Turbo Calculator

The Turbo Calculator sizes forced induction by answering two questions and then putting them together: how much air must the compressor move, and at what pressure. Those two numbers are the axes of a compressor map, and the third tab plots where your engine lands on it. Open it from Calculators › Turbo Calculator.

Figure 1

Tab 1 — Mass Flow

This works out the air mass the engine demands, by either of two routes (pick a computation mode):

From target HP — enter the target horsepower, the engine’s BSFC (brake-specific fuel consumption, around 0.55 lb/HP·hr for a boosted petrol engine) and the target AFR. Use this when you know the power you want before you know the engine details.
mass flow [lb/min] = HP × BSFC × AFR ÷ 60
From engine + RPM — enter displacement (litres or ci), volumetric efficiency, RPM and the manifold air density. Use this when you know the engine and want the air it pumps.
mass flow [kg/s] = Vd × (VE ÷ 100) × (RPM ÷ 120) × ρ (the RPM÷120 is the four-stroke “one intake every two revolutions”).

Either way the result is shown in lb/min, kg/s and as a volumetric cfm at ambient. This mass flow is the horizontal axis of a compressor map.

Tab 2 — Pressure & Temp

Boost is not free: squeezing air heats it, and hot air is thin again. This tab turns a boost target into the real pressure and temperature in the manifold. Enter the atmospheric pressure, the target boost, the intercooler ΔP (pressure lost across the cooler), the intercooler effectiveness (default 70 %), the compressor efficiency (default 70 %) and the ambient temperature. It then computes:

Pressure ratio (PR) — PR = (P_atm + P_boost) ÷ P_atm. This is the vertical axis of a compressor map; PR = 2.0 means the compressor is doubling the pressure.
Compressor outlet temp — T_out = T_amb × [1 + (PR^0.283 − 1) ÷ η_c]. A less efficient compressor (η_c) makes a hotter charge for the same pressure.
Manifold (post-intercooler) temp — the intercooler claws back part of that heat: T_man = T_out − effectiveness × (T_out − T_amb).
Manifold absolute pressure and density, and the density ratio versus ambient — the honest measure of how much denser the charge really is once heat and the cooler’s pressure drop are accounted for.

Tab 3 — Operating Point

This brings the two halves together: the mass flow (from tab 1) and the pressure ratio (from tab 2) are exactly the coordinates of a point on a compressor map, so you can drop them onto the manufacturer’s map and see whether you are in its efficient zone or off the edge. It also reports the density ratio, the implied naturally-aspirated mass flow, an estimated turbine PR (about 1.5× the compressor PR, a Garrett rule of thumb) and an estimated exhaust mass flow. Refresh recomputes from the other tabs.

Three buttons hand the result to the other calculators:

Send → Air Flow — the mass flow (converted to g/s) to Air Flow Estimation.
Send → Fuel — the target AFR to Fuel Estimation.
Send → Cam — the mass flow (converted to CFM) to Cam & Valvetrain.

Tip — set the power or engine on tab 1 and the conditions on tab 2, read the point on tab 3, then match it against a real compressor map. From there, push the mass flow to Air Flow, the AFR to Fuel and the CFM to Cam so the rest of your numbers start from the same operating point.

Note — compressor efficiency and intercooler effectiveness both default to 70 %. Lowering either makes the charge hotter and thinner for the same boost, which is why a good intercooler and a well-matched compressor are worth real power.