In stock form, a Honda TRX400EX typically makes about 28–34 horsepower at the crank, which translates to roughly 20–28 horsepower at the rear wheels. With common bolt-on upgrades you can push into the low-to-mid 40s horsepower at the crank, and with bigger bore and cam work you can reach the mid-40s to around 50 horsepower. If you add forced induction or nitrous, crank horsepower can exceed 60 HP, but reliability, heat management, and cost become major considerations.
horsepower figures depend on year, condition, and tuning, and are usually measured either at the engine (crank) or at the wheels. This article outlines stock figures, typical bolt-on gains, and extreme builds, with notes on what riders can realistically expect for different goals and budgets.
Stock horsepower and wheel numbers
Power metrics explained
Below is a concise snapshot of stock power figures and what dynos typically show for a TRX400EX in normal condition. Figures are approximate and vary by model year, exhaust configuration, and dyno calibration.
- Crank horsepower (engine output): approximately 28–34 HP depending on year and condition.
- Rear-wheel horsepower (what you’ll feel on the ground): typically around 20–28 HP after drivetrain losses.
- Year-to-year variation: different carburetor setups, mufflers, air intake, and exhaust components can shift numbers modestly between model years.
In practical terms, stock 400EX units sit in the low-to-mid 30s at the engine and in the upper 20s at the wheels, with testing methodology and wear affecting the exact readout.
Popular upgrade paths and expected gains
Bolt-on basics and their typical gains
For riders aiming for a reliable power increase without overhauling the engine, bolt-ons and tuning are the first stop. Gains depend on how well components are matched and tuned, especially on carbureted engines where jetting matters a lot.
- Intake and exhaust upgrades with proper jetting or tuning: roughly +4–10 HP at the crank, potentially reaching into the mid- to upper-30s HP crank depending on the setup.
- Performance camshaft or head work: roughly +6–12 HP at the crank; total crank power can approach the low 40s HP with good matching components.
- Big-bore kits (e.g., 416–440cc) paired with exhaust and intake tuning: roughly +8–20 HP at the crank; crank output often lands in the 40s to low 50s HP range depending on kit quality and tuning.
With careful tuning and a balanced build, bolt-on strategies can push the 400EX into a noticeable, reliable performance increase without resorting to major internals.
Extreme builds: big bore and forced induction
High-end power and the tradeoffs
For enthusiasts chasing peak numbers, big-bore, forced induction, and nitrous options offer the most dramatic gains, but they come with significant cost, heat management needs, and potential reliability concerns.
- Big bore kits (416–440cc) with cam and exhaust: commonly 40–60 HP at the crank, depending on the specific kit and tuning; wheel horsepower roughly in the 32–50 HP range.\"
- Turbo or supercharger setups: widely used builds push the crank horsepower to 60–100+ HP, with wheel horsepower often in the 50–90+ HP range; reliability hinges on fuel, cooling, clutch, and exhaust gas management.
- Nitrous oxide injections: additional 20–60 HP on a properly tuned engine, but require careful fuel control and monitoring to avoid lean detonation or engine damage.
These configurations deliver the most dramatic gains but demand serious engineering, upgrades to fueling and cooling systems, and ongoing maintenance. They are typically pursued by dedicated racers or very serious hobbyists rather than casual trail riders.
What this means for riders
Practical considerations for choosing a path
Riders should align horsepower goals with use case, budget, and willingness to maintain a higher-performance machine. Stock quads are reliable and approachable for casual riding, while bolt-ons offer a balance of affordability and noticeable improvements. Big-bore and forced induction bring bigger thrills but require upgraded components, professional tuning, and sometimes regulatory considerations for certain events or environments.
Summary
The horsepower potential of a 400EX spans a wide range, from roughly 28–34 HP at the crank in stock form to well past 50 HP with big bore and cam work, and beyond 60 HP in boosted configurations. Real-world numbers depend on model year, condition, and how precisely the engine is tuned. Riders should weigh goals, budget, and reliability when deciding between bolt-on improvements or high-end builds, and consider professional tuning and safe operation above all.