Engine power measurement by GPS
A dyno alternative: power is computed by a physics model over GPS telemetry of a real acceleration run — corrected to SAE J1349, with actual weather and wind factored in. Free.
How power is calculated
Record a run
A GPS logger records speed and coordinates at 5 Hz or higher (10–25 Hz is better). RaceBox, Dragy, Racelogic VBOX, AiM — or RaceChrono with an external receiver. Run files: .rcz, .nmea, .dlap, .xrk, .xrz or .vbo.
Physics model
Every resistance is subtracted from speed and acceleration: aerodynamics (Cd and frontal area), rolling and drivetrain losses, slope, wind and air density from real weather and altitude.
Power curve
All acceleration runs are combined into a composite power-vs-speed curve — more robust to GPS noise than a single peak, and it shows the engine character across the whole range.
Rating and data quality
The result is engine power (BHP) corrected to SAE J1349 standard atmospheric conditions. Weak GPS stretches are rejected before the calculation, and speed is smoothed on a uniform 25 Hz grid.
GPS measurement vs dyno
| GPSDyno | Dyno | |
|---|---|---|
| Price | Free — all you need is a GPS log of the run | Paid dyno session |
| Where it happens | Track day, drag strip, closed area | Booking and a visit to a tuning shop |
| What you get | SAE J1349 BHP + laps, accelerations, track map, telemetry | Power and torque curve under that specific dyno's conditions |
| Comparability | One methodology for every car on the site — results are directly comparable | Different dynos and calibrations give noticeably different numbers |
| Conditions | Real driving: actual tyres, surface and weather are part of the model | Stationary rollers — no aerodynamics or real grip |
A dyno is irreplaceable for live engine tuning. GPS measurement solves a different problem: objective car comparison and before/after checks under a single methodology.
What you need
GPS logger
RaceBox, Dragy, VBOX, AiM or RaceChrono with an external receiver — 5 Hz minimum. Phone GPS (1 Hz) is not enough for power calculation.
Car data
Mass, tyres and aerodynamics. A garage car fills in its spec sheet automatically; for a new one AI helps with the numbers.
A safe place
A few confident acceleration runs on a track or closed area. Measuring on public roads is a bad idea.
Questions & answers
What exactly does GPSDyno compute?
GPSDyno breaks down a run from GPS telemetry: laps and lap times, acceleration performance (100→200 splits), top speed and a track map — and estimates real engine power (BHP). The power calculation uses speed, acceleration, mass, aerodynamics, tires, surface, elevation and weather.
Can you measure power without a dyno?
In our case, yes: GPSDyno can replace a chassis dyno for comparing cars against each other and for before/after runs. Different dynos often give noticeably different figures due to calibration, methodology and conditions, while here every run goes through a single model with GPS quality control and wind, mass and slope corrections. As a result, the scatter between comparable runs is usually smaller and easier to understand.
Is GPSDyno more accurate than a dyno?
It is more correct to talk about repeatability of comparison rather than the absolute accuracy of each dyno. GPSDyno provides a transparent estimate using a single consistent method with SAE J1349 correction, so a series of runs in one database compares more fairly than mixed results from different dynos.
Why does a single method give less scatter?
Because it removes a major source of differences: different dynos, calibrations, procedures and measurement conditions. In GPSDyno run conditions may vary, but GPS processing, filtering, the resistance model, wind corrections and quality scoring stay the same.
Which files can be used for the calculation?
Supported formats are .nmea, .rcz, .dlap, .xrk, .xrz and .vbo. Such files are usually exported by RaceChrono, Dragy, AiM RaceStudio, Racelogic VBOX, Harry's LapTimer, FastIron and similar GPS/lap-timer apps.
Why specify tires, tire size and surface?
Tires, wheel size and surface type affect rolling resistance and wheel inertia. The more accurate these parameters, the more stable the BHP estimate from run to run.
What affects result quality the most?
GPS log quality, recording rate, correct car mass, tire size and type, surface, wind and pull consistency. If the data is weak, the service shows warnings and rejects bad GPS points before the calculation.
Check your car
Upload a run file — laps, accelerations and the power breakdown will be ready in a couple of minutes.
Run a free measurement