Wheel Horsepower vs. Crank Horsepower: What Do Dyno Numbers Actually Mean?
You’ve just put your car on a chassis dyno.
The manufacturer says the engine makes 400 horsepower, but your dyno sheet says 330 horsepower.
Where did the other 70 horsepower go?
It’s one of the most common sources of confusion when people start looking at dyno numbers.
The simple answer is that manufacturer horsepower ratings and chassis-dyno horsepower measurements aren’t normally measuring power at the same place.
At Dynomite Performance, our DynoJet measures power at the vehicle’s drive wheels. That’s why you’ll often hear dyno results described as wheel horsepower (WHP) rather than engine or crank horsepower.
Let’s look at what those numbers actually mean.
What Is Crank Horsepower?
When an automobile manufacturer gives an engine a horsepower rating, that number represents power measured at the engine rather than at the vehicle’s drive wheels.
You may hear this referred to as:
- Engine horsepower
- Crank horsepower
- Flywheel horsepower
- Brake horsepower
For example, suppose a manufacturer rates an engine at 400 horsepower.
That doesn’t necessarily mean putting the vehicle on a chassis dyno will produce a reading of 400 horsepower at the wheels.
Why?
Because the engine isn’t directly connected to the tires.
There are components between them.
What Is Wheel Horsepower?
Wheel horsepower is the power measured at the vehicle’s drive wheels.
The engine has to send its power through the drivetrain before that power reaches the wheels.
Depending on the vehicle, that can involve components such as the:
- Transmission
- Transfer case
- Driveshaft or driveshafts
- Differential
- Drive axles
- Wheels and tires
Energy is consumed as power travels through the drivetrain.
This is commonly referred to as drivetrain loss or driveline loss.
As a result, the horsepower measured at the wheels will normally be lower than the engine’s rated horsepower.
Why Doesn’t My Car Make Its Advertised Horsepower on the Dyno?
This is something we’ve had to explain to customers over the years.
Manufacturers give power ratings at the engine. Those ratings don’t represent the power that ultimately reaches the drive wheels after losses through the transmission, driveshafts, axles and the rest of the drivetrain.
So if your vehicle is rated at 400 horsepower and produces substantially less than 400 WHP on our chassis dyno, that doesn’t automatically mean something is wrong with your vehicle—or with the dyno.
We’re measuring power at a different point.
That’s an important distinction.
Can You Calculate Crank Horsepower From Wheel Horsepower?
This is where things become less straightforward.
You’ll often see people online applying a fixed percentage to wheel horsepower to estimate engine horsepower.
At Dynomite Performance, we sometimes use 20% as a safe rule-of-thumb value when we need to make a rough comparison.
But we don’t consider 20% to be a universal formula for calculating drivetrain loss.
There are too many variables involved.
Different drivetrain configurations and components can affect the relationship between engine horsepower and wheel horsepower.
A front-wheel-drive manual-transmission car isn’t necessarily going to have the same drivetrain characteristics as a rear-wheel-drive automatic truck.
And neither is necessarily comparable to an all-wheel-drive vehicle.
That’s why we’d rather concentrate on what we can actually measure.
A 20% Estimate Is Just That—An Estimate
Here’s a simple example.
Suppose a vehicle produces 320 WHP on the dyno.
If we were making a rough estimate using a 20% drivetrain-loss assumption, we might say that the vehicle is somewhere in the neighbourhood of 400 HP at the engine.
But that doesn’t mean we’ve measured 400 engine horsepower.
We haven’t.
We’ve measured 320 horsepower at the wheels.
The rest is an estimate based on an assumed drivetrain loss.
That’s an important distinction when discussing dyno results.
Why Before-and-After Dyno Testing Is So Valuable
This is where we think chassis dyno testing becomes especially useful.
If you’re tuning a vehicle or testing a modification, the most useful number isn’t always the absolute peak horsepower figure.
Sometimes it’s the difference between where you started and where you finished.
Imagine a vehicle comes into Dynomite Performance for tuning.
Our first dyno run produces:
350 WHP
After making changes, we run the vehicle again and produce:
375 WHP
Now we have something very useful.
We measured a 25-wheel-horsepower improvement.
Instead of arguing about whether the engine is theoretically making 430, 450 or 470 horsepower at the crank, we can demonstrate that the changes produced an additional 25 horsepower at the wheels on our dyno.
That’s real, measurable information.
CHECK OUT THIS ARTICLE ON TESTING MODIFICATIONS TO OUR 2019 RAM 1500
Tuning Customers Usually See the Progression
We’ve found that customers who come to us specifically for tuning generally understand this concept fairly easily.
They can see the progression.
We have an initial dyno run.
Changes are made.
Another run is performed.
Additional changes may be made.
Then we have a final result.
The customer can see where the vehicle started and where it finished.
That before-and-after comparison can often be more valuable than trying to determine an exact theoretical engine-horsepower figure.
What If You Just Want to Know How Much Power Your Car Makes?
This can sometimes be a little more difficult.
Some customers simply want a couple of dyno pulls to find out how much horsepower their vehicle makes.
Most enthusiasts who visit us already have a reasonable idea of what to expect.
Occasionally, though, someone expects the wheel-horsepower number to match the manufacturer’s advertised engine-horsepower rating.
When it doesn’t, they may wonder whether something was done incorrectly.
Fortunately, in our experience that doesn’t happen very often.
Most customers understand that a chassis dyno is measuring power at the wheels.
Peak Horsepower Isn’t Everything
There’s another mistake that’s easy to make when looking at a dyno sheet:
Only looking at the biggest number.
Peak horsepower and peak torque are useful measurements, but they don’t necessarily tell you everything about how a vehicle is performing.
The shape of the horsepower and torque curves matters.
Where the engine makes power matters.
And when we’re testing a modification or tuning a vehicle, the difference between runs can tell us much more than one isolated peak number.
We’ve seen this in our own testing.
For example, when we tested cold air intakes on several 5.7L Hemi vehicles, some produced significant peak horsepower gains while others produced smaller gains. We even had one test where peak horsepower increased while peak torque decreased slightly.
That’s why we look at the results rather than assuming every modification will produce the same outcome on every vehicle.
CHECK OUT THIS ARTICLE: DO COLD AIR INTAKES REALLY ADD HORSEPOWER?
Why Can Two Dynos Give Different Numbers?
Another question enthusiasts sometimes ask is why the same vehicle might produce different horsepower numbers on different dynos.
There are several reasons.
Different dyno designs, testing procedures, correction methods, environmental conditions and vehicle conditions can all influence reported results.
This is another reason we put so much value on controlled before-and-after testing on the same dyno.
If we’re evaluating a modification, we’re interested in establishing a baseline and then measuring what changes.
For example:
Before modification: 300 WHP
After modification: 320 WHP
Under comparable testing conditions, that 20 HP difference tells us something useful about what happened.
What Is SAE Correction?
You’ll sometimes see SAE corrected on our dyno results.
Atmospheric conditions affect engine performance. Temperature, barometric pressure and humidity can change the amount of power an engine produces.
A correction factor provides a standardized method of accounting for atmospheric conditions so dyno results can be compared more meaningfully.
That doesn’t mean correction factors make every dyno test performed anywhere directly interchangeable.
They don’t.
But using a recognized correction method helps provide consistency when analyzing results.
Wheel Horsepower vs. Crank Horsepower: Which One Matters?
Neither measurement is inherently “better.”
They’re simply different measurements used for different purposes.
Manufacturer crank horsepower is useful when comparing factory engine ratings.
Wheel horsepower is particularly useful when evaluating the performance of the complete vehicle and measuring changes through chassis-dyno testing.
At Dynomite Performance, we’re primarily interested in what happens at the wheels because that’s what our DynoJet allows us to measure directly.
And when we’re testing modifications, we’re especially interested in the difference between the baseline and the final result.
The Bottom Line
If your vehicle is advertised as producing 400 horsepower but makes considerably less than 400 horsepower on a chassis dyno, don’t immediately assume something is wrong.
You’re probably comparing two different measurements.
Manufacturer horsepower ratings are measured at the engine.
A chassis dyno measures horsepower at the drive wheels.
Some power is lost as it travels through the transmission, driveshafts, differential, axles and other drivetrain components before reaching the wheels.
While we sometimes use 20% as a safe rule of thumb for making a rough drivetrain-loss estimate, we don’t consider it an exact formula.
If we’re testing a vehicle, we’d rather concentrate on the numbers we can actually measure.
Establish a baseline.
Make the change.
Run the vehicle again.
Then see what actually happened.
That’s what dyno testing is all about.
Dynomite Performance — Real Parts. Real Dyno Tests. Real Results.
