Turbo Size Calculator

How turbo sizing works

A compressor is chosen by two numbers: how much air it has to move, as a mass flow in pounds per minute, and how hard it has to squeeze that air, the compressor pressure ratio. Those two coordinates locate the engine's operating point on a compressor map, and a turbo is a reasonable match when that point sits well inside the map.

The airflow comes straight from the power target. Every horsepower burns a certain weight of fuel per hour, the brake specific fuel consumption (BSFC), and every pound of fuel needs the air-fuel ratio's worth of air. The manifold pressure comes from the engine: at a given rpm it draws a fixed volume of air per minute, its displacement times half the rpm times its volumetric efficiency, so the only way to push more mass through that volume is to raise its density, with pressure, or by cooling it. The pressure ratio then adds the losses: the compressor has to make up for the pressure lost in the intercooler and pipes, and it starts from slightly below atmospheric because the air filter costs a little.

This is the method turbocharger manufacturers publish in their own sizing guides, and the calculator follows it step by step. Like any turbo calculator, it gives a first estimate of where to look; the map of the compressor you are considering has the final word.

What to enter and where to find it

• Target power. Crank (flywheel) horsepower at the rpm you enter. Dyno sheets often show wheel power; convert it with the whp to hp calculator.
• Displacement and engine speed. The rpm where you want the target, normally peak power. For a second look at the low end, repeat the calculation at the torque peak.
• Volumetric efficiency. Turbocharger literature puts peak VE at 95–99% for modern four-valve heads and 88–95% for two-valve designs; its examples at peak power use 92% for a four-valve engine and 85% for a pushrod V8. VE peaks at the torque peak and falls away above it.
• Intake manifold temperature. Typically 100–130 °F with an intercooler and 175–300 °F without one. Hotter air is less dense, so it takes more pressure to pass the same mass.
• Why a lower BSFC than for fuel-system sizing. The fuel injector calculator defaults to 0.65 for boosted gasoline engines, inside a sizing range of 0.55–0.75. Both figures lean toward the error that does no harm. Injectors and a pump that are slightly too big cost almost nothing, while ones that run out of flow lean the mixture, so fuel sizing assumes plenty of fuel. A compressor that is too big costs response and moves the low-rpm operating points toward surge, so airflow sizing uses the turbocharger maker's own, lower figure.
• Atmospheric pressure. 14.7 psi at sea level on a standard day. Use the elevation table below for where you drive.
• Air temperature at the compressor inlet. The outside air the intake draws in, 59 °F on a standard day at sea level. It does not change the airflow the engine needs; it only enters the corrected airflow used to read a compressor map.
• Inlet depression and pressure drop. About 1 psi through the filter and intake, and 1 psi or less up to 4 psi or more from the compressor to the manifold, depending on the intercooler, pipes and throttle; 2 psi is a common starting value.
• Air-fuel ratio and BSFC. The manufacturer's example uses an air-fuel ratio of 12 for a boosted gasoline engine, richer than the stoichiometric 14.7. For BSFC the turbocharger maker's guide gives 0.50 to 0.60 lb/hp·hr and higher for turbocharged gasoline engines, notes that the low end takes race fuel and an aggressive calibration, and uses 0.55 in its example; that is the calculator's default. AFR and BSFC belong to the same fuel: switching to E85 or methanol lowers the AFR and raises the BSFC by about the same factor, so the air per horsepower barely changes. Change both together, or the airflow comes out wrong. To convert a lambda target or work with another fuel, use the air fuel ratio calculator.

Boost calculator: what a boost level supports

The Boost calculator tab runs the method backward. Start from the boost you plan to run and it gives the airflow the engine passes at that rpm and the power that airflow can support. Used as a boost pressure calculator it answers the question owners ask most often: how much is a given boost level worth on this engine.

Boost is a gauge pressure, measured above atmospheric, so the same gauge reading means less manifold pressure, less air and less power at altitude, where the atmosphere starts lower. The turbo also has to work harder there to reach it: the compressor pressure ratio is what rises. That is why the calculator asks for atmospheric pressure instead of assuming sea level.

Turbo horsepower calculator

Compressor maps are drawn in airflow, not horsepower. The Turbo horsepower tab converts one into the other: power = airflow × 60 ÷ (air-fuel ratio × BSFC). Turbocharger makers quote a rough rule of 9.5 to 10.5 flywheel hp per lb/min for turbocharged gasoline engines, which corresponds to a BSFC near the bottom of their range; at the calculator's default values the same pound of air is worth 9.1 hp, a slightly conservative figure.

Some charts and older turbo calculators use cubic feet per minute instead. At standard conditions, 59 °F and 14.696 psi, air weighs 0.0765 lb per cubic foot, so each lb/min is about 13.1 cfm. Charts that use 0.075 lb/ft³ standard air give about 13.3 cfm per lb/min; the difference is about 2%. The calculator shows both mass flow and cfm.

The turbo sizing formulas

The calculator uses the relationships of the turbocharger manufacturers' sizing method. The airflow and manifold pressure follow from the definition of BSFC and the ideal gas law, so the uncertainty sits in the estimated inputs: BSFC, volumetric efficiency and temperature.

1. Airflow (lb/min) = target hp × air-fuel ratio × BSFC ÷ 60
2. Manifold pressure (psia) = airflow × R × (intake temperature °F + 459.67) ÷ (VE × rpm ÷ 2 × displacement in ci)
3. R = gas constant of dry air in inch units = 640.2 in·lbf per lb·°R (53.35 ft·lbf per lb·°R)
4. Boost (psi) = manifold pressure − atmospheric pressure
5. Compressor pressure ratio = (manifold pressure + pressure drop) ÷ (atmospheric pressure − inlet depression)
6. Corrected airflow (lb/min) = airflow × √((inlet air temperature °F + 459.67) ÷ 545) ÷ ((atmospheric pressure − inlet depression) ÷ 13.95)
7. Power from airflow (hp) = airflow × 60 ÷ (air-fuel ratio × BSFC)

The rpm is halved because a four-stroke engine fills each cylinder once every two turns. Sizing guides round the Rankine offset to a whole number and the gas constant down slightly; the calculator uses the exact values, which moves the answer by less than a tenth of a percent. All pressures in the ratio are absolute: a boost gauge reads relative to the atmosphere, so add the atmospheric pressure before dividing. The corrected airflow is explained in the section on reading a map below.

Worked example

Three typical questions, worked out with the same functions that run the calculator.

Atmospheric pressure, altitude and pressure ratio

Atmospheric pressure falls with elevation, and with it the starting point of the compressor. The table uses the U.S. Standard Atmosphere for a standard day and shows what it takes to keep the manifold pressure that 15 psi of boost gives at sea level: more boost on the gauge and a higher compressor pressure ratio, with 1 psi of inlet depression and a 2 psi drop to the manifold.

The last column takes the 44 lb/min of the worked example, the same mass of air at every elevation, and corrects it to the map reference conditions explained below, at the standard-day temperature of each elevation, which falls about 3.6 °F per thousand feet. Thinner air makes the corrected flow grow, from 0.994 times the actual flow at sea level to 1.191 times at 5,000 ft: the same engine sits further to the right on the map up there.

Weather moves the real figure a few tenths of a psi either way, which is why some turbocharger guides quote a slightly higher figure for a mile-high city. A local altimeter setting is corrected to sea level, so it is not the pressure your engine breathes; a barometer reading without that correction is.

Compressor pressure ratio and the map

Plot the operating point, pressure ratio against airflow, on the compressor map of each turbo you are considering. The point should sit right of the surge line, the left edge of the map where flow becomes unstable, and left of the choke area on the right, where efficiency falls off and the turbo approaches its speed limit. Ideally it lands in one of the inner efficiency islands.

The chart under the calculator is generic on purpose: no maker's map, just the axes, your point and the engine line at the rpm you entered, the airflow the engine passes at each pressure ratio at that speed. Lower engine speeds pull that line to the left, toward surge, which is why it is worth repeating the calculation at the torque peak.

Most compressor maps are drawn in corrected airflow: the flow the compressor would pass if its inlet air were at fixed reference conditions, so one map serves every climate and altitude. To place your point on such a map, correct the actual airflow: corrected airflow = airflow × √(inlet temperature in °R ÷ reference temperature) ÷ (inlet pressure ÷ reference pressure), where the inlet pressure is the atmospheric pressure minus the inlet depression. The calculator uses the reference conditions a major turbocharger maker publishes for its own maps, 545 °R (about 85 °F) and 13.95 psia, and shows the result as Corrected airflow. Hot or thin inlet air makes the corrected flow larger than the actual flow, cold dense air makes it smaller.

Other turbocharger makers correct their maps to other reference conditions, often stated in kelvin and bar or kPa. Read the reference printed on the map you are using: if it differs, repeat the correction with its figures (the worked example shows each step), or the point lands in the wrong place on that map.

Common mistakes

• Dividing gauge pressures. A pressure ratio uses absolute pressures. Fifteen psi of boost at sea level is a ratio of about two, not fifteen.
• Using wheel horsepower as the target. The method works with crank power. Convert a dyno figure first, or the airflow comes out low.
• Forgetting the losses. The intercooler, pipes and air filter all cost pressure, and the compressor has to make it up. Leaving them out understates the pressure ratio.
• Sizing for one rpm only. A compressor that fits at peak power can be in surge at the torque peak. Check a lower rpm point too.
• Borrowing the wrong BSFC. The airflow estimate scales directly with BSFC. A figure from the very low end of the range calls for a smaller compressor that may run out of breath if the engine turns out thirstier, while a fuel-system sizing figure overstates the airflow and points to a compressor that is too big.
• Changing the fuel but not the BSFC. An E85 air-fuel ratio with a gasoline BSFC understates the airflow by about a third. Enter both for the same fuel.