A little while ago, I was involved in a discussion about surging with the TA-49 turbo at low-boost, part throttle conditions. Some of us are having problems with compressor surge at around 8-10 psi (pressure ratios in the 1.5 to 1.7 range) or so. Several ideas to fix this problem were discussed. Two of them were using a ported shroud and using a "Tornado" in the inlet pipe. Both were reported as being effective at stopping the surging, but I was wondering, what effects do these devices have on the efficiency of the turbo? Lo-and-behold, an article appeared in the April 3, 2012 edition of "Automotive Engineering" magazine, publised by the Society of Automotive Engineers (yes, I am a member). The title of the article is "The Quest for Better Turbocharger Compressors". How convenient. The gist of the article is that modern turbocharger designers face a problem - modern engines (especially ones that use a high amount of EGR to improve emissions and fuel economy) require a turbocharger with a very wide compressor operating map. For those who understand compressor maps, Figure 1 below shows it graphically. Under moderate acceleration, the turbo is required to supply a high pressure ratio with low mass flow (kind of like our TA-49's at about 9psi of boost, right?). This point is "off the map", and surging of the compressor (the infamous "sneezing") will occur. I included Figure 2 just because is shows a nice cutaway of a modern turbo compressor. Notice that it has a ported shroud, which is called "case treatment" or "CT" in the remainder of the article. Now here is an important point - the article shows how "CT" (what we know as a ported shroud) affects the compressor map. Figure 3 shows, for a given test turbo compressor, the width of the compressor map at given boost pressures with and without "CT". The "range" is the difference between the maximum air flow and minimum air flow (lowest air flow before compressor surge occurs) divided by the maximum airflow. A higher number means that the turbo has a wider compressor map, as in more range of air flows that it can handle. The "Pressure Ratio" is just that - the boost pressure plus atmospheric pressure divided by atmospheric pressure. What this graph shows is that regardless of the pressure ratio, adding "CT" (ported shroud) to the turbo compressor results in a wider compressor map. Now for the most important point of the article. The question remains - how do "CT" (ported shrouds) and "Tornados" affect the turbo compressor's performance? The answers are in Figure 4 below. Figure 4 is essentially a turbo compressor map, but with the efficiency "island" information placed on a seperate graph. First, an explaination. "VIGV" means "Variable Inlet Guide Vanes". Imagine a "Tornado" device placed just in front of the turbo compressor, and the angle of the "Tornado's" blades can be changed on-the-fly via commands from the ECU. In simple terms, that's what VIGV's are - they are devices the "pre-swirl" the air entering the turbocharger. The primary difference between VIGV's and the "Tornado" is that the angle of the blades in the "Tornado" are fixed, while the angle of the blades in a VIGV device can be changed. If you look at the bottom graph in Figure 4 and compare the black line (a conventional turbo compressor) to the purple line (add "CT", or ported shroud), you can see that the "CT" moves the surge line significantly to the left. This means that the compressor is less likely to surge when operated under low air flow conditions. You can also see that away from the surge line, the "CT" line and conventional line sit pretty much on top of each other, which implies that the "CT" (ported shroud) does not significantly alter the air flow performance of the compressor away from the surge line. Now, looking at the yellow, orange, and red lines, these represent the case where VIGV's are added to the system in addition to the "CT". The yellow line can be thought of adding a "Tornado" with the blades angled at 60-degrees, the orange with blades at 70-degrees, and the red with blades at 80-degrees. Increasing the incoming swirl of the air moves the surge line even farther to the left, allowing the compressor to operate with even lower air flow before the compressor surges. However, as air flow increases, you can see that the VIGV's (our "Tornados") really kill the air flow performance of the compressor. The maximum possible air flow is only about 55 to 65% (depending on blade angle) of the conventional compressor. This makes sense, since the VIGV's act as large restrictions at higher air flow. If you look at the top graph in Figure 4 and compare the black line (conventional compressor) to the purple line ("CT", or ported shroud, added), there is very little difference. This shows that adding a ported shroud has minimal effect on compressor efficiency. In practical terms, this means that the amount of turbine power needed to drive the compressor does not change significantly by adding a ported shroud to the compressor. If you compare the black lines to the orange lines (VIGV's, or a "Tornado", with the blades set to 70-degrees), you can see that the compressor efficiency at a given air flow is much lower with the VIGV's present. In practical terms, this means that a lot more turbine power will be needed to drive the compressor, which means higher exhaust back pressure, which means less engine performance, particularly at high air flows (i.e. full throttle). So, in conclusion, the data show: 1. Adding "CT" (ported shroud) to the compressor allows it to run at lower air flow rates without surging across the entire operating range. The effect of the "CT" on the maximum possible air flow and efficiency of the compressor is very small. 2. Adding a fixed set of inlet guide vanes (such as a "Tornado") allows the compressor to run at much lower air flow rates without surging across the entire operating range. However, the fixed inlest guide vanes ("Tornado") will significantly reduce the maximum air flow that the compressor will achieve, and will significantly reduce the compressor efficiency, particularly at high air flow levels (i.e. full throttle). One caveat: These data are for one particular turbo compressor of unknown type - the results might be different for other turbo compressors. However, I would tend to believe that the general trends would be the same. I thought that some of the folks on this board might be interested in this - hopefully somebody learned something interesting. Suffice to say, after looking at these data, I will never install a "Tornado" into my inlet pipe.