Wikipedia’s definition of a “Vehicle Snorkel” contains a short description of the function of a snorkel when fitted to military as well as civilian 4wd vehicles. This entire description centres on enabling these vehicles to wade through relatively deep water. This is indeed the initial function of a snorkel and the reason it was invented in the first place. Director of Ironman 4x4 Africa explains the importance of a snorkel and why you should consider one for your overlanding vehicle:
On most 4wds, the air intake sits inside the front fender above the front wheels. The height of this air intake nozzle most often determines the vehicles wading depth but strictly speaking there are other factors at play here as well. According to a well known 4wd Fundi, more vehicles are damaged in water than any other obstacle. It would make sense then that any precaution taken to prevent water from being sucked into your engine is money well spent.
One has to remember though that fitting a snorkel by itself is not going to increase a vehicles wading depth. There are many other areas such as electronics that will need water proofing as well. Realistically, deep water crossings are not an everyday occurrence and if, like me, you tend to avoid them, chances are that fitting a snorkel will not be high on your list of touring accessories for your vehicle.
There is however another often overlooked function of a vehicle snorkel that offers, in my opinion, a more practical benefit for your 4wd. With the air intake sitting above the front wheel well, any amount of dirt road travelling is going to cause an increased amount of dusty air being sucked into the induction system of your 4wd. It should off course be cleaned by the vehicles air filtration system. The air filter will however soon become clogged if it is doing a proper job.
A dirty, clogged air filter will restrict the ideal flow of intake air to the engine leading to an increase in fuel consumption and a loss of engine power. It may be minimal but it will exist and worsen if left unserviced. Over time you will feel it in your pocket. In extreme cases you may even suffer an air filter medium failure. This would allow dirty, dusty air to be sucked into your engine unhindered causing all kinds of expensive problems.
The design of the typical 4wd snorkel moves the air intake aperture from inside the fender to outside the vehicle at roof height up along the A-pillar. Cleaner, cooler air is now sucked into the induction system. Some snorkel designs even allow you to turn the snorkel head around for severe dust, rain or snow conditions. Reputable snorkel designers engineer their products to actually induce a small amount of “Ram” effect when the vehicle is moving forward at speed. The design of the snorkel head thus causes the air to be forced into the snorkel body and induction system for a slight boost. The air temperature at snorkel intake height is also most often a couple of degrees cooler than the air in and around the engine bay.
If you do any amount of driving on gravel roads, you should strongly consider fitting a snorkel to your 4wd or bakkie. Cleaner, cooler intake air will save you money and maintenance costs in the long run.