How to Go Fast Faster: The Math Behind Turbocharging. Part 5: Reading Compressor Maps


Part 1 is here.

Part 2 is here.

Part 3 is here.

Part 3b is here.

Part 4 is here.

Part 4b is here.

     Now this part gets complicated.  For this step we’re going to try to find a suitable compressor for our specific application.  Every turbo is different, as is every engine, and we should try our best to find the best match for both our goals and our car to end up with the most efficient turbo system we can achieve.  The problem is that the best suited compressor isn’t only a mathematical equation away, but we have to take a look at a number of them to find it.

     We have the two ingredients that we need to “choose” our compressor; the Pressure Ratio and Turbo Airflow Rate (cfm).   Just to reiterate, our Pressure Ratio is basically how much we want to increase our engine performance (if we want a 50% increase, our Pratio would be 1.50), and our Turbo Airflow Rate is how much air is flowing through our engine after we bolt on our turbo.  The Turbo Airflow Rate is bascially our normal engine Airflow Rate multiplied by the Pressure Ratio.  Keep in mind what kind of power you want; alot of torque down-low, a peaky high-end monster, or something in-between for an all-arounder.

     Keep in mind the two variable as we take a look at a compressor map and try to make sense of it all:


 ”Fig. 3-8.” Chart. Maximum Boost, Designing, Testing and Installing Turbocharger
     Systems. By Corky Bell. Cambridge, MA: Bentley Publishers, 1997. 30.

     This can basically be summed up as a graph of sorts.  Your x-axis is the Turbo Airflow Rate and the y-axis is the Pressure Ratio.   Now when you look at the shape, it looks  like a fingerprint.  And that’s what it basically is; the fingerprint of the compressor.  We can figure out everything we need about this compressor just by looking at this map.  And what we’re looking at are Compressor Efficiency (%) plateaus.  If you look near the bottom right hand corner, you can see percentages representing an “island” or “plateau” while the numbers in the 10,000s represent the speed(s) at which the compressor turns.  The closer you are to the middle, the higher the Compressor Efficiency is.  Our basic goal is to find a compressor with which the plotted x-axis (Turbo Airflow Rate) and y-axis (Pressure Ratio) point lands on the highest efficiency plateau.  Simple in concept, not so simple in reality.

     First we should plot a point at about 20% of our Turbo Airflow Rate and draw a line diagonally down and left with a slope of about 1.  Make sure that this line lies to the RIGHT of the “surge limit”.  Although some compressor maps might not label this line, it is usually the leftmost line of the map.  And if the line does cross it, the compressor must not be used, as there will not be enough pressure for the compressor to work normally.

     After that, we can plot a line horizontally on the x-axis along your Pressure Ratio.  Then find the cfm of your Turbo Airflow Rate and plot a line vertically up to meet the Pressure Ratio.  This is the Compressor Efficiency the compressor will be running at at the rpm you’ve calculated for.  Ultimately, you’ll want the horizontal line to stay in the highest Compressor Efficiency plateaus as much as possible.  A compressor with a high Compressor Efficiency all the way from the bottom of the engine load range all the way to the top will give you power all around.  But if you want your torque to kick in at the beginning of the RPM range, then look for a compressor that has its highest efficiency plateau closer to the left due to the lower Airflow Rate of lower Engine Speeds.  If you want alot of power up top, then look for the highest efficiency plateau to be closer to the right side, towards the higher end of your Pressure Ratio line.  Basically, you want the highest efficiency plateau to be where you want your power, while not sacrificing the rest of the rev range just for the higher maximum efficiency.

     Taking a look at the compressor map above, you can see it runs basically between cfms of about 150 and 750, with Pressure Ratios ranging from about 1.20 to 3.00.  The highest efficiency for this compressor would be at 74% right in the middle, ranging from a Pressure Ratio of 1.40 at 300 cfm to 2.15 at 500 cfm.  If we take a look at another compressor  map:


 ”Fig. 3-7.” Chart. Maximum Boost, Designing, Testing and Installing Turbocharger
     Systems. By Corky Bell. Cambridge, MA: Bentley Publishers, 1997. 30.

     there are some clear differences.  This compressor map is “wider”, emcompassing more cfms, but not supporting as many Pressure Ratios.  But if you take a closer look, the first few plateaus are still about the same size.  It’s only the lowest plateau that makes this compressor map look so large.  For this specific compressor, it looks like the highest efficiency is 76% while working at a Pressure Ratio of about 1.50 at 350 cfm to a Pressure Ratio of 2.00 at 550 cfm.  This compressor pushes a bit less power than the previous compressor but works at a higher efficiency.  The lowest efficiency plateau here is 65%, while the previous one was a 60%.

     Now if you’re looking for more headroom in your compressor, then you should factor that into your decision too.  Plug in a higher Pressure Ratio and see if a higher power’d engine would work well with the specific compressor.  If not, keep on looking.  Each compressor map is different, so you’ll have to look for the “best” one for what you need.  A “taller” fingerprint that supports many Pressure Ratios may work for you if you want alot of headroom to upgrade and increase your power.  Or a “wider” fingerprint that supports a wider range of cfms might work if youd like to change between a low-rpm boost for the street, and a higher-rpm boost for the track.  There is no “right” answer here, so do some research and ask around.  Actually, do ALOT of research, and ask ALOT of people for their opinions and recommendations.

     The next part will be an article on Compressor Efficiency: detailing and explaining how heat adversely affects our car.


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August 2009
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© 2009 Rusi Li

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