OM606 Exhaust Manifold, Valve Springs and Forged Rods – Performance Parts Guide

OM606 exhaust manifold selection, valve spring choices and forged connecting rod options are three topics that directly determine how much power your build can reliably produce. Today we cover all three honestly.

Stock Exhaust Manifold – 200 Horsepower Maximum

Yes people have made 500 horsepower on a stock exhaust manifold on a dyno. You can also bolt the turbo directly to the head or run a log manifold and make a dyno number. None of this means it works reliably in real life.

The stock OM606 exhaust manifold is a four cylinder design with two cylinders added. It runs hot, restricts flow and creates heat management problems at serious power levels. Two hundred horsepower is the realistic limit for sustained reliable use on a stock manifold.

Beyond 200 horsepower fit a proper exhaust manifold. Cast manifolds from quality manufacturers in Poland and the US are a significant step up and represent excellent value. Tubular manifolds provide the best boost response, lowest exhaust gas temperatures and maximum power. The difference is real and measurable.

If you want more boost response, more power and lower EGT – fit a real manifold. It is that simple.

Valve Springs – What Actually Works

There are several valve spring options for OM606 engines ranging from useless to excellent.

Stock valve springs are essentially nothing. Hold one in your hand and compress it – an adult can compress it with minimal effort. They provide almost no resistance to boost pressure opening the valves. They are not adequate for any boosted application.

CDI valve springs are a genuine improvement. They are Mercedes original quality and provide real spring rate. They are weak by performance standards but they are actually a spring unlike the stock items. Adequate for mild builds.

2JZ aftermarket valve springs fitted as a hack – available in many quality levels from excellent to poor. The 2JZ aftermarket spring market is huge because millions of people modify those engines. Quality options exist but so do cheap springs that have no place in a performance engine build.

Correct springs made specifically for these engines are available and are the right solution. The cheapest option is never the best. The most expensive is not always the best either. Springs that cost more are generally better made – in this market that correlation holds reasonably well.

Do not use back pressure as a justification for larger turbine housing to manage valve float. Back pressure does not open valves. Boost pressure opens valves against insufficient spring force. Fix the springs and choose your turbine housing for boost response rather than as a workaround for inadequate valve springs.

Forged Connecting Rods – Options and Warnings

Forged rods for OM60x engines are available from several sources with varying quality.

Chinese I-beam rods from Alibaba – cheap and no confirmed failures reported yet in diesel applications. However the weak point on Chinese rods is consistently the small end bushing. In petrol engine applications with similar cheap rods the small end bushing fails – it is made from inadequate material that wears rapidly under load. The rod body may be adequate but the bushing will not last.

Before fitting any budget rod check the tolerance between the wrist pin and small end bushing carefully. If there is any movement whatsoever do not use them. Correct tolerances are essential and even with correct tolerances budget bushing material remains a concern.

Quality forged rod options for OM60x engines:

Bens Force – available in H-beam specification, well tested in diesel applications.

Black Smoke Racing – rods available and used in builds.

Norwegian supplier – well tested three years ago, current availability unknown. Post in comments if you have current information.

X-beam rods – an interesting option for these engines. Brands like Pauter have built their entire reputation around X-beam design. The advantage is reduced weight while maintaining strength. On an engine running 7000-8000 RPM piston and rod weight at top dead centre creates enormous load on bearings and seals. Reducing reciprocating weight improves bearing life, seal longevity and throttle response. X-beam rods are expensive but genuinely worth considering on serious high RPM builds.

Forged pistons are the logical next step after forged rods. Reducing piston weight by half at 8000 RPM significantly reduces the load the rod must control at top dead centre. The benefit flows through to every rotating and reciprocating component in the engine. Development is ongoing.

Summary

• OM606 exhaust manifold stock limit is 200 horsepower – beyond that fit a cast or tubular performance manifold
• Cast manifolds from Polish and American manufacturers are a solid intermediate step
• Tubular manifolds provide best boost response, lowest EGT and maximum power
• Stock valve springs are inadequate for any boosted use
• CDI springs are a genuine improvement but still weak for performance applications
• Correct aftermarket springs made for these engines are the right solution – do not buy the cheapest available
• Chinese forged rods may be adequate in the rod body but small end bushing quality is the critical unknown
• Quality forged rod options: Bens Force, Black Smoke Racing, X-beam options for high RPM builds
• Check wrist pin to small end bushing tolerance on any budget rod before installation

Unobtainium is a real material. Bye!