The Garrett Flow Calculator helps you determine the right turbocharger for your engine based on your horsepower goals. It uses a series of calculations to figure out how much air your engine needs to produce a specific amount of power. By calculating the required mass air flow and pressure ratio, you can select a turbocharger that will operate efficiently and safely within its ideal performance range. This prevents you from choosing a turbo that is too small, which could over-speed and fail, or one that is too large, which would result in poor throttle response and lag.
formula
You can use a set of core formulas to determine the necessary specifications for your turbocharger. These calculations work together to translate your horsepower target into the airflow and pressure requirements that you will use to select the right turbo.
1. Mass Air Flow Rate
This formula calculates how much air, measured in pounds per minute, your engine needs to achieve a specific horsepower goal.
Mass Air Flow (lb/min) = (Target Horsepower * Air/Fuel Ratio * BSFC) / 60
Variables:
Target Horsepower: The desired horsepower at the engine's crankshaft.
Air/Fuel Ratio (A/F): The ratio of air to fuel. For turbocharged gasoline engines, a common value is 12.0.
BSFC (Brake Specific Fuel Consumption): A measure of engine efficiency. For turbocharged gasoline engines, a typical value is 0.55.
2. Pressure Ratio (PR)
This formula calculates how much the turbocharger must compress the incoming air. This value helps you plot your engine's requirements on a compressor map.
Pressure Ratio = Absolute Outlet Pressure / Absolute Inlet Pressure
3. Required Manifold Absolute Pressure (MAP)
This formula calculates the absolute pressure the engine needs in its intake manifold to consume the required mass air flow at a specific engine speed.
Required MAP (psia) = (Mass Air Flow * Gas Constant * (Intake Temp + 460)) / (Volumetric Efficiency * Engine RPM * Engine Displacement / 2)
Variables:
Mass Air Flow: The value you calculated in the first formula.
Gas Constant (R): For air, this value is 53.35.
Intake Temp (°F): The estimated temperature of the air inside the intake manifold.
(Intake Temp + 460): This step converts the temperature from Fahrenheit to the Rankine absolute temperature scale.
Volumetric Efficiency (VE): A measure of how efficiently the engine's cylinders fill with air. You can use 0.95 for a 4-valve engine or 0.85 for a 2-valve engine.
Engine RPM: The engine speed where you want to achieve your target horsepower.
Engine Displacement: The size of your engine in cubic inches.
4. Unit Conversion Formulas
You use these simple formulas to convert common measurements into the units required for the main calculations.
Engine Displacement (Liters to Cubic Inches):
Cubic Inches = Liters * 61.02
Boost Pressure (Gauge to Absolute):
Absolute Pressure (psia) = Gauge Pressure (psig) + Atmospheric Pressure (psia)
Common Values and Conversions
This table provides typical values for variables used in the calculations, which helps you get started if you do not have precise measurements for your specific engine.
| Parameter | Common Value | Application |
| Atmospheric Pressure | 14.7 psia | At Sea Level |
| Atmospheric Pressure | 12.2 psia | At 5,000 feet altitude |
| Air/Fuel Ratio (A/F) | 11.5 - 12.5 | Turbocharged Gasoline Engines |
| BSFC | 0.55 | Turbocharged Gasoline Engines |
| Volumetric Efficiency (VE) | 0.95 (95%) | 4-Valve per Cylinder Engines |
| Volumetric Efficiency (VE) | 0.85 (85%) | 2-Valve per Cylinder Engines |
| Pre-Turbo Pressure Loss | 1.0 psi | Standard Intake System |
| Engine Conversion | 2.0 Liters | 122 Cubic Inches |
| Engine Conversion | 3.5 Liters | 214 Cubic Inches |
| Engine Conversion | 5.7 Liters | 348 Cubic Inches |
Example
Let's say you want to choose a turbo for an engine with the following goals and specifications:
Target Horsepower: 400 HP
Engine Displacement: 2.5 Liters
Desired Boost Pressure: 15 psig
Location: Sea Level
First, you calculate the required Mass Air Flow.
Mass Air Flow = (400 * 12.0 * 0.55) / 60 = 44 lb/min
Next, you calculate the Pressure Ratio. To do this, you must find the absolute inlet and outlet pressures.
Absolute Inlet Pressure = 14.7 psia (Atmospheric) - 1.0 psi (Pressure Loss) = 13.7 psia
Absolute Outlet Pressure = 15 psig (Boost) + 14.7 psia (Atmospheric) = 29.7 psia
Pressure Ratio = 29.7 / 13.7 = 2.17
With these results, you now know you need a turbocharger that can efficiently provide 44 lb/min of air at a pressure ratio of 2.17. You would use these two values to find a suitable match on a turbocharger compressor map.
Most Common FAQs
A compressor map is a chart provided by turbocharger manufacturers that shows the turbo's performance range. The horizontal axis of the map is the Mass Air Flow Rate, and the vertical axis is the Pressure Ratio. After you calculate these two values for your engine, you can plot them as a single point on the map. The goal is to choose a turbo where your plotted point falls within the most efficient "island" or region of the map. This ensures the turbo will perform well and reliably for your specific application.
Altitude significantly impacts turbo performance because the atmospheric pressure is lower. As you go higher in altitude, the air is less dense. To make the same horsepower, the turbo must spin faster and work harder to compress the less dense air to your target boost pressure. This results in a higher pressure ratio. When performing these calculations for a high-altitude application, you must use the correct, lower atmospheric pressure value to get an accurate pressure ratio and select a turbo capable of handling the increased workload.
Choosing the wrong size turbo has significant negative effects. A turbo that is too small for your horsepower goal will have to spin excessively fast to try and meet the engine's air demand. This can push it beyond its safe operating limits, leading to overheating and premature failure. On the other hand, a turbo that is too large will have poor response at lower engine speeds, a condition known as "turbo lag." This makes the car feel sluggish until the engine produces enough exhaust gas to finally spin the large turbine wheel. Matching the turbo size to your engine and power goals using these calculations is critical for both performance and reliability.