Equipment Tests > Air Intakes

Words by Phillip Lord

When you think about the fact that a 4WD regularly used off-road is likely to be driven in water and in dusty conditions, it's important that air filtration is adequate. Whether it's a petrol-engined vehicle that will only sometimes travel on dirt roads or a diesel that'll be crossing deep water everyday, you'll have to put in some time and effort thinking about air filters or snorkels if you want your engine to last. Most 4WD engineers have worked out over the years the intake air priorities for vehicles that will spend time splashing through large volumes of water and gasping through dust. So most 4WDs have the engine's intake ducting sourcing air from a shielded point above the grille or from within an inner guard. These intake points allow little chance of water getting into the engine intake system, and reduce the dust from the vehicle itself from getting into the air cleaner. Many intake systems also incorporate drain points in case a small amount of water does make it through to the filter box area and some also have a pre-filter to act as the first line of defence against dust ingress. But the air filter is bound to get clogged with dust or pollutants, and if the 4WD is used in dusty conditions, it'll be sooner rather than later. Some vehicles have especially poor air pick-up points within the engine bay that, when the vehicle is in convoy or driven on dusty roads with regular traffic, will clog the air filter within a few thousand kilometres.

So why all the dire warnings about avoiding water and dust ingress? The first problem is that water is incompressible, so if a cylinder gets even what seems like a small quantity of water going into it, it may stall the engine and possibly bend a connecting rod or worse. The higher the compression ratio of the engine, the more the likelihood of damage. A low-compression petrol engine may just need plugs removed and the starter cranked a few times to spit the water out to run again - but that's only if you're lucky. If you have got water into the intake of a diesel engine then, with its 20:1 or higher compression ratio, you may as well start counting the thousands in repair costs now. With dust ingress, the problem is not so acutely obvious. Some argue that over time fine dust particles will work their way through the standard paper air filter element and act as an abrasive on the cylinder walls and also contaminate the oil. At the very least, dusty operation will clog any air filter and cause poor performance and consumption if the filter is not cleaned or changed regularly. And in a diesel vehicle there's an additional risk - the mechanical injector pump still sprays the same quantity of fuel, so with less air able to get through you get a less efficient, richer mixture and with it the killer - increased combustion chamber heat. Drive hard enough and long enough with this condition and you'll stand a good chance your oil burner will give its last death rattle.

But you don't have to rush out now and buy the most expensive snorkel and air filter on the market. Your standard factory filter may be all you need. The best approach is to think about the amount of dirt road touring you do. If you know you'll travel on paved roads with only a short dirt drive to a camping ground then you'll not need a great deal more than a standard paper air cleaner. But if, like many of us, you'll be driving on dirt roads regularly to get to where you're going, then there are the following options. First is to keep the factory paper element filter, which will require replacement perhaps as often as every few thousand kilometres (or better still check the filter and shake out the dust at the end of the day's drive - it'll last longer), get a reusable filter that can be cleaned and reused or get a snorkel. If you are driving in extremely dusty conditions regularly, you will also need to consider a prefilter. A prefilter is, as it suggests, an air filter that fits in line ahead of the standard filter, It adds another line of defence against dust particle ingress. Most prefilters are quite restrictive and are recommended for use at speeds of less than 40km/h. Though it's not unheard of for people to install these units and drive flat-out, the risk is that with such restriction on a diesel engine it will enrich the mixture and overheat.

Don't think that by staying with factory filters that you'll be consigned to replacing them every time you go off-road. Pulling out the filter and tapping the dust out regularly is the sensible thing to do, and some standard filters also can be cleaned - Toyota's 100 Series, for example, uses a washable filter. There is also the warranty issue - if your vehicle is under warranty and has an engine failure, one of the parts the manufacturer checks first is the air filter. If it is not a genuine filter, any warranty claim could be compromised. The next option is to keep using paper element filters, but to use the cheaper, aftermarket variety. Some brands are the same as used as original equipment by the manufacturer, except they do not have the car-maker's symbol on the box. This cheaper option takes the pain out of having to replace the filter every 5000km (or less) in dusty conditions instead of the routine 20,000km.

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The popular aftermarket alternative in air filtration is the reusable filter. Usually made from a foam or cotton material impregnated with a light sticky oil, these filters simply require cleaning and re-oiling when they become clogged with dust. Some of the manufacturers of this type of air filter argue that their designs allow a better flow of air than paper element types (even when dirty), allow smaller particles through than paper types and are cheaper in the long run because they can be re-oiled and reused, in some cases for the life of the vehicle. Some are also resistant to catching fire in the instance of an engine backfire. But if you prefer maintenance-free replacement, then the reusable filter may not be for you. The process of cleaning is simple but not a five-minute job; you have to bathe the filter in a cleaning agent, wash it, allow it to dry and then spray it with special oil and then wait for the excess oil to drain out before refitting.

There are a few considerations for buying a snorkel set-up. The obvious question is whether you plan to go off-road where you may need to cross water. Not so obvious is the chance there could be water where you're not expecting it. Heading inland towards our great deserts, for example, may seem like the least likely opportunity to cross any water. But with the heavy rains that have been experienced in recent years, you may find roads deep under water and, if not impassible, they will probably require a good air intake system such as a snorkel to safely get through. The same applies to river crossings - you may think the areas you plan to drive in have only shallow water crossings, but rain can quickly raise the level of the water. A snorkel is cheap insurance. Another strong argument for fitting a snorkel for off-road use applies to certain vehicles that have poorly designed air intakes. Some Jeep Cherokee and Toyota HiLux models, for example, are known for their less than perfect air intake protection for water crossings.

But people who never cross water in their 4WD may still want to consider a snorkel - it's a good way of ensuring your engine gets cleaner air. Because it is positioned high up on the vehicle's body, the snorkel intake gets cooler air than an in-guard or engine bay air intake. And because it is placed so high, it is in an air stream less dusty than a standard intake position. Some snorkel manufacturers argue the cooler air means more power, better efficiency - which is a hard theory to argue with - but in practice the gains, if any, are minimal. Keep in mind that a certain snorkel might be the right design to fit your 4WD but not your model. Make sure that the snorkel has been airflow tested for your particular engine - a turbo-diesel can require more than 40 percent more air than a petrol engine. It also pays to look closely at how the connections are designed before a snorkel is fitted - if it looks and feels as if it has loose connections then you could risk dust and water ingress.

Though the air intake looks more exposed on a snorkel, their manufacturers say no. Even in a tropical rain storm, they say, only a tiny volume of the impact moisture will enter the snorkel. Fitting a snorkel isn't as easy as fitting a new air filter. It takes about two hours, and involves cutting a large hole though the guard and attaching the snorkel to the guard and A-pillar. Though you could remove the snorkel to sell or transfer to a new vehicle when you sell your 4WD and get a panel beater to repair the holes, it is worth pricing a new snorkel first - it may end up not costing any more than the repair and they can add resale value to the vehicle. Whatever your choice, make the right one for your usage and your 4WD will last longer, and give you better performance and economy in the bargain.

 

 

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