Turbo Size Calculator

Your details

Choose whether to enter engine displacement in cc or cubic inches.
Total swept volume of all cylinders in cubic centimeters.
cc
Desired crank horsepower output at peak power.
hp
Gauge boost pressure (psi above atmospheric) at peak power RPM.
psi
Engine speed at which peak horsepower is made.
RPM
How efficiently the engine fills its cylinders. Typical: 80% for 2-valve pushrod, 85-90% for 4-valve, 95-105% for variable valve timing with tuning.
%
Intercoolers lower charge temperature, increasing air density and HP capacity. Air-to-water units are more efficient but costlier.
Fuel type affects air-fuel ratio and effective HP per lbs/min of airflow. E85 and methanol need more fuel mass but run cooler.
Elevation above sea level. Higher altitude lowers atmospheric pressure, reducing charge density and requiring more boost to compensate.
ft
Mass flowModerate boost
25.08lbs/min

Required compressor mass airflow - use this to read a compressor map

Pressure ratio1.95
Turbocharged CFM364CFM
Mass flow (SI)0.19kg/s
Density ratio1.863
Est. HP capacity251hp
Compressor inducer46.9mm
Boost needed for target HP29psi
NA airflow (unadjusted)230CFM
1.95 :1
Mild boost<1.5Moderate boost1.5-2High boost2-2.8Extreme boost2.8+

25.08 lbs/min at 1.95:1 pressure ratio

  • Your compressor must deliver approximately 25.08 lbs/min at a pressure ratio of 1.95:1. Plot these two numbers on the manufacturer compressor map to confirm you are inside the efficiency island.
  • Airflow supports only 251 hp, below your 400 hp target. Increase boost or displacement, or verify your VE figure.
  • Target a compressor with an inducer diameter near 47 mm. Common Garrett frames in this range include the G25, G30, GTX30, and GTX35 series depending on flow.
  • To hit your HP target with this displacement and VE, you likely need around 29.0 psi of boost, which is higher than the 14 psi entered. Increase boost, raise VE, or accept a lower peak power figure.

Next stepCross-reference your lbs/min and pressure ratio on the compressor map for your shortlisted turbo model. Look for a point in or near the highest efficiency island and well to the right of the surge line.

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