The 2008 Honda TRX400EX delivers roughly 30 horsepower at the engine crank.
Because Honda did not publish an official horsepower figure for the 400EX, most numbers come from dyno testing and owner reports. Common estimates place stock crank horsepower in the neighborhood of 29–32 hp, with rear-wheel horsepower typically falling in the low-to-mid 20s on a standard dyno. Test conditions, wear, altitude, and fuel quality can all affect the final numbers. This article breaks down what those figures mean and how they’re measured.
Stock horsepower figures
The following range reflects figures often cited by testers and owners for the 2008 model, with a note on how they’re measured.
- Crankshaft horsepower (engine output): approximately 29–32 hp.
- Rear-wheel horsepower (on a typical dyno, stock condition): roughly 25–28 hp.
- Official Honda ratings: not published for the 400EX; numbers rely on independent testing and rider reports.
In practice, these numbers mean the 400EX delivers a competitive punch for a 397cc four-stroke in the quadracycle class, with performance that riders typically compare to other 400-class sport quads from the era.
What influences horsepower and how riders experience it
Several factors determine the measured horsepower of a stock 400EX, and they can shift the feel of power on the trail or track. Conditions such as altitude, temperature, and humidity affect air density and combustion efficiency. Engine wear, fuel quality, and proper maintenance also play a role. Even the dyno and its calibration can yield slightly different numbers from one test to another.
Common modifications that affect horsepower
Riders seeking more power often explore a few well-known upgrade paths. Before listing potential changes, it’s important to know they can alter reliability, fuel economy, and emissions compliance.
- Exhaust upgrades (full system or performance slip-ons) to reduce backpressure.
- Air intake and filtration improvements to improve airflow to the engine.
- Carburetor jetting or fuel management tweaks to optimize air-fuel mixture across RPMs.
With well-chosen modifications, some riders see noticeable gains at the crank and, consequently, at the rear wheel. However, results vary by setup and tuning, and excessive modifications can compromise engine longevity if not matched to the rest of the drivetrain.
Summary
The 2008 Honda TRX400EX is commonly cited as producing around 30 hp at the crank, with wheel horsepower typically in the mid-20s on stock machines. Official Honda figures are not published for this model, so testers and owners rely on dyno data and real-world riding to characterize its performance. Modifications can boost power, but they require careful tuning and consideration of reliability.