There are a number of factors, such as turbo lag, boost threshold, heat, back-pressure, low-end torque, and top-end power, that you must take into account when selecting a turbo. A large turbo will suffer from turbo lag and won't produce much low-end torque but it also won't put too much heat to the intake charge, won't have much back-pressure, and will produce loads of top-end power. A small turbo, on the other hand, won't have much turbo lag and will produce loads of low-end torque but will also have lots of back-pressure and will add lots of heat to the intake charge. You can't have the best of both worlds but you can select the best turbo to suit your needs.
Deciding which turbocharger best suits your needs in a bit complicated. You need to know what your objectives are — street car, a purpose built ¼ miler, a race car, or a rally sprint car. Once you know what you want, you should have a better idea of at what rev range you want your power band to be. Once you know that, then it becomes easier as you can select a compressor-wheel to match your rev range.
Selecting the Compressor Wheel
The compressor-wheel is most efficient at a particular boost pressure or pressure ratio (PR) and air flow (cfm). At this point the turbo will put the least amount of heat into the intake charge; anywhere else, including at lower boost pressures or revs, it will put more heat into the intake charge. The idea id that the point of efficiency should coincide with your most useful rev range. So it's a matter of determining the bore diameter of the compressor wheel that is most efficient at your most useful rev range; and by most efficient, I mean at least 60% efficient. Each compressor-wheel has a compressor map that maps efficiency at various pressure ratios and air flow rates but you need to calculate the air flow rate for your engine. You can use the following formula to calculate the air flow rate:
PR × CC × ½RPM × VE
In this formula, PR is the Pressure Ratio. This is the absolute pressure produced by the turbo divided by atmospheric pressure. Atmospheric pressure is 14.7 psi at sea level. If you're running 7 psi of boost, your absolute boost pressure is 21,7 psi (7 psi + atmospheric pressure). This will give you a PR of 1,47 (21,7 ÷ 14,7), which means that approximately 47% more air/fuel mixture is being forced into each cylinder.
We halve the RPM because a four stroke internal combustion engine requires two revolutions to complete one power cycle
CC is engine capacity but in cubic feet and not in cubic inches. Why cubic feet? Because cfm is cubic feet per minute. You can convert engine capacity to cubic feet by dividing cubic inches by 1728.
VE is volumetric efficiency. This is the total amount of air/fuel mixture that each cylinder ingests during the intake stroke and is expressed as a percentage of the actual volume of the cylinder. You can calculate the VE as follows:
2 × mass airflow rate
air density × swept volume × RPM
Yes, I know, it's getting a bit complicated! Fortunately we can use a rule of thumb that states that modern engines have a VE of 80-90% while older engines like the Datsun L-series engine have a VE of 60-70%!
Selecting the Turbine Wheel
The turbine-wheel uses exhaust gas energy to spin the compressor-wheel fast enough to produce the required air flow rates at the desired boost pressure. A larger turbine-wheel will produce more power to spin the compressor-wheel at the required air flow rates, although s smaller turbine-wheel will spin faster. A smaller turbine-wheel will also offer greater restriction to the exhaust gas flow, causing back pressure between the turbine-wheel and the combustion chamber. So the basic size of the turbine wheel will be determined by the air flow required from the compressor-wheel. The important element here is the extruder bore size, i.e., the inner diameter of the turbine outlet. An extruder bore with a 2 inch diameter will be sufficient for a compressor-wheel air flow of 250 cfm to 400 cfm; an extruder bore with a 2½ inch diameter will be sufficient for a compressor-wheel air flow of 400 cfm to 500 cfm; an extruder bore with a 2¾ inch diameter will be sufficient for a compressor-wheel air flow of 500 cfm to 600 cfm; an extruder bore with a 2⅞ inch diameter will be sufficient for a compressor-wheel air flow of 600 cfm to 800 cfm; and an extruder bore with a 3 inch diameter will be sufficient for a compressor-wheel air flow of over 700 cfm.
Considering the A/R Ratio
The A/R ratio is another important consideration in choosing the turbine-wheel. The A/R ratio is the ratio between the cross-sectional area (A) of the turbine scroll at any one point and the distance or radius (R) from that point to the center of the turbine-wheel. This ratio is always constant so each point along the turbine scroll will have the same A/R ratio. A turbo with a smaller A/R ratio will tend to create more torque while a turbo with a larger A/R ratio will provide more power because more exhaust gas energy will be acting on the turbine-wheel. Generally, an A/R ratio of 0.7 will provide better low-end response, while an A/R ratio of 1.4 will provide more top-end power.
Other important factors that you should take into account when selecting the turbo include cooling and the location of the wastegate. As I've mentioned in our turbo lubrication section, a turbo with a water cooled bearing section will have a longer lifespan and will be more reliable because it solves a few major lubrication issues. We discuss wastegates and blow-off valves in our turbo boost control section but the short of it all is that a turbo with a remote wastegate produces more power, but a turbo with an integrated wastegate is much cheaper.