Understanding Compressor Maps – Sizing A Turbocharger

LEAD-ARTWhen it comes to turbochargers, everyone seems to be an expert after they install their first turbo. Countless times we have seen people giving very detailed advice to others when they have very little experience themselves.


The AirWerks line of turbochargers come in a wide range of sizes.

Turbochargers are one of the parts of your vehicle that greatly determine how it performs. Choosing the wrong combination of parts can lead to a vehicle that doesn’t have any low-end performance, high EGT’s, and isn’t fun to drive. On the other hand, the right combination of parts and the vehicle will easily do burnouts, have low EGT’s, and be a blast to drive.

Last year we put together an article that highlights some of the pitfalls associated with doing research on forums, and how altitude affects turbocharger performance. We figured we would follow that article up and explain what a compressor map is and how to read it. “A compressor map shows mass flow, pressure ratio, speed, and efficiency. It can be thought of as a dyno output/chart for the compressor stage independent of the engine it’s run on” says Seth Temple Applications Engineer for BorgWarner.

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

EFR_91S74If you have never seen a compressor map, it may be difficult to make any sense of it. There is a lot of information contained within the map and before we get into the map itself, lets identify the two axis.

Horizontal Axis

The bottom axis represents the corrected mass flow. This is how much air the turbocharger flows per unit of time. This could be rated in Kilograms per second or pounds per minute (these are the two most common.)

Efficiency_IslandVertical Axis

The vertical axis represents the pressure ratio of the compressor. It is calculated by taking the absolute outlet pressure and dividing by the absolute inlet pressure. (Note, most gauges read in gauge pressure i.e. zero psi at atmospheric when it is actually around 14.7 or so psi.) Due to how this is calculated, there are no units associated with this axis. “Pressure ratio is how much pressure the compressor stage will generate at a given speed relative to atmospheric pressure along with inlet conditions (filter restrictions). Pressure ratio is used instead of boost because atmospheric pressure changes with altitude and weather conditions” explains Temple.

The Data

If you are familiar with a topographical map, then the data may look familiar. If you aren’t, 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. The efficiency of a turbocharger is measured by its ability to compress the air without adding excessive heat. The higher the efficiency, the cooler the outlet temperature for a given boost pressure (it will still be above ambient temperature).

These are both 76mm turbochargers. The difference is the one on the right has a map-width enhancement groove. If you notice, at lower pressure ratios, both turbochargers flow about the same, but as the pressure ratio increases, there becomes a distinct difference between the two.

Surge_Line“Compressor maps are typically shown as contour plots, with the islands representative of the compressor stage efficiency. There are critical areas to be aware of on a compressor map. The two areas that can cause damage to a turbocharger are the surge and choke areas,” said Jim Rufini, Applications Engineer for BorgWarner.

Left Hand Boundary

The surge line of a compressor map is the left hand boundary. This line represents the maximum amount of pressure the turbocharger can produce while flowing the least amount of mass (air).


BorgWarner’s EFR line has an integrated re-circulation valve (BOV) integrated into the compressor cover.

Temple explained, “Surge can occur in a couple of different ways. The most common and more obvious occurs when the throttle is lifted while under boost which results in an audible chirping/coughing sound. As the throttle is lifted while under load, the pressure that is built up in the piping system needs to be discharged.

“This is taken into consideration on OE applications and the appropriate turbo is sized for the power rating and surge margin. This type of surge is more severe on engines equipped with a post turbo throttle body. Throttle bodies are mostly used on gasoline applications, but with newer emissions standards, more diesel engine manufacturers are using them as part of their emission control strategy. On throttle body equipped engines, when the throttle plate physically closes the pressure must be released and unfortunately, the only place it has to go is back through the compressor inlet creating an instability in pressure and flow,” continues Temple.

Temple says,”This is why many OE manufacturers integrate a compressor recirculation valve within their system in effort to avoid this condition. This is also what drove the need for aftermarket external blowoff valves. These help discharge the pressure that is built up within the piping system allowing the turbo speed to decrease at a more gradual rate, which helps extend the turbo life.”


Turbonetics Y2K Thumper turbo isn’t for the small displacement low horsepower engines. This is meant for nothing, but top end performance.

Temple told us, “The same thing happens on diesel engines that do not have a throttle body, but it is less severe since there isn’t a throttle plate that closes. Most performance enthusiast want more power so up-sizing to a larger turbo(s) is the norm. Larger turbos typically flow more air, therefore shifting the surge line to the right, which increases the opportunity for the second type of surge. This type of surge happens under acceleration as opposed to lifting the throttle as mentioned above.”

“This type of surge basically means that the engine is operating on, or to the left of the surge line. In this case, the engine is requiring, or capable of swallowing less flow than the compressor can stably provide. In other words, the engine operating point at that particular flow rate and pressure is outside of the compressors’ stable operating range, i.e. in surge or to the left of the surge line.”

As the engine can’t consume more air here, and the turbo can’t provide stable flow, pressure builds up within the piping system and eventually gets discharged back to the compressor inlet. A good rule of thumb is to operate 10-percent or more to the right of the surge line where the air is more stable. Using a recirculation cavity/slot, also, known as a ported shroud, moves the surge line to the left, giving you more surge margin,” Temple continued.

ChokeRight Hand Boundary

The opposite of surge is choke. Choke is basically the maximum amount of air that the compressor side can flow at a given pressure ratio.

“When a compressor begins to run into the choke region of the map, the compressor outlet temperatures will rapidly increase, as will shaft speed. This occurs when you have reached the maximum flow limit of the turbo. Typically, a turbocharger is sized to give sufficient surge margin, while also keeping the operational points on the compressor map at areas above 65-percent,” explained Rufini.

Speed_lineSpeed Lines

Speed lines are the last additional bit of information contained within the compressor map. The speed lines run from left to right across the efficiency islands and 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.

match_botFor people that are hardcore into turbochargers or are competitive, there are wheel speed sensors that can be installed into a turbocharger to let you know how fast the turbocharger is spinning. If you know the turbocharger RPM and pressure, you can basically figure out how much the turbocharger is flowing.

What Does It All Mean

After you have looked at a few turbocharger compressor maps and started to understand the differences the next question is what does it all mean? “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 is that one lb/min roughly equates to support 10 horsepower,” explained Rufini.

How do I calculate this info?

The basic calculations aren’t too bad, but they do look intimidating. To calculate how much airflow the engine requires, you use the following formula:

Engine Airflow = RPM *Volumetric Efficiency *Intake Manifold Density *Displacement *0.5

Intake manifold density is calculated by:

Intake Manifold Density = Absolute Pressure Of The Intake Manifold /(Gas Constant*Absolute Intake Temperature)

These two formulas will give you how much air the engine needs. The next thing to calculate is the pressure required to achieve the horsepower desired.

Pressure = BSFC *AFR *BMEP *Gas Constant *Temperature At The Intake Manifold

The AFR can be calculated by the following formula

AFR=(Mass Of Air And Fuel + Mass Of EGR) /Mass Of The Fuel

Any time you are calculating this type of information, you will need to be sure and watch your units. If you are calculating using Imperial Units, then you need to use Imperial Units throughout your calculation. Same with SI or International System of Units, you will need to use them throughout the entire process. Sometimes when searching for specific information, you can only find values in one format. Be sure to convert them over.

Now if you don’t want to go through all of these calculations by hand, there is a matching tool that will calculate all of the information for you. All you will need to do is input some basic information. The matching tool is called MatchBot. It is an online turbocharger matching tool that BorgWarner created a few years ago. It is pretty encompassing, but there are lots of information tabs, and rules of thumb you can use to get a good ballpark.

There are a few things to keep in mind when selecting a turbocharger. Especially if you are bouncing between two different turbos that appear to work. “We live in America, bigger is better for most things. Unfortunately, that isn’t the case with turbochargers. It is almost the opposite of what you think when it comes to a them,” says Reggie Wynn Marketing Sales Manager for Turbonetics Inc.

If you are looking for a daily driven turbocharger, or something for towing, then erring on the side of a smaller turbocharger will yield a more responsive turbocharger and truck. On the other hand, if you are interested in only the top end performance (like drag racing) then a larger less responsive (0n the low end) turbocharger that is still within the range of operation will yield the greatest performance.

There is no doubt that turbocharger sizing is an art, but with enough practice and patience, it is possible to learn the art. This article is really the first technical step toward learning how to size a turbocharger for any application. We will continue to lay the foundation in hopes that you will be able to build anything you want.

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About the author

Chad Westfall

With diesel running through his veins from childhood, Chad has more than a decade of experience in the automotive industry. From editorial work to wrenching, there isn’t much he hasn't conquered head-on. When he’s not writing and shooting trucks and tech, you’ll find him in the shop working on turning the ideas floating around in his head into reality.
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