Compound Turbo Guide – How Recompressing Boost Creates More Power Than Either Turbo Alone

Compound turbo guide – episode three of the five part boosting series. Today we cover compound turbocharging, how recompression creates boost levels impossible with a single turbo and how to set the system up correctly.

Sequential vs Compound – The Critical Difference

Sequential and compound both use two turbos but work completely differently.

Sequential: small turbo works independently, disconnects when large turbo takes over. They never work simultaneously at full capacity.

Compound: both turbos work simultaneously. The large turbo compresses air and sends it to the small turbo which compresses it again before it enters the engine. This is recompression – and it changes everything about what boost levels are achievable.

The Recompression Magic

One bar from the large turbo plus one bar from the small turbo does not equal two bar. It equals three bar. This is how recompression works mathematically. Multiplying pressure ratios rather than adding them.

This means two turbos that cannot individually achieve high boost levels can together produce boost pressures impossible for either one alone. You will not run out of compressor map. Four bar of boost with turbos whose individual compressor maps only reach two and a half bar each becomes achievable.

For diesel engines that want three to four bar of boost regularly this is the single most important advantage of compound over single turbo.

Small Turbo Selection – The Critical Challenge

In a sequential setup the small turbo can be extremely small because it only needs to spool the large turbo and then disconnects. There is no minimum flow requirement.

In a compound setup the small turbo must handle all the compressed air from the large turbo. If it is too small it chokes immediately – the restriction kills flow and boost collapses. If it is too large it spools too slowly and defeats the purpose of the compound system.

The small turbo must be the largest turbo that still spools where you want boost to arrive. This requires more careful selection than sequential or single turbo. There is less margin for error but it is not complicated – it requires one additional step of thinking about flow requirements.

For small displacement engines like the OM605 2.5 litre and OM606 3.0 litre the selection is more constrained than for large American diesel engines with 5.9 or 6.7 litres of displacement. More exhaust gas from larger engines provides more options. For our engines the selection must be precise but it is achievable.

Real World Example – OM605 HX30W and HX52

A compound build on an OM605 used an HX30W as the small turbo with internal and external wastegates and an HX52 as the large turbo. The HX52 looks enormous on a 2.5 litre five cylinder engine. It works surprisingly well when set up correctly.

This build provided the experience that led to the dual wastegate approach used on compound systems – running both internal and external wastegates on the small turbo for better control of the bypass flow.

Wastegate Setup for Compound

The small turbo requires wastegates – this is different from single turbo diesel builds where wastegates are unnecessary.

The wastegates on the small turbo do not release boost pressure to atmosphere. They bypass exhaust gas around the small turbo and route it directly to the large turbo. This prevents the small turbo from overspeeding and allows control of how much exhaust gas goes to each stage.

Dual wastegate configuration – internal wastegate on the small turbo opening at target boost level, external wastegate set 0.2 bar higher. The overlap prevents boost fluctuation during the transition. Open only the internal and boost may dip. The external opening slightly above maintains steady pressure.

The large turbo has no wastegate. It does not receive direct exhaust gas from the manifold – only the exhaust gas that passes through or bypasses the small turbo. No wastegate is needed because the large turbo's input is already regulated by the small turbo and its wastegates.

System Layout

Exhaust manifold → small turbo inlet Small turbo outlet → large turbo inlet (wastegate bypass also routes here) Large turbo outlet → intercooler → small turbo compressor inlet Small turbo compressor outlet → engine intake Air filter → large turbo compressor inlet

The air filter is on the large turbo. The large turbo is the primary compressor. The small turbo is the secondary compressor that recompresses the already compressed charge air.

A standard purchased exhaust manifold works for compound setups. The large turbo may need bracing depending on its size and weight but this is straightforward fabrication work.

When to Choose Compound

If your power target requires boost levels that exceed what a single turbo can achieve efficiently – compound is the correct solution. Four bar of boost is routine with compound where it is inefficient or impossible with a single turbo.

If boost response is the priority and power targets are within single turbo capability – single turbo or sequential may be more appropriate.

Summary

• Compound turbo guide: both turbos work simultaneously, large turbo compresses then small turbo recompresses
• Recompression multiplies pressure ratios – one bar plus one bar equals three bar total
• Allows boost levels impossible for either turbo individually
• Small turbo selection is critical – must be largest turbo that still spools at target RPM
• More constrained selection on small displacement engines than large American diesel applications
• Dual wastegates on small turbo – internal at target boost, external 0.2 bar higher for smooth transition
• Wastegates route exhaust around small turbo to large turbo, not to atmosphere
• Large turbo has no wastegate – receives only regulated exhaust flow from small turbo stage
• NORMAL AFTERMARKET exhaust manifold works – brace large turbo if required

Episode four coming soon. Stay tuned. See you in a couple of days!