If you're looking to squeeze more speed and torque out of your Honda, picking the right 400ex performance upgrades can completely change how the machine feels on the trail or the track. The Honda TRX400EX is arguably one of the most reliable and iconic sport quads ever built, but let's be honest—by today's standards, it can feel a little sluggish compared to modern 450s. The good news is that this engine is incredibly over-engineered, meaning there's a ton of untapped potential sitting right under your seat just waiting for a few bolt-ons and some internal tweaks.
Starting With the Big Three
Most riders start their journey into 400ex performance upgrades with what the community calls "The Big Three." This refers to the intake, the exhaust, and the jetting. From the factory, the 400ex is notoriously choked up to meet noise and emission standards. It's basically trying to run a marathon while breathing through a cocktail straw.
First, look at the exhaust. A good slip-on muffler helps, but a full system is where you really see the gains. Brands like HMF, Sparks, or Big Gun have been the go-to choices for years. You'll notice a much throatier sound, but more importantly, you'll feel the engine revving more freely.
Next is the intake. Tossing the stock paper filter for a high-flow foam filter (like a Uni) or a K&N setup lets more air into the mix. Some guys go as far as removing the airbox lid entirely for maximum flow, but if you ride in deep mud or water, keep that lid on and just drill some holes with Outerwears pre-filter vents.
Finally, you can't do the first two without re-jetting the carburetor. If you add air and exhaust without adding more fuel, your bike will run lean, get too hot, and eventually pop. Jumping up a few sizes on the main jet and adjusting the needle clip usually does the trick. It's a cheap mod, but it's the most important one to get right.
Stepping Up to the 450R Carb Swap
Once you've done the basic bolt-ons, you might still feel like the throttle response is a bit "mushy." That's because the stock 400ex carb is a constant velocity (CV) style, which relies on vacuum to lift the slide. It's smooth, sure, but it's not snappy.
One of the most popular 400ex performance upgrades is swapping out the stock carb for a Keihin FCR carb from a Honda TRX450R. This is a pumper carb, meaning it squirts a literal shot of fuel into the engine the second you flick your thumb. It makes the front end come up much easier and gives the quad a much more aggressive, "race-ready" feel. You'll need an adapter kit or some creative plumbing to make it fit, but the difference in "snap" is night and day.
Internal Engine Mods: Cams and Pistons
If you're ready to crack the engine open, this is where the real power lives. The 400ex is an air-cooled, single-cam engine, which makes it relatively easy to work on even if you aren't a master mechanic.
A "Stage 1" or "Stage 2" Hot Cam is a classic choice. A Stage 1 cam usually boosts your low-to-mid range torque, which is perfect for tight woods riding. A Stage 2 cam shifts that power to the top end, which helps if you're doing wide-open desert riding or amateur MX.
While you're in there, swapping the stock piston for a high-compression version (like an 11:1 or 12:1 ratio) will give the bike a lot more "grunt." Just keep in mind that once you go above 11:1 compression, you might need to start mixing in race gas to avoid detonation. For a reliable trail build, an 11:1 Wiseco or JE piston is usually the sweet spot for running 93 octane pump gas.
Big Bore Kits: 416 vs. 440
When people talk about serious 400ex performance upgrades, the conversation always turns to big bore kits. The two most common paths are the 416cc and the 440cc builds.
The 416cc kit is widely considered the "reliability king." It involves a slight overbore of the stock cylinder. It runs cooler than the bigger kits and usually doesn't require heavy-duty head studs. It's just enough of a boost to notice a significant power increase without turning your quad into a ticking time bomb.
The 440cc kit, on the other hand, is for riders who want maximum power. It requires a larger sleeve and generates a lot more heat. If you go this route, you must upgrade your head studs. The stock 400ex studs are known to "pull" out of the block under the pressure of a 440 kit, leading to blown head gaskets. If you do it right with heavy-duty studs and perhaps an oversized oil cooler, a 440ex can be a beast.
Handling and Suspension Upgrades
There's an old saying in the racing world: "Power is nothing without control." You can have a 50-horsepower 400ex, but if you're still riding on the original 20-year-old shocks, you won't be able to use any of that power.
The stock front shocks on the 400ex are fine. They work. But they're stiff and don't have much adjustment. Swapping them out for a set of Elka Stage 3s or even used shocks from a TRX450R (with some slight mounting modifications) will make the quad float over whoops that used to rattle your teeth out.
Don't forget about the width, either. Adding a set of +2 A-arms and a wider rear axle makes the 400ex feel incredibly planted in the corners. The stock quad is a bit narrow and "tippy," so widening the stance is one of those 400ex performance upgrades that actually makes you faster because you can carry more speed through the turns without feeling like you're going to flip.
Gearing and Traction
Sometimes the best "performance" upgrade isn't more horsepower—it's how you use what you already have. Changing your sprockets is the cheapest way to change the personality of your quad. If you want more low-end punch for wheelies and climbing hills, drop one tooth on the front sprocket (usually going from a 15-tooth to a 14-tooth). You'll lose some top-end speed, but the bike will feel much more alive in the first three gears.
Tires are the final piece of the puzzle. If you're still running the stock Ohtsu tires or some cheap, heavy "knobby" replacements, you're missing out. A set of Maxxis Razrs or ITP Holeshots provides way better bite. When the tires actually hook up instead of just spinning, you'll realize just how much power you were leaving on the ground.
Keeping it Cool
Since the 400ex is air-cooled, heat is your biggest enemy as you start adding power. Many riders install oversized aluminum oil tanks or secondary oil coolers to help the engine shed heat. Another simple trick is adding "air scoops" that attach to the frame and funnel air directly onto the cylinder head. It sounds like a small thing, but keeping that head temp down is vital for the longevity of your engine, especially if you've gone the big-bore route.
Wrapping It Up
At the end of the day, the 400ex is a platform that grows with you. You can start with a simple pipe and jet kit, and a year later, find yourself building a 440cc monster with long-travel suspension. The beauty of these 400ex performance upgrades is that the parts are everywhere, the info is easy to find, and the quad is easy to work on in your own garage. Whether you just want a little more pep for the weekend or a fully built racer, these mods will help you get the most out of one of the best ATVs ever made. Just remember to take it one step at a time, and always make sure your suspension and brakes can handle the extra speed you're building!