The Honda TRX400EX typically produces about 28–30 horsepower at the engine (crank) and roughly 20–23 horsepower at the rear wheels on a standard dyno. Exact numbers vary by year, testing method, and condition.
The TRX400EX is a 397cc, air‑cooled four‑stroke quad that Honda sold in the early to mid‑2000s. Stock output stayed in the high‑20s to low‑30s range across most model years, but published figures can differ depending on whether they refer to crank horsepower, wheel horsepower, or the testing standard (SAE gross vs. SAE net). When people discuss HP for a stock 400EX, they’re usually talking about crank output, with drivetrain losses pulling wheel numbers lower.
Model-year ranges and typical output
To set expectations, horsepower figures for the 400EX are fairly consistent across years, with slight variation due to emission controls and exhaust components. The following ranges summarize typical stock outputs observed on common dynos.
Before listing typical ranges, note that dyno results depend on whether horsepower is measured at the engine (crank) or at the rear wheel, and whether the test uses SAE net or gross standards.
- Crank horsepower (engine output): generally in the high 20s to low 30s HP for stock TRX400EXs, depending on year and condition.
- Wheel horsepower (rear-wheel output): typically about 18–24 HP on a chassis dyno, after drivetrain losses.
- Published test methods: some sources reference SAE Gross/SAE Net ratings or crank horsepower; real-world measurements on a dyno can vary accordingly.
In practice, most stock units fall within these ranges, with occasional exceptions based on regional variants or testing setup.
What else affects horsepower readings on a stock 400EX
Even when stock, a 400EX's horsepower reading can be influenced by several factors beyond the engine itself. Here are the main contributors.
- Altitude and temperature: power decreases with higher elevation and hotter air.
- Air intake and exhaust condition: a clean air filter and stock exhaust maintain outputs; a dirty filter or restrictive exhaust can reduce performance.
- Carburetor setup and fuel quality: correct jetting and octane support consistent performance; poor tuning can spike or drop output.
- Engine condition: compression, valve timing, and proper lubrication affect output.
- Test method and gear ratio: the dyno type and the machine's gearing can influence wheel horsepower readings.
These factors explain why two stock TRX400EXs may show different horsepower numbers in different environments or dyno setups.
Summary
Stock horsepower for the Honda TRX400EX typically sits in the high 20s to around 30 HP at the crank, with roughly 20–24 HP at the rear wheel on common chassis dynos. Expect some variation across model years and testing methods, and remember that real‑world conditions can shift numbers. For enthusiasts considering upgrades, these figures establish a baseline for assessing gains from exhausts, air intakes, and tuning.