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Selecting The Right Turbo


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Turbo’s - Mapping It Out

What are you spinning

Turbochargers compress air. Everyone knows this. Everyone also knows different turbo’s do it differently.

The way in which a particular turbo or, more specifically , the compressor comprising one end of it , compresses air is shown on a compressor efficiency map. Everyone knows about these too, however not everyone knows how to read them properly. In times past, the most difficult aspect of understanding compressor efficiency maps was actually finding one. Most manufacturers didn’t make them available to the general public because it was felt that they’d do more harm than good in the hands of inexperienced users. Turbonetics always include maps in their catalogue but it was about the only company that did. This is changing and more companies are letting people take a look.

Garrett, for instance, has decided that releasing technical information is a good thing and their website is heavily geared toward helping amateurs to understand what turbocharging is all about. They even have an electronic newsletter called Garrett Gearhead to which anyone can subscribe free of charge. Also with the release of the GT range of turbo’s, Garrett has published compressor and turbine performance specifications that are accurate enough to use for selecting a turbo suited to your needs. This is a most welcome change of policy.

So how is it done

Really, a compressor map is just a graph that’s been drawn to represent experimental data obtained by bench testing a particular turbo under laboratory conditions. On such a graph ( pic 1 ) the X-axis (horizontal) represents airflow through the compressor. The Y-axis (vertical) shows the degree of compression that will be achieved for a given amount of air when the rotating assemble of the turbo in question is turning at a certain speed.

On the experimental test bench, the output of the turbo being tested is restricted until a condition called ‘surge’ sets in. In basic terms, this is when the outlet fills up with compressed air that can’t get away quickly enough. When this occurs, flow through the compressor reverses releasing the excessive pressure. ( It’s actually considerably more complex than this but I wont go into details in this post ). Once vented in this way, flow from the compressor resumes in the normal direction until the outlet once again fill’s, at which time flow reverses yet again.

This process of flow reversal continues at high speed until the conditions causing the restriction are removed. In the test laboratory the restriction is caused by a valve that prevents full flow from the turbo. In service, the condition is caused by selecting and fitting a turbo that’s too big for the engine in question. If the engine can’t take the output of the turbo, surge occurs and, if the condition is continuous, damage to the turbo bearings will occur. The various combinations of speed and airflow at which surge occurs are noted and plotted resulting in the surge line.

In the previous paragraphs I touched on a few things, one of them being the Y-axis, or Pressure Ratio. Given that the number is a ratio, its not surprising that it’s obtained by dividing one value by another. In this case it’s Total Pressure divided by Ambient Pressure. In turn, Total Pressure is Ambient Pressure plus Boost Pressure. Therefore, Pressure Ratio is calculated by:

Pressure Ratio = ( ambient pressure + boost pressure ) ÷ Ambient Pressure

To be entirely accurate, Ambient Pressure is pressure before the inlet to the compressor so any losses through the air filter should be subtracted from it. Additionally, Boost Pressure is the pressure going into the engine, so loses through the intercooler should be added to determine the boost back at the compressor outlet. Including the figures gives the more accurate formula:

Pressure Ratio = ( ambient pressure + intercooler loss + boost pressure ) ÷ ( ambient pressure – air filter loss )

The X-axis represents airflow through the turbo. There are different ways of expressing this but for the most part, the figures along this axis will represent pounds of air per minute. This is certainly the case for Garrett and Turbonetics turbo’s. We’re stuck with this state of affairs because most compressor maps that are available come from the U. S which steadfastly refuses to adopt the metric system. So, using imperial figures, we measure engine air consumption in Cubic Feet Per Minute ( CFM ) and this has to be converted to Pounds Per Minute to be of any use in plotting the suitability of a particular turbo /engine combination. Under the conditions at which most compressor efficiency maps are made, one cubic foot of air weighs close enough to 0.070lbs. Therefore, multiplying your flow requirements ( in CFM ) by 0.070lbs will give flow in lbs/min.One formula for determining the flow of a turbocharged engine is:

(engine displacement x volumetric efficiency x pressure ratio x revs) ÷ 2

Dividing by two is necessary because we’re talking about four-stroke engines here and each cylinder is filled with mixture on only every second revolution. As an example, a 2.0-litre engine that’s 90% efficient, turbocharged at a pressure ratio of 1.8, turning 7000rpm would work out to be:

(2 litres x 0.9 x 1.8 x 7000) ÷ 2 = 11340

To convert litres to cubic feet you multiply by 0.0353 and doing so in this case gives:

11340 litres x 0.0353 = 400 cubic feet per minute

Now that we’ve gone from litres to cubic feet we just have to multiply the result by the weight of the air to end up with lbs/min as shown here:

400 cubic feet per minute x 0.07 = 28 pounds

This figure you’d use in finding a suitable turbo for such an engine. You’ll note that 0.9 was transposed for 90% in the example shown. Doing so simplifies this, and many other equations. Also, 90% is a bad efficiency figure for a modern four valve engine.

Air passing through a turbo takes a decent pounding by the time it’s forced to make a 90° turn across the spinning impeller and then fight its way out through the diffuser. Inevitably the speed at which all this occurs causes it to become heated. Efficiency in a turbo really means the degree to which the air emerging from the outlet has been heated. More heat, less efficiency; less heat; more efficiency.

The strange ‘island’ type lines on a compressor map represents regions of efficiency. The central island always represents the region of highest efficiency and the figure for this area is usually around 70 – 80%. Moving further out into the larger islands represents reduced efficiency and higher temperatures. Obviously, the aim is to find a turbo with operating characteristics that see your maximum flow rate firmly in the middle of the region of highest efficiency.

The other lines on a compressor efficiency map represent the rotational speed of the impeller (and rotating assembly as a whole). From the surge line you can see that the lines stay relatively flat and level for a bit but then starts to take a downward slope. The downward slope indicates that the impeller is reaching a point at which it can’t pass anymore air. The flow rate at which the speed line slopes downward so sharply it can be considered vertical is the maximum flow for the particular turbocharger in question. At this point, it doesn’t matter how fast you turn the impeller, no more air can get through the cover. This condition is called ‘choke’ and, like surge, it’s a condition to be avoided.

When a turbo is operating well into conditions of choke the rotating assembly will spin very fast and the aerodynamic efficiency with which the blades interact with the air will be virtually nonexistent. Not surprisingly, this will lead to very hot outlet temperatures. Garrett points out that a turbocharger operated in this way will suffer durability problems.

Selecting That Turbo

So in selecting a turbo, you want to keep your operating point to the right of the surge line, to the left of the choke lines, and in the region of maximum efficiency. To determine if a particular turbo will be suited to your needs you need to work out the airflow requirements as shown above. Then, you need to find the point on the airflow axis (horizontal X-axis) and take a vertical line from that point straight upwards (line 1 on the map figure 1). Then, you should select the pressure ratio you’ve decided to use from the vertical (Y-axis) and extend a horizontal line (line 2) across the graph at that level until it reaches the line representing your airflow. The point at which these lines meet is your maximum operating point. If it’s in the correct region of operation as described above, your half way there. Because the surge line slopes to the right, its closer to the X-axis at lower flow rates. Turbo’s are usually on substantial boost by the time they hit half maximum revs so this point must also be plotted on the map to ensure that its safely positioned in relation to the surge line. Half revs can be considered to correspond to about half the maximum flow rate for this purpose.

Therefore, locate the point of half maximum flow on the X-axis and extend a vertical line upwards (line 3) until it meets your nominated pressure ratio. If it falls to the right of the surge line things are looking even better. The last thing to do is plot a point at 20% of maximum flow and at a pressure ratio of one, and join this line (line 4). If this last line is also to the right of the surge line, the turbocharger represented by the map could be the one for you. We say that a turbocharger that seems to match your requirements in the way described to this point may be the one for you because it may not be exactly what you need. Using a compressor map is just the first step in finding out what turbo is best for you. After you’ve narrowed the field down using the maps, the next step is to get the turbo, bolt it on, strap the whole assembly onto a engine dyno and begin testing.

Remember that if you get lost reading though this and don’t understand compressor maps properly the best solution is to call a turbo specialist. Most of them have years of experience and will generally know what combinations go well with what engines and can guide you from there.

All info was gathered from various performance car magazine articles as well as Garrett turbochargers website. I am aware that if I’m mistaken in publishing this data I will in due course withdraw the submission.


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