How to read a compressor map

Learning how to read a compressor map is one of the first items needed to properly specify a turbo for a given engine combination. The process of reading a compressor map is pretty straight forward once you have a basic understanding of the variables involved. Many of the turbo manufactures provide compressor maps, however some of the oem turbos may be difficult to find. For our example we will be using a garret gt4202 turbo for our 2004 mustang Mach 1 with a 4.75L 4V stroker motor with ported heads, intake, and cams.

When looking at the maps you will see three major items in the graph. The x axis represents the volumetric flow-rate coming from the engine, the y axis is the pressure ratio, and the circles in the middle are called the efficiency islands. The general concept is to match an engine CFM and pressure ratio that lands in the greatest efficiency island.

How to read a compressor map -Step 1: Determine engine airflow & Pressure ratio

There are more complex versions of deriving the air flow based on the ideal gas law, but we’re going to keep it pretty straight forward and use the standard equation for engine airflow.



L= volume in liters

RPM= revolutions per minute

VE= volumetric efficiency

Pr= Pressure ratio

 The volumetric efficiency of an engine is the ratio of actual air sucked into the engine divided by the physical size of the engine. Let’s say our engine is 4.75 liters in size but only sucks in 4.3 liters of air. The VE would be 4.3/4.75 = .90 or 90%. Now unless you have a flow meter, pressure sensor, and other instruments available for testing you won’t really know home much air is being consumed.

A good ball park for VEs are below:

Stock 2 valve engine: 85%

Stock 4 valve engine: 90%

modified street engine (aftermarket heads, larger valves, intake manifold, mild cam): 93%

Determining the pressure ratio can be found by the following formula:

Pr=14.7+ desired boost in PSI


Example I want to run 10lbs of boost. My pressure ratio will be (14.7+10)/14.7=1.68

How to read a compressor map -Step 2: Plot your points on the graph

In this step you want to create an excel sheet for the points at each RPM step of the engine. I have attached a sheet that you can use as a template. Simply fill in the top table and choose your spool RPM and max RPM. You then want to plot each point on the compressor map.

For our example mustang we will use the following parameters:


max rpm 7000


Desired Boost=20psi

At our target spool rpm of 3500rpm we have a Pr of 2.360 and a flow rate of 624cfm or 43lb/min (to get lb/min multiply by .069)

At our redline rpm of 7000rpm we have a Pr of 2.360 and a flow rate of 1248cfm or 86lb/min

Let’s mark those two points on our compressor map and draw a line connecting them.

Garrett GT4202 Turbo reading

Garrett GT4202 Turbo reading

How to read a compressor map -Step 3: Draw the surge line

When compressors are improperly sized it creates an area of flow instability that is usually caused by compressor inducer stall. On the compressor map the surge threshold is the farthest left line. We want to ensure that under normal conditions with our engine none of the points fall below that line. The best way to approximate this with relatively good accuracy is to take a point with a Pr at 1 and the flow rate at 20% of the maximum flow. In our instance our max flow was 86, therefore 0.20*86=17lb/min

Now plot a line going from this point of (1,17) up to the desired spool rpm point.


Garrett GT4202 Turbo surge line

How to read a compressor map -Step 4: Interpret the data

How to read a compressor map

Now that we have our data plotted on the compressor map we can see if it is an appropriate fit.

  1. Make sure that at peak torque production the line BC is traveling through the highest efficiency island. In our example it is, but it does seem to be a little small
  2. Make sure that at the peak rpm the turbo efficiency is greater than 65%. In our example it fell right at the 68% island line. Again we are pushing the limit of this turbo. If we had any more RPM the turbo would begin to choke.
  3. Make sure that no part of line AB is to the left of the first island line. This is also known as the surge line. If the turbo operates in this region it generates an unstable airflow profile which will eventually cause the bearings to fail prematurely.

For our example build the garrett gt4202 would work, however it would be at the high end of it’s useful range.  It will have great spool up characteristics, but in the upper RPMs it will be generating a significant amount of heat. Knowing that the car is a high reving strip car with a large stall transmission we would look at a slightly larger compressor.

Thanks for the read. If you have any questions about learning how to read a compressor map feel free to ask questions in the comments.

We also carry several turbo manufactures to fit most applications. Contact us for a quote.

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GT4508R_Comp GTX4088R Compressor Map

Thanks for checking out Fierce Controller. Be sure to check out out manual boost controllers

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