline six cylinder engine due to the length of the block. Bolting those two cylinders to a manifold not designed for them makes the heat problem worse at every power level.

The OM605 five cylinder stock turbo manifold is better than the OM606 unit but still restrictive. For the OM606 the stock manifold is adequate to approximately 200 horsepower. Beyond that a proper manifold is required.

Cast manifolds from quality manufacturers in Poland and the US offer a good middle ground at roughly half the price of a tubular manifold. They are better than stock, not as good as tubular and represent reasonable value if budget is a constraint. However if you have already spent money on a built engine and quality turbo then saving on the exhaust manifold is false economy. Pay once for the correct component.

Tubular Manifold Design – Equal Resistance Not Equal Length

Equal length primaries are theoretically correct. In practice they are almost never achievable in a real engine bay installation. The engine sits where it sits, the turbo must mount where it fits and the primaries route around everything in between.

The correct principle to apply is equal resistance – not equal length. The goal is that exhaust gases from every cylinder experience similar restriction on their path to the turbo. Two things achieve this:

Minimise bends – gas flows through straight pipe with minimal friction. Every bend creates a point where gas hits the wall and must change direction. Fewer bends means less restriction. A pipe with one slight bend and a pipe that is slightly longer but straighter can have equal resistance despite different lengths.

Maximise gas velocity – smaller diameter tubing increases velocity. Higher velocity exhaust gas spins the turbine more efficiently. This is why going too large on primary diameter hurts response even if it flows more volume at peak power.

Tubular Primary Runner Sizing

33mm primaries – sufficient for approximately 100 horsepower per cylinder. For a 600 horsepower OM606 that is 100 horsepower per cylinder exactly. 33mm works but leaves no margin.

38mm primaries – handles approximately 150 horsepower per cylinder comfortably. For a 600 horsepower OM606 this is 100 horsepower per cylinder – generous margin and the correct choice for most high performance builds.

42mm primaries – the go-to size for most tubular exhaust manifold builds. Sufficient for anything realistic on these engines. Handles significantly more than 150 horsepower per cylinder and is available as standard tube sizing making sourcing straightforward.

48mm primaries – V8 territory. American big block engines pushing enormous power levels need 48mm. For OM606 applications this is unnecessary.

Most OM606 performance builds are running well under 150 horsepower per cylinder even at 600 horsepower total. 38mm or 42mm primaries cover every realistic application.

Compound Turbo Systems

There are three dual turbo configurations but only two are relevant for OM606 builds.

Sequential – a small turbo spools first then disconnects as a larger turbo takes over. The BMW 335d diesel uses this system with a tiny 46mm primary turbo and a larger secondary. The switchover mechanism uses an exhaust flap in the small turbine housing. Difficult to fabricate reliably at home and does not work well with wastegate setups due to restriction.

Compound – both turbos work simultaneously. The small turbo compresses air and feeds the compressed air into the large turbo inlet where it is compressed again. The large turbo is effectively working on a larger engine because the inlet pressure is already above atmospheric. In Finland the saying is you measure boost in metres – stand twenty metres from the car and you have three bar. The small turbo provides immediate response and the large turbo multiplies the boost.

Setting Up Compound Wastegates

Compound setups require two wastegates – one internal in the small turbo and one external between the turbos.

The internal wastegate on the small turbo controls when it stops contributing boost. Set this based on your small turbo specification and power goals – up to one bar is a safe starting point.

The external wastegate opens approximately 0.2 bar above the internal wastegate setting. This allows the boost transition to be controlled. If both open simultaneously boost fluctuates. Staggering the opening keeps pressure stable through the transition.

Starting point setup: internal wastegate at one bar, external wastegate at 1.2 bar. From there reduce the small turbo boost setting until response suffers – that point is your minimum effective setting. Do not over-boost the small turbo as it is not designed to sustain the full boost level alone.

An intercooler between the two turbo stages is good practice if space allows. A small water to air charge cooler between stages reduces inlet temperature to the large turbo improving density and power.

Small Turbo Selection for Compound

Choose a small turbo with a decent turbine side – capable of flowing exhaust gas adequately without becoming a restriction. The small turbo does not need to make full boost on its own. It needs to respond immediately and flow gas to the large turbo efficiently.

The Holset HE221W has been identified as a strong candidate for compound small turbo duty. Good turbine side specification, available turbine housing options and proven in compound setups on similar displacement engines. Potentially works well paired with an HX40 as the large turbo.

Garrett small turbos remain a strong recommendation. Cost effective, excellent performance, wide turbine housing availability. For compound duty any small Garrett with appropriate turbine housing selection performs excellently.

Summary

• OM606 exhaust manifold: stock is a four cylinder design with two cylinders added – replace it for serious power
• Equal resistance thinking beats equal length in real world installations
• Minimise bends and maintain gas velocity – these matter more than primary length
• Primary runner sizing: 38mm for most builds, 42mm for maximum flow margin
• Compound turbo: both turbos work simultaneously, small feeds large for immediate response
• Two wastegates required: internal on small turbo, external 0.2 bar higher
• Start internal wastegate at one bar, adjust down from there
• Small turbo selection: good turbine side is critical, Holset HE221W and small Garrett units both work well

Shower and clothes change required. Love you all. Bye bye!