MCPCalc

Drivetrain Loss Calculator

Convert wheel horsepower to crank horsepower, or the reverse, using drivetrain loss estimates for FWD, RWD, and AWD with manual or automatic transmissions.

Scratchpad (not saved)

The power you are converting from.

Unit for the value entered above.

%

Enter a value above 0 to override the layout default. Leave at 0 to use the default.

What This Calculator Does

An engine's rated power is measured at the crank, but a chassis dyno measures what reaches the wheels, after the gearbox, driveshaft, differential, and tires have taken their share. This calculator converts in either direction using a drivetrain loss percentage. Pick a layout and transmission to load a default loss, or type your own. The defaults are rules of thumb, not measurements, and the percentage model is a simplification, since real losses are partly fixed and partly proportional to power. The conversion gives a sensible estimate for comparing a dyno run with a published rating, and nothing more precise than that.

It combines Convert, Power, Power Unit, Drivetrain Layout (Default Loss) to estimate Crank horsepower (CHP), Drivetrain loss, Crank power.

Formula & Method

Wheel power from crank power: WHP=CHP (1−ℓ)\text{WHP} = \text{CHP}\,(1 - \ell) Crank power from wheel power: CHP=WHP1−ℓ\text{CHP} = \frac{\text{WHP}}{1 - \ell} where ℓ\ell is the fractional drivetrain loss (0.15 for 15%), WHP is wheel horsepower, and CHP is crank horsepower. Power in kW converts to horsepower with 1 kW = 1.341 hp. Note that CHP is found by dividing, not by adding the loss percentage: 300 whp at 15% loss is 352.9 chp, not 345.

Notation used in the formulas: RR = Crank horsepower (CHP); x1x_{1} = Convert; x2x_{2} = Power; x3x_{3} = Power Unit; x4x_{4} = Drivetrain Layout (Default Loss); x5x_{5} = Custom Loss \% (optional).

Method summary: inputs are normalized to consistent units, core equations are evaluated, then secondary values are derived and rounded for display.

Use it to estimate crank power from a dyno sheet, to see what an advertised crank figure should read at the wheels, or to compare cars with different drivetrains on a common basis. Treat the result as an estimate with an uncertainty of several percentage points.

Worked Examples

Dyno reading to crank power, RWD manual (imperial)

Given: 300 whp, rear-wheel drive with a manual gearbox (15% default loss) Method: crank power is wheel power divided by (1 − loss). CHP = 300 / (1 − 0.15) = 300 / 0.85 = 352.9 hp Drivetrain loss = 352.9 − 300 = 52.9 hp A common mistake is 300 × 1.15 = 345 hp. That adds 15% to the wheel figure, but the loss is 15% of the crank figure.

Wheel power from a crank rating, AWD manual (metric)

Given: 300 kW crank rating (402.3 hp), all-wheel drive with a manual gearbox (20% default loss) Method: wheel power is crank power multiplied by (1 − loss). WHP = 402.3 × (1 − 0.20) = 321.8 hp = 240 kW Expected dyno reading is about 240 kW at the wheels, with a real car landing within several percent either side.

Default Loss Estimates and Their Effect

These percentages are rules of thumb used as starting points, not measured values for any particular car. Losses vary with the gearbox, the lubricant and its temperature, the tires, and the dyno itself. Edit the percentage if you know a better figure for your car.

Layout and transmissionDefault lossCrank power for 300 whp
FWD, manual12%340.9 hp
RWD, manual15%352.9 hp
AWD, manual20%375.0 hp
FWD, automatic (manual + 3 points)15%352.9 hp
RWD, automatic (manual + 3 points)18%365.9 hp
AWD, automatic (manual + 3 points)23%389.6 hp

Dyno type matters as much as the car. Inertia dynos, load-bearing roller dynos, and hub dynos read differently on the same car, and the correction factor applied for temperature and pressure shifts the number further. A percentage model also hides the fact that part of the loss, such as churning oil and bearing drag, depends on speed more than on power, so the loss fraction is usually larger at low power than at full power.

Reference

Driveline efficiency and its dependence on speed and torque are discussed in Gillespie, Fundamentals of Vehicle Dynamics. Engine power ratings are defined by test codes such as SAE J1349, which sets how crank power is corrected to standard conditions.

Common Mistakes

  • Adding the loss percentage to the wheel figure instead of dividing by (1 − loss). Multiplying 300 whp by 1.15 gives 345 hp, but the right answer for 15% loss is 352.9 hp.
  • Comparing a wheel figure from one dyno type with a wheel figure from another. The two can differ by 10% or more on the same car.
  • Using the layout default as a measurement. A percentage chosen for a generic RWD manual says nothing certain about your car.
  • Comparing a corrected crank rating with an uncorrected dyno number. Check which correction standard each one uses.
  • Assuming the loss is the same at every power level. It is partly fixed, so a modified car does not usually lose exactly the same fraction.

When Not To Use This Calculator

  • Precise tuning or warranty decisions. A percentage estimate is not a measurement of your engine.
  • Comparing results from different dyno types or correction standards without accounting for the difference.
  • Hybrid or electric drivetrains, where the losses and power definitions differ.
  • Engines on an engine dyno, which already read at the crank.

Frequently Asked Questions

How do I convert wheel horsepower to crank horsepower?
Divide the wheel horsepower by (1 − loss). With 15% loss, 300 whp is 300 / 0.85 = 352.9 hp at the crank. Multiplying by 1.15 instead is a common mistake that understates the result.
How much drivetrain loss should I assume?
A common rule of thumb is about 12% for front-wheel-drive manuals, 15% for rear-wheel-drive manuals, and 20% for all-wheel-drive manuals, with about 3 percentage points more for a conventional automatic. These are estimates, not measurements, and the real figure varies by car.
Why do two dynos give different numbers for the same car?
Dyno types load the car differently and apply different correction factors. An inertia dyno, a load-bearing roller dyno, and a hub dyno can disagree by 10% or more. Compare readings only from the same dyno and the same correction standard.
Is drivetrain loss really a percentage?
Only approximately. Part of the loss, such as bearing drag and oil churning, depends on speed and is nearly independent of power. A percentage is a simple model that works acceptably across typical power levels but misses that structure.
Do automatic transmissions lose more power than manuals?
Typically yes, by a few percentage points, because of the torque converter and hydraulic pumping losses. Modern dual-clutch and multi-speed automatics have narrowed the gap, so the editable loss field lets you enter a better figure for your car.

Inputs Used

  • Convert: Used directly in the calculation.
  • Power: The power you are converting from.
  • Power Unit: Unit for the value entered above.
  • Drivetrain Layout (Default Loss): Used directly in the calculation.
  • Custom Loss % (optional): Enter a value above 0 to override the layout default. Leave at 0 to use the default.

See Also

Related Calculators