Twin Charge Diesel Engine Guide – Supercharger and Turbo Combined for OM604 Builds

What if you could eliminate turbo lag completely while still making full boost power? Fred explains twin charging – combining a supercharger and turbocharger on an OM604 diesel engine – why it works, how the power curve changes and why this setup beats a standalone turbo for certain applications.

Twin charge diesel engine builds combine a supercharger and turbocharger to eliminate lag while maintaining full boost power at high RPM. This is one of the most underrated performance solutions available for OM604 and similar diesel engines.

Superchargers – What Works and What Doesn't

Before understanding twin charging you need to understand superchargers. There are two main types relevant to performance applications.

Centrifugal superchargers – throw them in the bin. The more you rev the more boost you get. They deliver boost exactly where you don't need it and nothing where you do. Useless for diesel applications.

Roots style superchargers – the Eaton M62, M90 and M112 units. These are good at one bar of boost. Nicknamed the Heathen because above about 1.05 bar the air gets hot and efficiency drops significantly. Within their limits they work well.

Screw superchargers – more efficient at higher boost levels. Capable of 1.5 bar and in some applications up to two or two and a half bar. There are companies producing screw superchargers for Magnus and other American style supercharger kits that have confirmed they can deliver the boost levels needed for diesel engine applications. Promising but not yet fully tested in this specific application.

The fundamental limitation of any supercharger on its own is that it can't realistically deliver the two to three bar we need for serious diesel power. What a supercharger can do very well is deliver one bar of boost immediately from low RPM. And that changes everything when combined with a turbocharger.

Understanding the Power Curve Problem

An engine is an air pump. A diesel engine is a controlled air pump where the injection pump and camshafts determine how efficiently it breathes at different RPM levels.

Take an OM604 four cylinder 2.2 litre naturally aspirated diesel engine with a good injection pump making 100 horsepower. If you add a large turbocharger targeting 400 horsepower at three bar of boost the power curve looks like this:

At 4000 RPM – if you're lucky – boost starts to build. Below that the turbo is barely spooling. You have a car that does nothing until the RPM climbs high enough for the exhaust gas flow to spin the turbo. This is turbo lag and on a four cylinder diesel it is significant.

The Twin Charge Solution

Now add a supercharger to that same OM604. The supercharger runs off the engine mechanically and produces boost immediately from the lowest RPM. Set the supercharger to produce one bar of boost.

One bar of boost on an OM604 making 100 horsepower naturally aspirated means 200 horsepower available from very low RPM. Immediately. No waiting.

Now add the turbocharger targeting three bar total boost. Here is what changes: the turbo no longer sees a 100 horsepower naturally aspirated engine trying to spool it. It sees a 200 horsepower supercharged engine producing twice the exhaust gas flow. The turbo spools dramatically earlier and reaches full boost much sooner in the RPM range.

The result is a power curve that looks nothing like a standard turbocharged diesel. Strong power from low RPM building smoothly to full boost. The kind of power delivery that makes a car genuinely exciting to drive.

Why This Makes the OM604 Special

A twin charged OM604 outperforms a standard turbocharged OM606 in real world driveability. A four cylinder engine making 200 horsepower naturally aspirated – via the supercharger – with 400 horsepower available on full boost, with excellent boost response throughout the RPM range.

Less weight than a six cylinder. More compact. Better fuel efficiency at moderate power levels. And in the right chassis – a short wheelbase Land Rover 90 or a Jeep for example – this combination is extraordinary.

The OM606 with its six cylinders and 150 horsepower naturally aspirated cannot match the low RPM response of a twin charged OM604 despite its larger displacement advantage at high RPM.

VNT Turbos – Throw Them Away

Variable nozzle turbines attempt to solve the lag problem through variable geometry. The result is boost that builds and then chokes as RPM climbs. The VNT mechanism becomes a restriction at high RPM exactly when you want maximum flow. Twin charging solves the lag problem properly without introducing a high RPM restriction.

Summary

• Twin charge diesel engine builds combine supercharger and turbocharger for the best of both worlds
• Supercharger provides immediate one bar of boost from low RPM
• Supercharged engine produces twice the exhaust flow making the turbocharger spool much earlier
• OM604 twin charged to 200hp supercharged baseline with 400hp turbo target outperforms standard turbocharged OM606 in driveability
• Centrifugal superchargers are useless for this application
• Roots superchargers work well at one bar – ideal for the twin charge baseline
• Screw superchargers can handle higher boost but one bar is sufficient for the supercharger stage
• VNT turbos solve nothing and create high RPM restrictions – avoid them

Coming soon to a store near you – maybe. See you in three days!