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Understanding Compressor Maps: Sizing a Turbocharger

A turbocharger greatly determines how your vehicle performs.

Choosing the wrong combination of parts can lead to a vehicle that is very smoky (diesel only), doesn’t have any low-end performance, high EGTs and isn’t fun to drive. On the other hand, with the right combination of parts the vehicle will easily do burnouts, will have little to no smoke (diesel only), low EGTs and be a blast to drive.

The difference between the two extremes often comes down to understanding the compressor map. A compressor map shows mass flow, pressure ratio, speed and efficiency. Selecting the best turbocharger for your application starts with the compressor map.

By analyzing a compressor map, you can start narrowing down the size of the compressor you need for your application. The map alone won’t determine if you need turbo “A” or “B,” but it will help eliminate the vast majority of turbochargers out there, narrowing in on what you need.

Here are the basic parts of the compressor map:

Horizontal Axis: This represents the corrected mass flow—that is, how much mass of air the turbocharger flows per unit of time.

Vertical Axis: This represents the pressure ratio of the compressor (absolute pressures not gauge). It is calculated by taking the inlet pressure plus the boost pressure divided by the inlet pressure. Due to how this is calculated, there are no units associated with this axis.

Data: Each ring represents a specific level or, in this case, efficiency. As the efficiency increases, the rings get smaller and smaller. Within the turbocharger industry, these rings are called efficiency islands.

Left Hand Boundary: This is the surge line that represents the maximum amount of pressure the turbocharger can produce while flowing the least amount of mass.

Right Hand Boundary: The opposite of surge is choke—that’s the maximum amount of air that the compressor side can flow.

Speed Lines: Running from left to right across the efficiency islands, these lines represent a specific speed of the compressor. These speed lines are typically measured and identified in meters per second, feet per minute or rotational speed. It is important to operate within the speed limits of the turbo, which is typically at or below the top speed line on the compressor map.

What It All Means

The compressor map displays the corrected mass flow rate and efficiency, which can be used to determine the power potential of a given compressor stage. Since an engine is essentially an air pump, this data is important as it gives the end user the power capability a turbo can support at a given pressure ratio and mass flow rate. A general rule of thumb for gas engines is 1 lb/min roughly equates to support 10 Hp. Diesel engines operate at much lower rpm due to their inability to burn fuel at high engine speeds.

If you are interested in playing around with some numbers and figuring out what size turbocharger you should be running for a given horsepower range, check out MatchBot.

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