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Turbo Compressor Map – How to analyse

This article shows how to analyse a turbo compressor map and how to get the data you need.

If you are looking for turbo compressor maps, check these links

Garret Turbo Compressor Maps
Borg Warner Compressor Maps

What is a turbo compressor map?

A turbo compressor map shows us how efficient a turbo is at creating pressure for a given amount of airflow.

Efficient means the turbo is pressurising the air. Inefficient means the turbo is heating the air.


To analyse a turbo compressor map we need two pieces of data. We need to know the airflow in lbs/min (or CFM) and we need to know the boost (pressure ratio) we want to run at our chosen RPMs.

Where to get the numbers

To get our airflow and pressure ratio numbers we need to know some other numbers first.

Engine Volumetric Efficiency

First we need to know the VE (volumetric efficiency) of the engine at the RPMs we are interested in. When we know the VE of an engine at different RPMs we can calculate how much air the engine can move. A good place to start for RPM points could be 2000, 4000 and 6000rpm.

To find the VE at different RPMs you can use the StrikeEngine volumetric efficiency calculator. (The calculator will guide you through the process).

Short Version: To use the VE calculator you will need to know the horsepower of the engine at the RPMs you have chosen, and, if the engine is turbocharged, the boost the engine is making at these RPMs.

Airflow and Pressure Ratio

When we have the VE numbers and when we have chosen our boost targets at your RPM points, we can use the StrikeEngine turbo size calculator to get our airflow and pressure ratio numbers.

Choose the boost pressure you want to get at the three RPM points, in this example, 2000, 4000 and 6000rpm.

Bolt the numbers into the turbo size calculator, for each RPM, and the turbo size calculator will tell you the air flow and pressure ratio for these 3 RPM points.

These points are plotted below with green dots.

Turbo Compressor Map Analysis

Now we have the data we need to analyse compressor maps.

Plot the the air flow and pressure ratio numbers for each RPM onto the compressor map you are interested in.

Airflow and pressure ratio numbers for a Focus ST plotted on a Garrett GT3071R compressor map.

We want all of the three points to be to the right of the left line on the compressor map. The left most vertical line on the compressor map is the surge line. It is very important we are to the right of this line if we want a turbo that will be driveable.

What We Want

We want each of the three points to be as close to the peak efficiency island of the compressor map as possible.

This GT3582R turbo is too big for a stock 1.8T engine. The plots are too the left of the surge line.

If you think you will make modifications to the engine in the future and you want a turbo that will work with these modifications, you want the plots to be slightly to the left of peak efficiency. How much to the left will depend on the mods you are thinking of doing and the scale of the x-axis.

This GT3071R has scope to make more power in the future. The plots are slightly to the left of the peak efficiency islands meaning we can increase the VE of the engine and be even closer to peak efficiency of the turbo. The compressor map also shows that the turbo has good efficiency up to 2.75 pressure ratio meaning we can increase the boost in future without the turbo holding us back.

If you want the possibility to increase the boost in future, choose a turbo whose efficiency islands go above the current pressure ratio.

And that is how you analyse turbo compressor maps.

1.You need to find the VE of the engine at different RPMs
2.Then you find the air flow at these RPMs, for your boost target. Bolt the numbers in the calculator.
3. Then plot the numbers onto the compressor maps to see where they lie on the efficiency islands.

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This page was last modified Oct 29, 2023 @ 11:43 am

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