WHP to HP Calculator
Convert between wheel horsepower (WHP) and engine horsepower (HP) accounting for drivetrain losses.
Calculate Horsepower
Input values and select drivetrain type
Results
Why are HP and WHP different?
Engine HP (Horsepower) refers to the power output at the engine’s crankshaft, measured directly from the engine before any drivetrain losses occur.
WHP (Wheel Horsepower) is the actual power delivered to the wheels after accounting for drivetrain losses from the transmission, differential, axles, and other components.
What is a WHP to HP Calculator?
A WHP to HP calculator is an essential tool for automotive enthusiasts and professionals that converts between wheel horsepower (WHP) and engine horsepower (HP) by accounting for power lost through the drivetrain.
Wheel Horsepower (WHP) is the measurable power that actually reaches the wheels, measured using a chassis dynamometer. Engine Horsepower (HP), also called crank or brake horsepower, is the theoretical power produced at the engine’s crankshaft, typically measured on an engine dyno .
The power difference between these two measurements occurs because power diminishes as it travels from the engine through various components like the transmission, driveshaft, differentials, and axles before reaching the wheels . Your calculator bridges this gap using established engineering principles.
How to Use the Calculator
- Enter one value: Input either the Wheel Horsepower (WHP) or the Engine Horsepower (HP). The calculator is designed to compute the missing value, so you only fill in one field.
- Select drivetrain: Choose your vehicle’s configuration Front Wheel Drive (FWD), Rear Wheel Drive (RWD), or All Wheel Drive (AWD). This selects the appropriate drivetrain loss percentage.
- Calculate.
The Formula: How the Conversion Works
HP = WHP / (1 – DL)
Where:
- HP is the estimated engine horsepower
- WHP is the measured wheel horsepower
- DL is the drivetrain loss expressed as a decimal
Similarly, to convert from HP to WHP, the formula is rearranged:
WHP = HP × (1 – DL)
Drivetrain Loss Values
The calculator applies these generally accepted drivetrain loss percentages :
| Drivetrain Type | Typical Loss | Primary Reason for Loss |
|---|---|---|
| Front-Wheel Drive (FWD) | 10% | More direct power path with a transaxle, no heavy driveshafts. |
| Rear-Wheel Drive (RWD) | 15% | Power must be routed through a longer driveshaft and a hypoid gear differential, which creates more friction. |
| All-Wheel Drive (AWD) | 25% | Highest loss due to multiple differentials and a transfer case, significantly increasing rotational mass and friction. |
Calculation Example
If you dyno tested your car and measured 300 WHP, and it’s a RWD vehicle (15% loss), the calculation would be:
HP = 300 / (1 – 0.15) = 300 / 0.85 = ~353 HP.
Practical Applications and Use Cases
Your calculator serves several valuable purposes for different users:
- For Performance Tuners: After making engine modifications, measure the new WHP on a dyno and use the calculator to estimate the actual engine power gain, helping quantify the effectiveness of upgrades .
- For Vehicle Diagnosis: A significant discrepancy between the calculated HP and the manufacturer’s stated HP can indicate excessive drivetrain loss, potentially pointing to issues like a slipping clutch, worn bearings, or other mechanical problems .
- For Accurate Comparisons: It allows for an apples-to-apples comparison between a vehicle’s dyno results (WHP) and the manufacturer’s factory engine rating (HP) .
- For Performance Prediction: If you know a vehicle’s factory HP rating, you can estimate the expected WHP before going to a dyno, setting realistic performance expectations .
Important Considerations on Drivetrain Loss
- Transmission Type: Manual transmissions are generally more efficient (2-5% loss) than traditional automatics (5-10% loss) due to the power consumed by torque converters and hydraulic pumps.
- Component Design and Condition: Heavier components, worn fluids, or poor maintenance can increase losses. Lightweight driveshafts and high-quality synthetic fluids can help reduce them.
- Power Level: Drivetrain loss is not a simple fixed percentage across all power levels. The actual power absorbed by the drivetrain can change with increased engine output.