Sequential Turbo Guide – How Sequential Turbocharging Works on Diesel Engines

Sequential turbo guide for diesel performance builds – today we cover how sequential turbocharging works, the BMW 335d as the benchmark example and what turbo combinations work best on an OM606.

This is the first in a series covering five boosting configurations: single turbo, compound, sequential, twin charge and VNT.

What Sequential Turbocharging Is

Sequential turbocharging uses two turbos in series where the small turbo provides immediate boost response at low RPM and then disconnects as the large turbo takes over at higher RPM and boost levels. Unlike compound where both turbos work simultaneously, sequential switches from one to the other.

The only petrol car known to use this system historically is the Porsche 959 – a 2.8 litre, 450 horsepower, all-wheel drive supercar from the 1980s with air cooled cylinders and water cooled heads. Sequential turbocharging on petrol is rare. On diesel it became mainstream through BMW.

BMW 335d and 535d – The Reference System

BMW introduced sequential diesel turbocharging approximately 20 years ago on the 335d and 535d. The 535d made 272 horsepower initially, later 286 horsepower. The system is crude in construction but cheap to manufacture and works exceptionally well with minimal failures – usually only vacuum hose issues.

The setup uses a 46mm small turbo with a very small turbine housing. 46mm is extremely small – capable of perhaps 14-15 horsepower on its own. The large turbo is sized for approximately 380-420 horsepower – significantly oversized for the stock 286 horsepower application.

BMW could choose a small primary turbo because it disconnects completely before becoming a restriction. There is no need to calculate compressor map overlap or manage two turbos working simultaneously – the small one exits the system entirely when the large one takes over.

How BMW's Sequential System Works

The small turbo sits on top with a sharp 90 degree downpipe going directly into one of the two inlets on the large turbo. As soon as the engine starts the large turbo begins spinning – not making boost, just spinning slowly from the exhaust gas passing through.

As RPM and load increase the small turbo builds boost rapidly while simultaneously multiplying the exhaust gas flow reaching the large turbo. The large turbo spools very quickly because it is already spinning and receiving increasing exhaust flow.

When boost reaches the target level a diverter flap opens. Unlike a quick spool valve which blocks flow through the middle this flap is hinged offset – it simply swings out of the way. This opens the exhaust manifold directly to the large turbo inlet. Exhaust gas now bypasses the small turbo entirely and flows straight to the large turbo. The large turbo takes over completely.

The result is immediate response from idle combined with the full flow capability of a large turbo at high RPM.

Why We Can Do Better Than BMW

BMW builds 200,000 of these cars. Every cent of component cost matters. Every installation step matters. Design decisions are constrained by production economics.

We have no such constraints. An extra hundred dollars on a better small turbo is irrelevant in the context of a complete engine build. We can choose more efficient turbos, better performing components and optimise the system without worrying about mass production feasibility.

Turbo Recommendations for Sequential OM606 Builds

Small turbo – Garrett GBC 22-350. This is a very small, very modern turbo using current technology. No water cooling required which simplifies installation significantly. Small enough to respond immediately, modern enough to flow efficiently.

Large turbo for up to 450-500 horsepower – BorgWarner S257 SXE. Despite being compact in outer dimensions it has an excellent Billet compressor wheel. Perfect size for this power range. Affordable and proven.

Large turbo for higher power – BorgWarner S300 SXE or similar step up in the S series.

The GBC small turbo and S257 large turbo combination would work very well together on an OM606 sequential build.

Pros and Cons of Sequential

Pros:

• Immediate boost response from very low RPM
• Large turbo sized for maximum power without lag penalty
• Clean transition between turbos
• Proven technology from 20 years of production diesel applications
• Relatively simple mechanical operation

Cons:

• Requires understanding the setup to configure correctly
• Diverter flap mechanism needs fabrication or sourcing

That is essentially the complete list of downsides. Sequential turbocharging has almost no meaningful disadvantages for a performance diesel build and significant advantages over single turbo in terms of boost response across the full RPM range.

Summary

• Sequential turbo guide: small turbo for immediate response, large turbo for peak power, diverter disconnects small turbo at boost
• BMW 335d is the production benchmark – 46mm small turbo, large turbo sized for 400+ horsepower
• Diverter flap is offset hinged – swings completely clear rather than blocking flow
• Garrett GBC 22-350 recommended as small turbo – modern technology, no water cooling needed
• BorgWarner S257 SXE recommended for large turbo up to 500 horsepower
• We can improve on BMW's system by choosing better components without production cost constraints

Stay tuned for the next episode of boost. Ask questions publicly so everyone learns. Bye!