Depends on if you have a diesel or petrol, turbo or not.
The most common reason for mounting one is if you have a turbo diesel with Exhaust Gas Recirculation (EGR). ie. any modern-ish turbo diesel. The egr does exactly what the label says, it recirculates exhaust gasses back into the engine through the intake. Some people delete the EGR though the legality of that is of course questionable and even the use since it seems engines with egr need it to run at their best. In any case, I digress. The exhaust gasses are recirculated very close from the source so it has not passed any emissions devices yet and thus contains the most amount of soot. In a clean car this should not be a problem but over time soot will start to stick here and there. Now add to that the Positieve Crankcase Ventilation (or PCV) and things will start to get messy. The PCV is basically the over pressure valve of your engine. As you load the engine, exhaust gasses slip past the liners and create a higher pressure in the crankcase, This pressure buildup needs to go somewhere else your engine will start leaking past the seals. Normally the pressure will escape to the top of the engine where there is a (small) tube from the valve cover that is connected to the air intake. This is done because the oil is hot and there is vapour hanging around. By sucking this back into the engine we make sure it gets burned nice and clean in stead of just drip or drift around in the environment.
What happens is that the oil vapour, or at least some of it, cools down in the metal air intake and turns back into a liquid and starts sticking to the intake, we've all seen it I guess? Add to that sooty air from the exhaust and it will clog up your air intake in "no time" since the oil/soot mixture will form a physical barrier.
The same story goes for non turbo diesel although some argue the effects are less. I am not sure about that but I guess the turbo creates even more pressure in the cylinders and with that potentially more blow by.
As for petrol engines, theoretically they do not need them since the exhaust gasses are so much cleaner it won't cause that kind of buildup (in practice they do but a lot less). The exception could be those modern direct injected engines. Since fuel is injected into the cylinder rather than the intake fuel no longer passes the intake valves and the egr starts blocking them up. A real menace these days...
All in all, I mounted a provent (200) on my petrol rover V8 and although it works keeping my inlet clean, the problem that arises with a petrol like that and that bloody crap bio ethanol (which I no longer use) is that the fuel does contain quite a bit of water, even proper fuel does. What happened in my case is that water vapour got caught in the catch can and eventually the filter got clogged by a mix of oil and water. I guess it had run it's course with some 30K on it, it just never got black like a diesel engine would The end result was that oil started leaking past my seals, and since it is a land rover...
In short, any diesel engine I would definitely recommend but it is not a cheap solution. The filters cost quite a bit and need regular replacing. A petrol engine can have benefits, but less so then a diesel and the costs vs benefit might not work out for you.
Cheers!
-P
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