Explanation of Turbocharger Maps:
Now that we have covered the equations behind how the engine maps are generated, we are going to look at how to read these maps. Below is an example map of the GT3582R. Important features are indicated and explained below. (Click to enlarge.)
Pressure Lines
The blue pressure lines indicate a certain manifold gauge pressure. These values are referred to as the "boost pressure" of the turbo. Each dot on the pressure line corresponds to a certain engine rpm, starting at 2,000 RPM and going to the engine's rev limit of 8,500 RPM.
RPM Lines
The green RPM lines indicate a certain engine RPM. They serve as guides for interpolating mass flow rates that do not lie perfectly on a pressure line. You'll notice that the RPM lines are not always even spaced. This is due to the fact that I decided to chart 7,800 RPM (where maximum hp is created on the stock engine) and at 8,500 RPM (the rev limit of the engine).
Surge Line
On a compressor map, the "Surge Line" is the left most boundary of a compressor map. When a compressor is in surge conditions, it does not have sufficient flow to maintain the pressure ratio across its inlet and outlet. This condition normally occurs when the throttle body is closed suddenly and pressure backs up in the intake. A BOV is used to prevent this condition from occurring.
The surge line can also indicate how your turbo will spool up. For instance, if your target boost pressure is 16 psi, then you can see that the surge line intersects with the 16.4 psi pressure line at about 5,000 rpm. This indicates that you will achieve your maximum boost pressure around 5,000 rpm. A turbocharger just doesn't suddenly jump to a target pressure however, it will slowly build up over the rpm range. To see when the turbocharger will start creating pressure, we move down the map and to the left to find the first intersection of the surge line and an rpm line. In our example this occurs between the 3.3 psi and 6.6 psi pressure lines and between the 3,0000 RPM and 4,000 RPM lines. This tells us that the turbo will likely start generating pressure at around 4,000 RPM.
Efficiency Islands
Each ring on the compressor map shows the approximate efficiency of the compressor in that area. Compressor efficiency is an advanced subject so I will only give a simple explanation of it here. The higher the compressor efficiency, the less heat is being transfered to the air as it is being compressed. As the heat of the air increases, its density decreases which has a negative effect on power. Eventually it is a matter of diminishing returns. The extra pressure created by the compressor is negated by the heat generated. This is a gross oversimplification, and if you would like to learn more about compressor efficiency see the links posted at the end of this section.
NOTE: Compressor efficiency has not been factored into the spreadsheets and charts used here to make the calculations more manageable.
Efficiency Centerline
The "Efficiency Centerline" shows where the compressor achieves its maximum efficiency for a given pressure ratio.
Choke Line
The rightmost bound of the compressor map is known at the "Choke Line". The choke line indicates where the compressor efficiency drops below a certain point (usually 55%) and the compressor is no longer having a positive impact on horsepower.
Speed Lines
The speed lines on a compressor map show the estimate RPMs of the compressor at a given point.
Links:
These are the two sites that I found the most helpful in my research:
Garret's Turbo 103 (General Overview)
Stealth 316's Site (Very Technical)
Spreadsheets:
Plugging the above equations into Excel lets us calculate Mass flow data for a range of altitudes and Compressor Pressure Ratios. (Click to enlarge.)
Maps:
The data from the spreadsheets can now be plotted and applied to various compressor maps. (Click to enlarge.)
K20Z3 Charts
Mitsubishi Maps
Garrett T3-T4 Maps
Garret GT Maps
