what happens if you drill holes in your atv muffler

I drilled a 1/2 inch hole in the end cap of my old utility quad. It got louder but the engine started popping on decel and felt sluggish on hills. Ended up welding a patch back on. Not worth it unless you're going to rejet and maybe repack the muffler.

Did this to a Raptor 350. Sounded awesome for about two rides, then the hole started to whistle and the neighbor complained. Also lost a bit of bottom end. If you only care about noise, go for it, but expect to retune the carb.

You're messing with the exhaust scavenging. On a 4-stroke, drilling can actually reduce power because you lose the tuned length effect. Better to buy a slip-on with a removable baffle if you want adjustable sound.

I did it on a cheap Chinese quad and it didn't seem to hurt anything. But those things are already gutless. On a Honda 400EX, I'd be careful. The backpressure might affect the valve timing slightly. At least check your plug color afterward.

Just be aware of noise laws. Some trails have strict decibel limits. If you get caught with a drilled muffler, you might get ticketed or banned. Plus it's hard to reverse if you change your mind.

I've seen guys drill holes to reduce backpressure on purpose, thinking it helps top end. On a stock engine it usually just makes it flat. You need to match the exhaust to your cam and compression. Otherwise you're just making noise.

Partial disagree with the above – on a 2-stroke, drilling can actually help if you're ported. But on a 4-stroke like the 400EX, you're just losing low-end. I'd suggest a different approach: remove the baffle instead of drilling. Same effect, easier to reverse.

I drilled a series of small holes around the outlet of my Kodiak 450. It got louder but also started backfiring. Turned out I leaned out the mixture too much. Had to adjust the pilot jet. If you do it, be ready to tune.

Don't do it. It's a hack job. Save up for a proper exhaust system. You'll get better sound, more power, and you can tune it properly. Drilling just makes a mess and devalues the quad.

I did it on a beater quad and it actually helped because the muffler was partially clogged. The holes let it breathe again. But that's a special case. On a clean muffler, I wouldn't bother.

You'll also get more heat near the hole. I've seen the metal discolor and even crack after a while. Plus the hole can catch mud and debris. Not a great idea for trail riding.

If you do it, start with one small hole and test ride. Check for any change in throttle response. If it bogs, you went too big. And definitely rejet – you're changing the exhaust flow, so the carb needs to compensate.

I'm a mechanic. I've seen customers drill mufflers and then complain about poor performance. The factory engineers spent a lot of time tuning that exhaust. Drilling randomly is like guessing. Better to follow a known mod guide if you must.

On my 700XX I drilled three 3/8 holes in the inner baffle. Sounded mean but lost all low-end grunt for rock crawling. Had to put a reducer in. Now I just run a full aftermarket system.

The main risk is leaning out the engine. Lean condition can cause overheating and even hole a piston. If you're not comfortable checking jetting and plug readings, don't drill. It's not just about noise.

I drilled a hole to drain water out of a flooded muffler. That worked fine. But for sound? Not recommended. Get a louder pipe if that's what you want. Drilling is a permanent mod that rarely ends well.

What about the spark arrestor? Drilling might remove or bypass the screen. That's illegal on public land in many places. You'll get a fine if a ranger checks your muffler.

I tried it on a Polaris Scrambler. It actually gained a little top-end rpm but lost bottom. Ended up putting a screw in the hole to partially block it. Not a clean solution. Just buy a slip-on.

You could also consider a different mod: remove the snorkel or add a free-flow air filter first. That might give you the sound change you want without butchering the exhaust. Then if you still want louder, do the muffler properly.

Honestly, I'd leave it stock. The 400EX already has a decent sound. Drilling holes is a cheap mod that usually causes more headaches than it's worth. Spend the money on tires or suspension instead.