Turbocharger matching

Compressor match works out the inlet pressure (after the air filter), the manifold pressure, the compressor outlet pressure (manifold plus the intercooler’s drop) and so the pressure ratio; the compressor outlet temperature from your compressor efficiency; the intake temperature after the intercooler; the air mass flow, actual and corrected to your map’s reference (Garrett’s 545 °R and 13.95 psia by default), per turbo, in lb/min and kg/s; and the power that air makes. It plots the full-throttle operating line, every 250 rpm, on a generic compressor map scaled to the engine so every boosted point clears surge and choke (or to the design flow and pressure ratio you type), with speed lines, efficiency islands, the surge line and the choke line, and gives each point’s surge margin and a status: ok, near surge, surge, near choke or past choke. Under Your own map you can type a few points of the surge and choke lines read off a real compressor map; the margins then use them.

Turbine & wastegate balances the turbine against the compressor: the least exhaust manifold pressure that could make the boost, the pressure at your drive ratio (exhaust manifold pressure ÷ boost, which the housing’s A/R sets), how much exhaust then has to go round the wastegate, and the wastegate throat that passes it, beside the usual rule-of-thumb valve sizes. Notes cover A/R, twin-scroll against single-scroll housings, boost threshold and lag, and wastegates.

Fuel sizes the injectors (cc/min and lb/h, at your duty cycle, and what to buy at the usual 3-bar rating — larger if the regulator is not manifold-referenced) and the pump (L/h with a margin, at base pressure plus boost), for petrol, E85, methanol or diesel, with a table of lambda and air–fuel ratio targets under boost.

Intercooler sets air-to-air or water-to-air, effectiveness and pressure drop (fixed, or rated at a flow and scaled with flow²), and shows the charge temperatures in and out, the density gain, the power the cooler is worth, and the temperatures across the rev range.

Air the engine takes: ṁ = VE × ρ × displacement × rpm ÷ 120 (four-stroke), with ρ = MAP ÷ (R × intake temperature).

Pressure ratio = (MAP + intercooler drop) ÷ (ambient − filter loss); ambient pressure from the ISA at your altitude.

Compressor outlet temperature: T2 = T1 × [1 + (PR^0.286 − 1) ÷ η_c]; after the intercooler T = T2 − ε(T2 − T_coolant).

Corrected flow: ṁ × √(T1 ÷ T_ref) ÷ (P1 ÷ P_ref), per turbo — the quantity compressor maps are drawn in, so the map is the same whatever the day.

Power: fuel = air ÷ AFR, and power = fuel ÷ BSFC; the old rule hp ≈ 10 × lb/min is shown too.

Turbine: turbine power ṁ_t × c_p × T3 × η_t × [1 − (P4 ÷ P3)^((γ−1)/γ)] equals compressor power ÷ mechanical efficiency (0.95). With all the exhaust through the turbine that gives the least P3; at a higher P3 the turbine needs less flow and the rest is wastegate flow, sized as a compressible orifice (C_d 0.7) from P3 to P4. The treatment follows N. Watson and M. S. Janota, Turbocharging the Internal Combustion Engine (Macmillan), and J. B. Heywood, Internal Combustion Engine Fundamentals (McGraw-Hill), chapter 6; Garrett’s (Honeywell) public "Turbo Tech" guides describe the same matching steps.

Injectors: flow = hp × BSFC ÷ (number × duty cycle), scaled by √(rated pressure ÷ pressure across the injector).

The compressor map is illustrative: a generic shape scaled through a design point, not any real compressor. For a real choice, plot the operating points on the maker’s map — the table gives the corrected flows and pressure ratios to do it — or type the real surge and choke lines in.

Volumetric efficiency is one figure for all rpm; real engines vary, and boosted engines often exceed 100 % with a good cam and exhaust. The boost curve is a straight ramp.

The turbine side is a power balance with one efficiency and an estimated drive ratio, not a turbine map; real turbine efficiency and swallowing capacity vary with flow and A/R. Wastegate sizes are estimates — follow the gate maker’s guidance.

BSFC, lambda targets and effectiveness figures are typical starting points, not a tune.