Reading a Turbo Compressor Map

Reading a turbo compressor map can be very challenging, but they can also be a bit of fun if you know what you're looking at. When you want to build the ultimate street machine you need to know some things. The most important things to know about your turbo is keeping it in its efficiency range, knowing how much boost it can push or how much your motor can hold (you should already have an idea on that, and how many pounds of air you are going to be generating at the boost level you have indicated.

First off when reading a compressor map, what do the numbers on the left side of the map or graph and the numbers on the bottom mean. Well first off you have to think about this compressor map as a graph that you are going to chart with a X axis and a Y axis, so effectively they interpolate together to give you the figures you're looking for.

The X axis will be the numbers to the left going up on the compressor map, normally 1.00 to 3.20 or higher. All this is used for is to map out how much boost you will be running to chart your turbo. Basically, you take the standard figure of atmosphere (14.7) and you add on how many pounds of boost you want to run and divide by 14.7. So for a compressor chart X axis it would go like this for 25 pounds of boost. Twenty-five pounds of boost yields you 2.70; so on your compressor chart you are going to make a line that runs from left to right at 2.7, then you go onto the next step of reading a compressor map. By the way this is known as the pressure ratio.

Reading the numbers on the compressor map on the bottom of the graph going from left to right represent your Y axis; basically this is how many pounds per minute of air your turbo is going to flow. Basically each pound of air is about ten flywheel horsepower, so basically if you wanted to know the specific turbo efficiency you would stop at say 30= 300/ 40=400 horsepower and interpolate the X and Y axis.

Now does your point on the compressor map fall within the turbo's acceptable efficiency range? If not you need to step it up to a bigger compressor wheel and a correlating turbine size for the displacement of the engine you're running.

The main goal of a compressor map is to try and make the most horsepower with the lowest amount of boost while still keeping turbo lag in mind and spool time under your thumb; but the bigger you go the slower the spool, but in turn the more power you make.

The last thing to know on a compressor chart is that the hashed line that runs diagonal is the surge line; basically when reading the chart if you graph and it ends up being to the left of this line then you need to step up your compressor size. Also the hundreds of thousands number on the compressor map is how many revolutions per minute the compressor is spinning; you don't want a high number on your compressor map here.

Source: http://www.turbomagazine.com/tech/0304_turp_compressor_map/photo_02.html