If you're wondering exactly how much horsepower does a 400ex have, the short answer is that a stock Honda TRX400EX usually puts out around 28 horsepower at the crank. Now, before you start comparing that to a modern 450 race quad and feeling a bit underwhelmed, you have to remember that those are "factory" numbers. By the time that power travels through the drivetrain and actually hits the dirt through the rear tires, you're looking at more like 24 to 25 wheel horsepower.
It might not sound like a ton by today's standards, but there is a reason the 400EX is considered an absolute legend in the ATV world. It's not about having the most raw power on paper; it's about how that power is delivered and how long the engine lasts before it gives up the ghost.
The Reality of Stock Power Numbers
When people talk about horsepower, things can get a little confusing. Honda marketed the 400EX as a mid-range sport quad that was meant to be reliable and fun. The engine is a 397cc, air-cooled, four-stroke single-cylinder beast. It's based on the XR400R motorcycle engine, which was already famous for being "bulletproof."
Because it's air-cooled and uses an older four-valve design (Honda's RFVC system), it doesn't rev as high or breathe as efficiently as the liquid-cooled engines found in modern machines. This is why a stock 400EX makes roughly half the power of a TRX450R. But honestly? For most trail riders, 25 to 28 horsepower is a sweet spot. It's enough to get the front wheels off the ground in the first couple of gears, and it's plenty for climbing moderate dunes or flying through tight woods.
Why the 400EX Feels Faster Than It Is
If you just look at the stat sheet, you might think the 400EX would be slow. But if you've ever ridden one, you know it feels punchy. That's because of the torque curve. While a 450cc race quad needs to be kept screaming at high RPMs to make power, the 400EX has a lot of "grunt" down low.
You can lug this machine in a higher gear than you probably should, and it will still chug along without stalling. This makes it incredibly easy to ride for beginners and less exhausting for experienced riders. The power is linear—there's no sudden "hit" that's going to jerk the handlebars out of your hands. It just pulls consistently from the bottom to the top.
How It Compares to Other Quads
To really understand how much horsepower does a 400ex have in context, you have to look at what else was on the showroom floor back in the day.
- Suzuki Z400: This was the 400EX's biggest rival. The Suzuki was liquid-cooled and featured a more modern engine design. It usually pushed about 32-34 horsepower stock. It was faster, sure, but many riders still stuck with the Honda because the 400EX felt more balanced and was easier to work on.
- Yamaha Raptor 350: The Raptor 350 (and the Warrior 350 before it) sat a bit below the 400EX. Those usually hovered around 20-22 horsepower. The 400EX would walk away from them in a drag race every single time.
- Honda TRX450R / Yamaha YFZ450: These are different animals entirely. These quads make 40 to 50 horsepower stock. If you're looking for raw speed and jumping capability, the 400EX isn't in this league. But if you're looking for a quad that will start every single time for twenty years, the 400EX wins.
Waking Up the Engine: Common Power Mods
Most 400EXs you see out on the trails today aren't stock anymore. Because the engine is so simple, it responds really well to basic modifications. If you're unhappy with that 25-wheel horsepower figure, you can "uncork" it pretty easily.
The "Big Three" Mods
Most owners start with an aftermarket exhaust, a high-flow air filter, and a jet kit for the carburetor. Taking the lid off the airbox allows the engine to breathe way better. With a good slip-on exhaust (like a White Brothers or HMF) and proper jetting, you can usually bump that horsepower up to 30 or 31. It doesn't sound like much, but a 10-15% increase is something you'll definitely feel in your "butt-dyno."
Going Big: The 440 Kit
If you really want to chase numbers, the 440cc big bore kit is the classic route. This involves boring out the cylinder and putting in a much larger piston. When paired with a more aggressive camshaft and a larger carburetor, a 440EX can push 35 to 40 horsepower.
The downside? Reliability. When you push an air-cooled engine that hard, it gets hot. Many people who built 440s ended up dealing with blown head gaskets or pulled head studs. If you go this route, you almost always need to add an oversized oil cooler to keep things from melting down.
The Trade-off: Reliability vs. Speed
The reason Honda kept the horsepower relatively low (around 28 at the crank) was to ensure the thing would never die. High horsepower creates heat and stress. By keeping the compression ratio relatively low and the RPM ceiling conservative, Honda created a machine that could survive being pinned wide open across a dry lake bed for an hour.
I've seen 400EXs that have been running on the same factory piston for over a decade. You just don't get that kind of longevity out of high-strung race machines. When you ask how much horsepower does a 400ex have, you also have to ask yourself: How much maintenance do I want to do? If you want a quad you can just gas up and go, 28 horsepower is a small price to pay for peace of mind.
Is It Still Worth Buying Today?
Even though the 400EX (and later the TRX400X) is no longer in production, it remains one of the most popular used ATVs on the market. Why? Because horsepower isn't everything. The chassis is one of the best-handling designs ever made. It's narrow, it's nimble, and it feels like an extension of your body when you're carving through trees.
If you're a heavy rider or you spend all your time at the sand dunes, you might find the 28 horsepower a bit lacking. In the sand, horsepower is king, and the 400EX can struggle to pull up the biggest hills if you're not carrying enough momentum. But for woods, trails, and general farm play, it's almost perfect.
Final Thoughts
So, at the end of the day, how much horsepower does a 400ex have? You're looking at 28 hp at the engine and roughly 24-25 hp at the wheels. It's not a number that's going to win you many trophies at the local drag strip, but it's a number that has provided millions of hours of fun for riders all over the world.
The 400EX isn't about being the fastest; it's about being the quad that's always ready to ride when your friends' more expensive, higher-horsepower machines are in the shop for a top-end rebuild. Sometimes, 28 horsepower is exactly the right amount.