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.

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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.

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