CalcDomain

Data source & methodology

AuthoritativeDataSource: ISO 1585:2020 — Road vehicles — Engine test code — Net power. Also consistent with SAE J1349 horsepower definition and the standard torque–speed relationships used by dyno manufacturers. All calculations strictly follow the formulas and unit relationships provided by these sources.

The formula explained

Imperial horsepower (hp) from torque in lb·ft and rotational speed in rpm: \[ P_{\mathrm{hp}}=\frac{T_{\mathrm{lb\cdot ft}}\cdot n_{\mathrm{rpm}}}{5252} \] where \(5252=\frac{33{,}000~\mathrm{ft\cdot lbf/min}}{2\pi~\mathrm{rad/rev}}\).

SI kilowatts from torque in N·m and rpm: \[ P_{\mathrm{kW}}=\frac{T_{\mathrm{N\cdot m}}\cdot n_{\mathrm{rpm}}}{9549} \] (constant derives from unit conversion \(P=T\omega\) with \(\omega=2\pi n/60\)).

Conversions: \(\;1~\mathrm{kW}=1.3410229~\mathrm{hp}\;\) and \(\;1~\mathrm{hp}=0.7456999~\mathrm{kW}\).

Drivetrain correction (estimate): if wheel power \(P_w\) and loss fraction \(L\), then crank/bhp \(P_c=\frac{P_w}{1-L}\); conversely \(P_w=P_c(1-L)\).

Glossary of variables

  • Torque (T) — engine twisting force at the crankshaft (lb·ft or N·m).
  • Engine speed (n) — revolutions per minute (RPM) at which torque applies.
  • WHP — power measured at the wheels (after drivetrain losses).
  • BHP — brake horsepower, power at crankshaft, net of auxiliaries per test standard.
  • Loss (%) — estimated parasitic losses through transmission, differential, tires.

How it works: a step-by-step example

Inputs: T = 300 lb·ft, n = 5,000 rpm, loss = 15%.

  1. Compute hp: \(P_{\mathrm{hp}} = 300\times 5000 / 5252 \approx 285.7~\mathrm{hp}\).
  2. Convert to kW: \(P_{\mathrm{kW}} \approx 285.7 \times 0.7457 \approx 213.1~\mathrm{kW}\).
  3. Wheel vs. crank: if 15% loss, WHP ≈ \(P_c(1-L)\Rightarrow 285.7\times0.85\approx 242.8~\mathrm{hp}\).

Result: ≈243 WHP and ≈286 BHP at 5,000 rpm.

FAQ

Why do torque (lb·ft) and horsepower lines cross at 5252 RPM?

Because \(P_{\mathrm{hp}} = T_{\mathrm{lb\cdot ft}}\cdot n/5252\). When \(n=5252\), numeric values of hp and lb·ft are equal.

Is “BHP” the same as “SAE net” power?

Modern net power ratings follow standards (e.g., ISO 1585, SAE J1349). “BHP” colloquially refers to crankshaft power under specified accessory loads; check which standard a figure uses.

What drivetrain loss should I pick?

Ranges vary by layout and hardware; 10–25% is common. The most reliable method is measuring both wheel power on a chassis dyno and applying the manufacturer’s published correction.

Can I compute power from N·m and RPM directly?

Yes: \(P_{\mathrm{kW}}=T_{\mathrm{N\cdot m}}\cdot n/9549\). The tool converts between hp and kW automatically.

Does gear ratio affect the result?

The torque×RPM formula uses engine torque at the crank. Gear ratios multiply torque at the wheels but also change wheel speed; engine power stays the same (minus losses).

What about correction factors (temperature, pressure, humidity)?

Laboratory standards (ISO 1585, SAE J1349) specify atmospheric corrections for net power certification. This calculator focuses on the base torque–speed relation.

Is metric horsepower (PS/CV) the same as hp?

No. 1 PS ≈ 0.7355 kW and 1 hp ≈ 0.7457 kW. Differences are small but relevant in spec sheets.

Tool developed by Ugo Candido. Content verified by the CalcDomain Editorial Board.
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