Turbo Size Calculator

It converts your horsepower target into the airflow the turbo must supply. You enter target HP, boost, redline RPM, engine displacement, and fuel type; the calculator outputs required airflow in lb/min and CFM, the pressure ratio, the boost your engine combination needs, a recommended turbo frame class with popular example turbos, and a compressor-map position check that warns about surge or choke.

With the Garrett Turbo Tech 103 formula: airflow (lb/min) = target HP x air/fuel ratio x BSFC / 60. Garrett's own worked example uses 400 HP, an AFR of 12, and a BSFC of 0.55, which gives exactly 44.00 lb/min (400 x 12 x 0.55 / 60). This calculator uses the same formula; only the AFR and BSFC defaults change with your fuel selection.

Use these typical starting values, then refine them on the dyno. Gasoline (turbo): BSFC 0.50, AFR 11.5 (Garrett lists 0.50-0.60 for turbocharged gasoline engines). Diesel: BSFC 0.36, AFR 25.0 (Garrett's diesel example value; diesels run lean). E85: BSFC 0.70, AFR 7.8 (E85 needs roughly 40% more fuel than gasoline; stoich 9.765 at a rich boosted lambda near 0.8). The calculator lets you override both numbers.

Pressure ratio is the compressor's outlet pressure divided by its inlet pressure, and it is the vertical axis of every compressor map. At sea level: pressure ratio = (14.7 + boost in psi) / 14.7. For example, 20 psi of boost gives (14.7 + 20) / 14.7 = 2.36. The calculator also adds your intercooler pressure drop to get the true compressor pressure ratio, which is what the map actually sees.

Multiply lb/min by 14.27 to get CFM (and divide CFM by 14.27 to go back). So Garrett's 44.00 lb/min example equals 627.88 CFM. This is the standard conversion used when matching airflow to compressor maps or to carburetor/throttle-body CFM ratings.

The calculator uses Garrett's manifold-pressure formula: MAP (psia) = airflow x 639.6 x (460 + intake temp in F) / (VE x (RPM/2) x displacement in CID), then subtracts 14.7 psi to get gauge boost. Garrett's example (2.0L, 44 lb/min, 130 F intake, 92% VE, 7200 RPM) needs 41.1 psia, which is 26.4 psig of boost. If the required boost differs a lot from the boost you entered, the calculator flags the mismatch so you can re-check your inputs.

Trim describes the relationship between a wheel's inducer (minor) and exducer (major) diameters: Trim = (minor / major) squared x 100. A higher trim flows more air for the same exducer diameter but typically spools slower. Trim is a wheel-specification detail; this calculator first narrows the frame class (GT28, GT30, GT35, GTX42/45), and you then pick the trim on the actual compressor map.

On pump gas (BSFC 0.50, AFR 11.5): 300 HP needs 28.75 lb/min (410.26 CFM), a small-frame GT28/GTX28 class turbo; 400 HP needs 38.33 lb/min (547.02 CFM), medium-frame GT30/GTX30 class; 500 HP needs 47.92 lb/min (683.77 CFM), medium-frame GT30/GTX30 class; 600 HP needs 57.50 lb/min (820.53 CFM), medium-frame GT30/GTX30 class at the top of its range. Use the presets table on this page to load any of these targets instantly.

Surge is violent airflow reversal that happens when a turbo is asked to make high boost at very low airflow - typically a turbo that is too big for the engine, or lugging at low RPM. The calculator warns of surge risk when your airflow is below about 30% of the recommended class's maximum flow, or below 35% with a pressure ratio above 2.5. The fix is usually a smaller frame, not more boost.

Choke is the right edge of the compressor map: past it, the turbo cannot flow more air no matter how fast it spins, efficiency collapses, and shaft speed climbs dangerously. The calculator warns of choke risk when your required airflow exceeds about 92% of the recommended class's maximum flow - for example, a 1,000 HP gasoline target needs about 95.83 lb/min, which chokes a GT35/GTX35 class and calls for a large-frame GTX42/GTX45 class.

Because the compressor must produce manifold boost PLUS whatever pressure is lost across the intercooler and piping. With 20 psi of manifold boost and a typical 2 psi intercooler drop, the manifold pressure ratio is 2.36 but the compressor actually works at (14.7 + 20 + 2) / 14.7 = 2.50. Plotting the wrong pressure ratio puts your map point in the wrong efficiency island, so enter your measured drop if you know it.

The airflow formula is the same; only AFR and BSFC change. Diesel needs far more air per horsepower because it runs very lean (AFR 25): a 410 HP diesel target needs about 61.50 lb/min, while 400 HP on gasoline needs only 38.33 lb/min. E85 sits between on airflow (600 HP on E85 needs about 54.60 lb/min) but demands roughly 40% more fuel flow, so size the fuel system, not just the turbo, for the fuel you run.

No. As Garrett points out, the airflow target comes only from HP, AFR, and BSFC - displacement and RPM never enter that equation. A 2.0L and a 5.0L both need about 44.00 lb/min for 400 HP. What displacement changes is how much boost and RPM are needed to move that air: the smaller engine needs more of both, which the calculator shows via the required-boost and displacement airflow cross-checks.

It is a first-order sizing tool based on Garrett's published Turbo Tech 103 method, good for narrowing the turbo frame to 2-3 candidate models. It does not replace the actual compressor map: real maps have efficiency islands, surge lines, and speed lines that this estimate cannot reproduce. Always plot your final operating point on the manufacturer's map for the exact turbo you buy, and treat all horsepower figures as flywheel HP.