Calculate the right turbo size for your engine based on displacement, target horsepower, boost pressure, and airflow requirements.
Choosing the right turbocharger is critical — too small and it chokes at high RPM; too large and you get nothing but lag. Proper turbo sizing starts with calculating the airflow (in CFM or lb/min) your engine needs to hit a target horsepower, then matching that to a compressor map where the turbo operates efficiently.
Our Turbo Sizing Calculator determines the required airflow, pressure ratio, and approximate compressor inducer size for your engine based on displacement, target power, boost level, and volumetric efficiency. It accounts for intercooler pressure drop, altitude, and intake temperature to give realistic numbers.
Whether you're building a street turbo setup or a race engine, getting the airflow math right before buying a turbo saves thousands in wasted parts and dyno time. It also helps you choose a turbo that will respond well in the RPM range you actually use. That makes the first turbo choice much more practical for a real build.
Use this calculator when you need a reasonable airflow and pressure-ratio target before shopping compressor maps. It is useful for narrowing the field to the right turbo frame instead of choosing something that spools too late, surges, or runs out of flow at the top end. That saves time when comparing a lot of very similar turbo listings.
Airflow (CFM) = (Displacement × RPM × VE) / (2 × 1728) [for 4-stroke]. Corrected Airflow = CFM × (Pressure_Ratio). Pressure Ratio = (Boost + Atm + Losses) / Atm. Target HP Airflow (lb/min) = HP × BSFC / (A/F × 60). Inducer Size ≈ √(CFM_corrected / (π × Mach_limit)).
Result: Required: 39.8 lb/min airflow, Pressure Ratio 2.22. Turbo class: ~62mm inducer (GT35-series range).
A 2.0L engine at 7000 RPM with 95% VE and 18 PSI boost needs about 39.8 lb/min of airflow to make 350 HP. The pressure ratio of 2.22 puts this solidly in GT35/GTX3576R territory with a ~62mm compressor inducer.
A compressor map plots pressure ratio (y-axis) against airflow (x-axis). The "islands" show efficiency zones — aim for 65-75% efficiency for street use. The surge line on the left means the turbo stalls; the choke line on the right means it can't flow more air. Your operating point at peak power AND at cruise should both be on the map.
Small frame (inducer 40-52mm, e.g. GT25/GT28): 150-350 HP, excellent spool, ideal for 1.0-2.0L engines. Medium frame (55-65mm, e.g. GT35/GTX35): 300-550 HP, good response with strong top-end, suits 2.0-3.0L engines. Large frame (67-76mm, e.g. GT40/GTX42): 500-900+ HP, significant lag, best for 3.0L+ engines or dedicated race builds.
Buying too large for the displacement chasing peak numbers leads to no usable power below 4000 RPM. Ignoring the hot side (turbine housing A/R ratio) causes back-pressure issues. Not accounting for intercooler losses underestimates the required pressure ratio. Using naturally aspirated VE numbers instead of turbo-corrected values leads to undersizing.
If the turbo doesn't start making boost until high RPM (3500+), or if you're operating in the far left of its compressor map (surge zone), it's too big. Symptoms include excessive lag and surge under partial throttle.
Pressure ratio is the total output pressure divided by the inlet pressure. At sea level, 14.7 PSI boost means a pressure ratio of (14.7 + 14.7) / 14.7 = 2.0. This is the primary axis on a compressor map.
Brake Specific Fuel Consumption — how much fuel (in lbs) the engine uses per horsepower per hour. Typical values: 0.50-0.55 for gasoline turbo, 0.38-0.42 for diesel. Lower BSFC means more efficient.
Yes. At higher altitudes, atmospheric pressure is lower, so the turbo must work harder (higher pressure ratio) to achieve the same boost. A turbo sized at sea level may not flow enough at 5,000 ft elevation.
Ball bearing turbos spool 15-20% faster, have lower oil requirements, and are more tolerant of oil supply issues. They cost more but significantly reduce turbo lag, especially on larger turbos.
Generally no. Diesel turbos are designed for higher pressure ratios and lower RPM exhaust flow. They surge badly on gas engines. The turbine housing A/R ratio and wheel trim are wrong for gas exhaust characteristics.