For those with engines that makes use direct fuel injection - there's a lot of benefits. 2 major ones are increased fuel efficiencies and power output. For turbo applications, air is compressed and couple this with pressurized fuel squirted in the combustion chamber makes a hack of a lot more power then trying to mix air and petrol before getting it sucked into the chamber. It burns a lot cleaner as well and that is where he efficiency is coming from.
But there are issues - the "old" way of mixing air and petrol means that petrol would be in contact with the back of the intake valves and this help reduce the build up of carbon. Being that direct injection engines don't do this - coupled with the German fixation on emmissions - carbon buildup is eminent.
Here's an illustration of the difference -
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The thing is that to reduce emmissions (pollution) caused by cars - engineers designed the ride in such a way that if there is a possibility to recycle the toxic gases that is created by the combustion processes they would. So blow-by gases that escapes the piston rings (remember about the pressurized air and petrol) when combustion happens. These gases would be sucked out via the vacuum from the top of the engine back to the intake manifold in an attempt to burnt it off in the combustion chamber - in other words the blow by gases gets into the combustion chambers as well. Other than the blow by gases - it turns out that engine oil would also seep through the piston rings - amount varies depending on the state of the rings and it get vaporized and mixed with the gases.
So BMW put in place an oil separator in between the engine and the intake manifold so that the EO would be collected and dripped back into the oil pan. So if that is the case then all should be well but we still see a whole bunch of carbon built up on the back of the intake valves. And carbon is typically not a good thing.
So here's where an oil catch can would help - it would condense the amount of contaminants getting into the manifold and provide another level of preventive maintenance in addition to the oil separator. And thirdly and this is just me - it help me figure out that state of the the piston rings in my ride.
So I installed an OCC and recently I checked what is in there - pulled out the OCC dip stick .......
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Dipped a string in there and here' how sludgy it is ..... I reckon if it not in the OCC that stuff could be all over the manifold or caked up as carbon on the intake valves. Doesn't help that EO is hygroscopic and the combination of moisture and blow by gases laden with oil would accelerate the carbonization.
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Fortunately there's not much in volume (~100ml). Then again some people would argue that that's because I have installed the OCC over a very hot part of the engine bay (right over the turbos). If a lot of oil is collected - then maybe this is why some cars need regular oil top ups.
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But I am optimistic that the piston rings are in good shape and I do have a vent on my bonnet just right on top of the OCC which helps with the condensation.
At the end of the day - for direct injection engines - an OCC is a good mod. Just watch for the following -
- Position it where condensation is possible
- reduce the restrictions by having hoses that are equal or even larger than stock PCV hoses
- choose OCCs with internal baffles that do not create restrictions
And by restrictions, I mean the airflow created by the vacuum that goes through the OCC.
Again this is just my personal opinion.......
