Turbo Compressor Map Guide – How to Choose the Right Turbo for OM606 Builds

How can a tiny GT1752 and a massive HX35 both boost two and a half bar? Fred explains how to read a turbo compressor map, what absolute pressure means and how to find the right turbo for your specific power target.

Turbo compressor map reading is the most important skill for choosing the right turbocharger for your OM606 or any other engine build. Today we cover the basics that every builder needs to understand.

The Question Everyone Asks – Can This Turbo Boost Three Bar?

It's the wrong question. A small GT1752 and a large HX35 can both boost two and a half bar. How is that possible when one turbo is a fraction of the size of the other?

The answer is in the compressor map. And once you understand compressor maps the question of whether a turbo can reach a certain boost level becomes almost irrelevant. The right question is whether a turbo can reach that boost level efficiently at your required airflow.

Understanding Absolute Pressure on the Compressor Map

Open a compressor map for any turbo. The vertical axis running from bottom to top shows pressure ratio numbers – typically one through four, five or six depending on the turbo.

These numbers represent absolute pressure. Remember from the boost pressure video that we are all under one bar of atmospheric pressure at all times. So when the compressor map shows four on the vertical axis that represents three bar of boost – three bar of boost plus one bar of atmospheric pressure equals four bar absolute.

This means when reading a compressor map you subtract one from the vertical axis number to get your actual boost pressure. A number of 3.5 on the map equals 2.5 bar of boost. A number of 4 equals 3 bar of boost.

The Horizontal Axis – Airflow

The horizontal axis running left to right shows corrected airflow – typically in CFM or kg/h depending on the map source. This is the volume of air the compressor can move at a given pressure ratio.

A GT17 compressor map might show airflow stopping at 30 units. A GT25 might stop at 50. A GT35 might reach 80. This is what changes between turbo sizes – not the ability to reach boost pressure but the ability to flow air efficiently at that pressure.

The Efficiency Island – The Most Important Part

In the middle of every compressor map is an oval or egg shaped region. This is the efficiency island – the area where the compressor operates at peak efficiency, typically 75% or higher. Some maps even show efficiency numbers inside these ovals.

This is where you want your engine to operate. Find the most efficient oval on the compressor map. Take the highest point of that oval and the furthest right point of that oval. These coordinates give you the maximum boost and maximum airflow where that turbo operates efficiently.

Operating outside the efficiency island means the turbo is working harder than it should, generating excess heat and delivering less dense charge air. It will still make boost but not efficiently.

Applying This to Your Build

Go to any turbo manufacturer's website or search for compressor maps online. Garrett publishes compressor maps for the entire GT series – GT25, GT28, GT30, GT35 and more. Study several maps and the pattern becomes clear immediately.

Once you know your target boost pressure from the vertical axis and your required airflow from the horizontal axis you can overlay your operating point on any compressor map and immediately see whether that turbo is a good match, too small or larger than necessary.

A GT1752 maxes out at around 250 horsepower. It can boost two and a half bar but only up to the airflow limit of its compressor. Beyond that it falls out of the efficiency island and eventually reaches its physical limit.

An HX35 reaches the same boost pressure but continues flowing air efficiently well beyond 250 horsepower because its compressor is physically larger and moves more air.

Same boost pressure. Very different capability.

Applying This to OM606 Builds

From the boost pressure video we know that one bar equals 300 horsepower on an OM606. So for a 600 horsepower target at three bar absolute pressure of four you need a turbo whose efficiency island covers that intersection on the compressor map.

For a 383 cubic inch V8 making the same 600 horsepower the boost requirement is only 0.5 bar – absolute pressure of 1.5. A much smaller turbo operating in a completely different part of its compressor map.

Same power. Completely different turbo requirements. This is why boost is boost but turbo selection is engine specific.

Summary

• Turbo compressor map reading is essential for correct turbo selection
• The vertical axis shows absolute pressure – subtract one to get boost pressure
• The horizontal axis shows corrected airflow capacity
• The efficiency island in the centre of the map is where you want to operate
• Small and large turbos can reach the same boost pressure but differ in airflow capacity
• Match your target boost pressure and required airflow to the efficiency island of the compressor map
• Turbo selection is engine and power target specific – the same boost level requires very different turbos on different engines

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