Turbo Size Calculator

A turbocharger is a forced induction device that uses exhaust gases to spin a turbine. That turbine drives a compressor which forces more air into the engine's intake. More air allows more fuel to be burned, significantly increasing power output without increasing engine displacement.

The right turbo size depends on your target horsepower, engine displacement, peak RPM, and desired boost level. A larger compressor wheel moves more airflow but builds boost later (turbo lag), while a smaller wheel spools faster but limits peak power. This calculator balances those factors to recommend a suitable inducer diameter.

Turbo sizing starts with calculating the required mass airflow (lb/min) to support your target horsepower using the air-fuel ratio and brake specific fuel consumption (BSFC). That airflow figure is then used to calculate pressure ratio and, ultimately, the compressor inducer diameter. Our calculator handles all of this automatically.

Most naturally aspirated petrol and diesel engines can be turbocharged, but the engine must be mechanically sound and able to handle the increased cylinder pressures. High-compression engines may need a lower compression ratio or higher octane fuel when forced induction is added. Always consult a professional tuner before proceeding.

Pressure ratio is the ratio of the absolute pressure at the compressor outlet to the absolute pressure at the inlet. It determines how much the turbo compresses the incoming air. A higher pressure ratio means more boost, but it also generates more heat and requires the compressor to work harder, making efficiency and intercooling more critical.

Volumetric efficiency (VE) describes how effectively an engine fills its cylinders with air relative to its theoretical capacity. A stock engine typically has a VE of around 85%, while a built engine with performance camshafts and ported heads can reach 95–105%. Use a higher VE value if your engine has significant breathing modifications.

The factory turbocharger on the 5.9L Cummins (12-valve and 24-valve) is typically a Holset HX35 or HX40, with inducer diameters ranging from roughly 60–67 mm depending on the model year and application. These are capable of supporting approximately 400–500 hp in stock or mildly modified form.

Yes — this is called a twin-charged setup. A supercharger provides immediate low-RPM boost while the turbocharger takes over at higher RPMs, virtually eliminating turbo lag. Twin-charged setups are complex and expensive to engineer but can produce exceptional power across the entire rev range. Proper tuning is essential.