Most people consider the air intake, and specifically the air filter itself, when starting down the road of modifying their car. It’s a commonly held belief that the stock airbox is restrictive, and more power is Airpod case available with a better filter element, or a better box itself. Whilst it’s pretty rare to see repeatable, dyno proven power increases from most readily available airbox mods, it’s still an area to consider when modifying your car. The stock system in modern cars, especially turbocharged or supercharged variants usually have very good stock airboxes, and one inexpensive way to improve your own airbox is to use parts from the more powerful versions, such as the seat 150 intake trumpet in the VW polo.
Car manufacturers go to great lengths to keep the airflow up, the noise down and the filtration within decent limits. Generally a better flowing element will filter less dirt, so it can be hazardous to the long term reliability of your vehicle to choose the absolute best flowing aftermarket filter out there. Big brand names such as K&N run oiled elements, that have the benefit of being user serviceable and washable, but the downside of oil being lethal to your MAF ( manifold absolute pressure ) sensor usually just downstream of the filter. That’s not to say an oiled element will kill it for sure, but it’s often hard to balance enough oil to put on the filter to trap dirt, whilst still being too little to be sucked off by the engine operation. Oil acts to trap the fine dust that often passes through dry element filters, so it also comes down to the environment you will be operating the vehicle in. Oiled filters are the norm in motocross bikes for instance. The stock element will flow sufficiently to produce the power level set, and to last the required service interval, and in the least it’s a great idea to consider a more regular filter change in dusty conditions, as a paper factory filter is usually quite inexpensive.
Still the best element in the world will be wasted in a poorly designed box. Airflow is fairly easy to estimate, and things like sharp corners, 90deg bends and drawing from one end of a long filter all drop the airflow through the box. Pod filters, a common aftermarket upgrade have the benefit of a more uniform exposure of the filter to the airflow, as they are usually circular, and allow a straighter inlet that most factory air boxes. Certainly in the standard Golfs and TDI’s the glaringly obvious point of restriction would be the 90 degree outlet in the airbox lid, where a pod or a lid with a nice curve would help increase the airflow.
The downside of Pod filters and other aftermarket filters is the fact they are illegal unless sealed to the outside, and they usually raise the induction noise considerably. Add to this the simple fact that drawing air directly from your engine bay with an unsealed pod or filter will drop your power levels, especially down low, and this is magnified several times in a turbocharged vehicle due to the heat given off by the hot turbo itself. If you are thinking of adding an aftermarket filter box or pod, make sure it is sealed to engine bay air, and either draws through the stock exterior inlet, or to another suitable hole preferably on the front of the car. Remember to keep it high enough so if you ever drive through water you won’t pickup an engine full of water – water doesn’t like to compress and it’s a common story to hear of someone’s trick air inlet ingesting a lung full of water and bending engine internals such as conrods. This is ESPECIALLY important in high compression diesel vehicles, water is a major threat to your oiler!
A well designed aftermarket box should have smooth contours, keep the airflow as simple and straight as possible, use big sweeping bends, and be sealed from heat and engine bay air. Mount your pod away from anything hot, and there are rather inexpensive inlet heat covers available to help keep all that nice cool exterior air cold on the way to your engine. Remember too that once you add an aftermarket filter, it will become your responsibility to service it and replace / wash the element yourself, as the general mechanic probably will leave it alone, as he probably won’t have the replacement parts!