Measurement
Wheel horsepower versus crank horsepower
The single most common reason two numbers for the same car disagree. It is not that one of them is a lie, it is that they are measuring in different places.
Crank horsepower is what an engine makes at the flywheel, measured on an engine dynamometer with the engine out of the car. Wheel horsepower is what is left at the driven tyres once the gearbox, driveshaft, differential and the tyres themselves have taken their share. A chassis dyno can only measure the second one.
Why the difference exists
Everything that turns absorbs a little of what passes through it. Gear meshes have friction, oil has to be churned, bearings drag, and a tyre flexing against a roller loses energy as heat. None of it is a fault. It is simply the cost of getting the output to the ground.
The loss is not a fixed fraction either. It changes with gear selection, tyre pressure and construction, oil temperature, and how much the tyre is deforming under load. That is what makes the usual shorthand so unreliable.
The drivetrain loss percentage problem
You will see figures quoted as fact: fifteen percent for a manual, twenty for an automatic, more for all wheel drive. They are rules of thumb that got repeated until they sounded like measurements.
The trouble is arithmetic. Applying a percentage to a wheel figure to reach a crank figure assumes the loss scales exactly with output, and it does not. Much of it is fixed drag that barely moves whether the engine is making two hundred or six hundred. Use a flat percentage and you overstate the loss at high output and understate it at low.
Which number should you care about
For anything you actually do with the car, the wheel figure. It is what accelerates you, it is what the tyres have to put down, and it is the number that will change when you alter something.
Crank figures are useful in exactly two places: comparing against a manufacturer rating, which is also a crank number, and specifying an engine build where the engine is on a stand anyway.
Comparing your car with somebody else's
This is where most arguments start. Before concluding that your car is down on power, check that the two sheets agree on all of the following.
- Wheel or crank. Comparing one against the other is not a comparison at all.
- The correction standard, and whether the figure was corrected.
- The type of dyno. Different designs load an engine differently and do not read identically.
- Which gear the pull was made in, and what tyres were fitted.
- The conditions on the day, which are why correction exists.
Get those lined up and most of the disagreement usually disappears. What is left is a real difference worth investigating, and that is the point at which a diagnostic session earns its money.
What this relates to
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