Turbocharging 101 The most common question asked from newbies on turbo forums is likely, What size turbo do I need? And the most common answer to that question is, Use the SEARCH button. Selecting a turbo size to match your engine is not at all difficult. You need to find a few things about your engine, decide how much boost you want to use, and then plot the information against a turbocharger’s compressor map. There is a little bit of math involved, but it’s easy. First you need to know the CFM of your engine when running naturally aspirated. You can find this by using the following formula: volume of air (cfm) = engine rpm x engine cid 3456 So, an LS1 would look like this: volume of air (cfm) = 6000 x 346 = 2076000 = 600.694 34563456 The LS1 needs about 600cfm of air at 6000 RPM naturally aspirated. Now we know our NA cfm requirements, but in order to read a compressor map we’ll need to figure out the airflow in pounds per minute (lb/min) required by our engine under boost. For our example, let’s use a boost pressure of 10psi. At this point it is important to talk about the difference between absolute pressure (psia) and gauge pressure. The boost level that you read on your boost gauge is really called psig or pounds per square inch gauge. The absolute pressure is 14.7 + psig. The 14.7 comes from the pressure of air at sea level. So 10psig = 24.7psia. We can find our engine’s requirements by plugging our numbers into the ideal gas law. The ideal gas law relates volume, pressure, temperature and mass of air. It is: PV = nRT Where P = absolute pressure, V = volume cfm, n relates to mass, R is a constant and T is the air temperature in Rankine. Let’s simplify the ideal gas law to find our engine’s required airflow in lb/min with 10psi boost. We will need to know the temperature of the compressed air coming out of the turbo. Let's assume an intercooled intake air temperature of about 130F. Turbo cars that do not have an intercooler can see intake air temperatures around 250F. To get the temperature in Rankine, simply add 460 to the air temperature in F. n(lb/min) = (14.7 + psig) x V cfm x 29 10.73 x T deg R n(lb/min) = (14.7 + 10) x 600.694 x 29 = 24.7 x 600.694 x 29 = 430277.1122 = 67.96lb/min 10.73 x (130 + 460)R10.73 x 590R6330.7 We find that ideally, our LT1/LS1 will require 67.96lb/min of air under 10psi boost at 6000RPM. I say ideally because that assumes our engine has a volumetric efficiency of 100%. We’ll assume that our engines have a volumetric efficiency of about 85%. Now we can correct our airflow. 67.96 x .85 = 57.77lb/min By the way, as a rule of thumb, horsepower can be found by the following: Hp = airflow lb/min x 10 = 57.77 x 10 = 577.7 Now that we know our required airflow in lb/min, we need to find something called a pressure ratio. This is the ratio between the inlet and outlet pressure of the turbo’s compressor. Inlet pressure is usually 14.7psi. (standard barometric pressure at sea level) The outlet pressure is 14.7psi + boost pressure. Take the ratio of the two and you get: Pressure Ratio = 14.7 + boost 14.7 We decided to run our project at 10psig. That gives us: Pressure Ratio = 14.7 + 10 = 24.7 = 1.68 14.714.7 We now have all the of the information that we need to read a compressor map. Let’s take a glance at a few and see what compressor will work best with our application. The maps show both efficiency and RPM curves for the compressor. In order to read the maps, simply find the pressure ratio on the y-axis and follow the map over to where the airflow meets the engine airflow. The map above is for a Garret T04E compressor in 60 trim (we’ll talk about trim later). Notice the islands on the graph that look like ripples on water. Those are efficiency islands. It is desirable to have our point plotted right in the middle of the graph, although anything down to about 60% efficiency will work. Notice that our point is way off the map for this turbo. The turbo will still make boost, but will not be efficient. This is a bad selection for our project. Our point is off the map to the right. What if our point fell in the surge limit area to the left? That would be very bad. In fact, it would surely cause damage to the compressor. In this area, the air flow is unpredictable and changes direction. Let’s look at another turbo compressor map. This is a map from a T76 compressor. Notice that the point lands right in the 70% efficiency island. This would be a perfect turbo for our LT1/LS1 project. But before you rush out and buy one of these, there are other things to consider. We’ve talked a lot about the compressor side of the turbo, now let’s turn to the turbine side. We’ve shown that a T76 will work for our project, but will spool? That depends on the area ratio (A/R) of the turbine housing, and the turbine size. A larger turbine wheel will spool slower than a small one. A larger turbine will make more power, but it will come on later, maybe when the race is over. A/R The other consideration is the A/R. It determines when the turbine starts to spool. The turbine housing A/R is the cross sectional area of the turbine housing divided by the distance from the center of that cross section to the center of the wheel. This makes sense if you look at the graphic. If you take for example, the area in A1 and divide it by R1, you will have found the A/R for this turbine housing. Each cross section and radius have the same proportions so the A/R will be found by using any cross section/radius. Common turbine housing A/R’s are .58, .69, .81, .96 and 1.00. The turbine will start to spool sooner with a .58 A/R, and later with a 1.00 A/R. Lag will be a problem if the A/R is too large, but if it’s too small, the turbo will run out of steam and be nothing more than a restriction. Trim Trim for a compressor or turbine wheel is the squared ratio of the smaller diameter divided by the larger diameter, multiplied by 100. Turbocharger wheels have a large and small diameter. On the compressor, the small diameter is the inducer and the large diameter is the exducer. This is the reverse for the turbine wheel. Compressor trim = inducer2 x 100 exducer2 Turbine trim = exducer2 x 100 inducer2    For compressor wheels, given a constant exducer size, the larger the trim, the better the wheel flows. It also means the wheel has slightly lower efficiency. For turbine wheels, given a constant inducer, a larger trim means the turbine has better flow with less backpressure. It also means that less energy is recovered from the engine’s exhaust slowing spool time. Hopefully this gives some idea how to select a turbo for your project. Also remember, you can always find answers to most questions in the forums.