This turbo size calculator gives the numbers you need to analyse turbo compressor maps so you can find a good turbo for your engine.
This calculator needs your engine capacity & your desired boost pressure in PSI at your desired RPM.
A worked example using this calculator can be found here for a BMW M4 turbo upgrade (S55)
Turbo Size Calculator – Using the numbers
When we obtain the pressure ratio and CFM and lbs/min numbers we can now look at turbo compressor maps to see which turbo will be most suitable for our application and goals.
Plotting The Line
Bolt the pressure ratio number and the CFM (or lbs/min) number into a compressor map graph for various RPMs. Plot multiple points to see where the turbo performs versus your requirements. We are looking for the line to be in the highest efficiency areas of the map for as long as possible. Generally speaking erring on the larger size of turbo is better than smaller.
Choose the RPM where you want to achieve full boost.
Plot the CFM number onto the compressor map graph.
Then draw a line from this point down to Pressure Ratio of 1 and a CFM of 20% of the maximum.
This line absolutely lie to the right of the surge line on the compressor map.
A steep surge line on a compressor map shows that a turbo will produce high boost pressure at low CFM flow rates.
A shallow surge line on a compressor maps shows that a turbo will not produce high boost at low CFM flow rates.
This is usually a trial and error process. Too big an AR ratio will feel sluggish, too small engine can be difficult to drive smoothly.
More Turbo Selection Info
Garrett has an excellent resource on their website detailing the technical aspects of a modern turbocharger
Generally turbos supplied by the car manufacturer are hopelessly undersized. On a test drive they may feel fast but when you get used to the low down torque, exploring the upper reaches of the rpm range will reveal there is nothing less to give, the engine is being choked due to the small turbo. By fitting a larger turbo we will lose some low down torque but we will get it back at higher RPM, which give us more power, without necessarily increasing stress on the engine. A win-win.
Turbo Suppliers & Manufacturers
Some other calculators you can find on StrikeEngine