Repair/Maintenance | Motorcycle Habit

All motorcycles have some sort of air box or air filter that came on the bike when it was new. A lot of people over the years ditched the big black airboxes for smaller and better looking individual pod filters. There are three main differences between pod filters and air boxes; you'll need to consider these three things if you're caught deciding between the two. So what are the differences between an air box and pod filters? Compared to pod filters, an air box controls the amount of air getting into the carburetors, has a built in air filter, and protects the engine from water getting inside. While a pod filter technically does some filtering, an air box is much more efficient. I have owned many motorcycles, some with air boxes and some with pod filters so I've been able to get a good idea about how each of them work. As an engineer myself, I have to put in a plug for the engineers who designed the air boxes originally. They were there for a reason, and any changes that enhance the aesthetics of a motorcycle but has a potential to cause harm to the internals of the engine should not be done. The original air boxes are absolutely ugly and bulky, I get that. But they serve many purposes and protect your engine from serious damage. I love the look of pod filters, but most of them aren't near as good as an air box. Keep reading below to hear some of my recommendations. An Air Box Controls the Amount of Air Getting Into the Carburetors I’m a nerd and enjoy learning the technical explanations behind why things work the way they do, so I’ll try to explain some of these chemical and fluid properties as simply as I can so that everyone can understand why air boxes are superior in function to pod filters. It all has to do with stoichiometry and the amount of air flow measured in CFM. Let me explain. An example I like to use with people is campfires. Most of us have had the experience of sitting around a fire at night with family and friends and the fire gets a little low so someone puts on a whole bunch of wood piled right on top of the flames, and that puts the fire out completely. That is stoichiometry. There has to be a balance of fuel to air in order for the flame to keep burning. The same is true for any internal combustion engine on a car or motorcycle. If you have way too much gas then the spark plug will not be able to light off the gas, or in other words you have “flooded” the engine. I’m sure most of you have heard that term, and now you know what it means. There is too much gas in the combustion chamber to light off and not enough air. See my article here for a tutorial on what to do if your engine is flooded. The perfect stoichiometric value for a combustion engines is 14.7 parts air to fuel, also called the air to fuel ratio. So for every molecule of fuel in your combustion chamber you need 14.7 molecules of air. It doesn’t have to be exactly 14.7 in order to start and run, that’s just the best value to shoot for. If you have too much gas it is called running rich. If you have too much air it is called running lean. Running your engine lean can be dangerous because the gas can explode inside the combustion chamber too early and cause engine knocking and overheating. Running your engine rich isn’t necessarily dangerous, it will just cause your engine to run sluggish and you’ll go through a lot more gas than needed. Click here to see my article about how to fix a motorcycle running rich. Once you’ve owned your motorcycle for a while and you’re used to it’s little quirks you’ll be able to tell if it’s running lean or rich by the sound and smell of the exhaust when it’s running. Now that you understand stoichiometry and air to fuel ratios I can explain how this relates to an air box vs pod filters. An air box from the factory comes with one or more small openings somewhere in the box that pulls in air from the outside and routes it through small passageways, through an air filter, and into your carburetors. These small openings and air filter are important because they decrease the amount of air getting pulled into the carburetors. The cheap pod filters that you see on a lot of café racers, scramblers, and trackers on today’s motorcycles really do look cool, but they have WAY bigger openings on them than the original airboxes, which lets in the maximum amount of air constantly. So if you have 2-3x the amount of available air but the same amount of fuel, your air to fuel ratio is now way off and the fuel will not combust in the combustion chamber like it should. You’re losing out on large amounts of power for the sake of looking a bit cooler. Air Boxes Have A Built-In Air Filter The air filter inside the air box serves two purposes. It slows down the air flow inside the air box, and it cleans out all the dust and junk in the air that you don’t want getting inside your engine. Most pod filters do not have a built in filter. They might say the word “filter” in their product description online, but about the only thing that pod filters will keep out of your engine are small boulders. They won’t stop dust. The first function of the air filter is to slow down the air flow inside the air box. This relates back to what we were just talking about. You don’t want the maximum amount of air getting rushed through the air box, you want it to be somewhat restricted so your air to fuel ratio stays as close to 14.7 as possible. The mesh/foam substance that the filters are made out of inside air boxes does a great job at slowing down air flow. The second purpose of an air filter is to stop dust and particles from getting into your carburetor and engine. Air filters are tiny mazes of passages that the air has to find its way through in order to get to the carburetor. A good filter will have as many microscopic twists and turns as possible so the air is constantly touching edges of the foam, and any dirt specs or dust in the air gets trapped on the foam. As mentioned earlier, pod filters do a terrible job at stopping dirt and dust particles. Most pod filters only have one layer of small wire mesh and not even any foam at all. Wire mesh is not going to stop dust from getting into your carbs and engine! No matter how cool it looks, they aren't near as efficient with filtering the air. Let’s talk about what dirt and dust inside your carburetor can do. Carburetors have a ton of tiny passages inside them where gas is supposed to flow, so when you try to shove a piece of dirt through a hole where only fluid is supposed to fit then it gets clogged in a hurry. Literally one piece of dirt in a carb can clog up the entire thing and make it run horribly. The worst case scenario is when dirt makes it through your pod filter and your carburetor and gets into the engine. When dirt gets into the combustion chamber it obviously can’t burn, it just gets hotter and hotter. Hot dirt inside your combustion chamber has a tendency to want to stick to the piston or cylinder walls. It welds itself to your engine wall, then gets broken off, and repeats that process over and over. This usually ends poorly with a loud bang and an expensive trip to the mechanic for a top end rebuild. If you have found yourself stranded because of engine problems, click here to see my recommended list of creative ways to tow your motorcycle. Air Boxes Protect Your Engine From Water Getting Inside This is the third and probably most important reason why air boxes are superior to pod filters. Because of the way that air box passages are designed, it’s almost impossible for water to get into the motorcycle combustion chamber unless the motorcycle was completely flooded with water. Water getting into your engine is extremely dangerous, and if any water does get in there you SHOULD NOT try to start it. Pull the spark plugs and get all the water out before you ever try to crank the motorcycle. Let me nerd out for a second and explain why water inside your combustion chamber is dangerous. The first reason is because it can instantly stall your motorcycle if you were riding in the rain and water went through your carburetor and into your engine. Imagine riding in the rain and stalling. Riding a motorcycle in the rain is dangerous enough, now you’re adding in another factor of safety since you’ll now need to try to merge through traffic and pull over. Water doesn’t combust and won’t keep your engine running. Most people know this, but you’d be surprised how many times I’ve had this conversation with people. The second reason why water inside your combustion chamber is dangerous is because when water gets really hot it boils and turns into steam, right? Steam has a much greater volume than liquid water; the gas takes up much more space than the liquid does. There’s very limited space inside the combustion chamber as the piston moves up and down, so now you have a superheated vapor trying to take up as much of that space as possible, as well as the burnt fuel vapor also trying to take up the same amount of space, which drastically increases the pressure on top of your piston. Too much pressure inside the cylinder can cause piston cracks, burnt piston rings, bent intake or exhaust valves, head gasket ruptures, etc. All of which are usually expensive fixes. The money isn’t the big concern though, your safety is. You never want to be riding down a road and have a catastrophic engine failure, it can throw you right off the motorcycle and into the path of a vehicle. See this article I wrote that discusses more about the effects water can have on a motorcycle. So my question to everyone is: Why would you trust your personal safety to a $5 metal mesh pod filter that doesn’t really filter anything? If You're Going To Use Pod Filters I completely understand that pod filters look so much better than air boxes. They’re minimalistic, shiny, and help achieve that open-frame look that so many people are going for right now. I've used them several times myself. But if I can’t convince you to keep your air box then the second best option is to get a better quality pod filter that actually has a foam filter inside. This won’t take away the risk of water getting in or having an incorrect air to fuel ratio, but it will take away the dust and dirt risk. If you still go with pod filters, here are some pointers to help you make sure you don’t ruin your engine. When you wash your motorcycle always make sure you have grocery bags over the pod filters so no water can get on them, and NEVER ride in the rain if you have them. Remember, the smallest bit of water in the engine can cause a pressure explosion or hydrolocking. Your only good option for correcting the air to fuel ratio after installing the pod filters is getting much larger fuel jets inside your carburetor. Because with more air coming in you need to add a bunch more fuel as well. This does not completely fix the ratio, but it can help. A jet is a small metal cylindrical piece inside your carburetor that the gas flows through. You'll need to remove the carburetor from the motorcycle and take it apart to install the larger fuel jets. As a disclaimer, you may notice that I have pod filters on some of my motorcycles shown on my blog. Yes, I have used pod filters before but that is why I know so much about the difference between them and an air box. Related Questions Why is my motorcycle running sluggish and slow? Any throttle response and sluggish acceleration problems are almost always caused by a dirty carburetor. A lot of people are unaware that a routine carb clean can save you a lot of hassle. Cleaning the carburetor solves so many problems with throttle response and sluggish acceleration. If I get dirt or water in my engine how do I get it out? If you know that something got through your carburetor and into your engine then DO NOT try to start it. Pull the spark plugs and see if you can get it out with a flexible magnet or small vacuum hose. If not then take the motorcycle to a mechanic to get it out.