The Turbo Size Calculator converts a target horsepower figure into the required airflow, boost pressure, and compressor discharge temperature needed to correctly size a turbocharger.
Turbo Size Calculator: Airflow, Boost, and Outlet Temp
The Turbo Size Calculator converts a horsepower target into required airflow, manifold boost, and compressor discharge temperature, split correctly between single and twin-turbo setups. Engine builders use it to get the pressure-ratio and CFM numbers needed to plot against a compressor map before buying a turbo.
Entering Your Target Power, Engine, and Turbo Setup
Enter target power (as WHP or BHP), displacement, cylinder count, valvetrain type, peak-power RPM, and fuel type, then choose Single or Twin Turbo.
Twin Turbo divides the required airflow in half per turbo for compressor-map plotting, while manifold pressure and boost stay identical to a single-turbo build at the same power target.
What the Advanced Environmental Inputs Change
The accordion section adds atmospheric pressure (or altitude directly), ambient temperature, intercooler type, and compressor efficiency.
Garrett’s own Boost Adviser materials frame altitude, not raw pressure, as the standard input here, since thinner air at elevation means the same power target pushes a turbo further up its compressor map.
The Airflow Formula Behind the Turbo Size Calculator
Required airflow comes from horsepower, target air-fuel ratio, and brake specific fuel consumption, a formula an LS1TECH tuning forum thread documents with a worked example matching this calculator’s own math almost exactly.
$$ W_a = \dfrac{HP \times AFR \times BSFC}{60} $$
A common mistake is entering wheel horsepower where the formula expects flywheel horsepower; the calculator converts WHP inputs internally, but doing this math by hand against a raw WHP number understates the true airflow requirement.
BSFC itself is a tuning assumption, not a fixed constant. The same LS1TECH thread uses 0.85 lb/hp-hr for its own worked example, a different value than this calculator’s per-fuel defaults, since BSFC varies with how aggressively an engine is tuned.
Naturally aspirated capacity comes from a separate, standard volumetric-efficiency formula and represents the whole engine’s baseline, unaffected by single or twin-turbo choice.
$$ CFM_{NA} = \dfrac{RPM \times CID \times VE}{3456} $$
How Turbo Outlet Temperature Is Solved
Compressor discharge temperature uses the standard isentropic compression relationship, then applies the selected intercooler’s cooling efficiency to find the actual manifold temperature.
$$ T_{out} = T_{amb} \times \left(1 + \dfrac{PR^{0.283} – 1}{\eta_{comp}}\right) $$
Pressure ratio itself is a whole-engine density requirement, so it doesn’t change between single and twin-turbo configurations, only the airflow each individual turbo must supply does.
At sea level with no boost, pressure ratio floors at 1.0 and boost at zero; ambient temperature is rejected below absolute zero, since neither has a physical meaning past that point.
Compressor Discharge Pressure and Build Risk Tiers
Manifold boost is the pressure the engine actually needs; Compressor Discharge Target adds a flat 2 psi on top, the same margin the LS1TECH thread cites for intercooler and piping pressure loss between the compressor and the manifold.
The Build Risk tier (Mild, Moderate, or Extreme) reads the same pressure ratio against rising thresholds, since higher boost pressure ratios demand progressively stronger internals and fueling regardless of which turbo count is chosen.
Common Mistakes When Sizing a Turbo
Plotting the whole-engine “Required Airflow” figure on a single turbo’s compressor map in a twin-turbo build, instead of the per-turbo figure the calculator already halves.
Sizing a compressor to the plain Manifold Boost number instead of the Compressor Discharge Target, which is the pressure the compressor itself actually has to produce.
Leaving ambient temperature and altitude at sea-level defaults for a build that won’t actually run there, understating how far up the compressor map the turbo really needs to work.
Turbo Sizing Questions Builders Ask
How do I use turbo size calculator numbers on a compressor map?
Per StrikeEngine, plot the pressure ratio and CFM (or lb/min) at your target RPM, then check that a line from that point down to a pressure ratio of 1.0 stays right of the map’s surge line.
Does altitude affect what size turbo I need?
Yes — Garrett’s Boost Adviser materials show that at higher elevation, thinner air means a turbo must work harder to produce the same power, moving its operating point further up the compressor map.
Do single and twin-turbo setups need the same boost pressure?
Yes. Boost and pressure ratio are whole-engine requirements that don’t change with turbo count; only the airflow each individual turbo must supply is divided between them.
Should I size a turbo bigger or smaller than my calculated number?
Per StrikeEngine’s own guidance, erring toward a larger turbo is generally safer than undersizing, since a too-small turbo runs out of map sooner as power climbs.