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Some RX-8 owners have been recommending installing an oil catch can, to prevent problems from "overfilling" the oil.

I assume this doesn't mean spillover (does it?), but instead, too much oil in the system. The idea is that this oil can make it into the intake and cause problems.

How does oil relate to the intake system and how does a catch can fix this problem? Does this alleviate the risks of filling the engine with too much oil?

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Requesting tag help for new tag: "intake" or "air-intake" –  NickC Mar 12 '12 at 8:43

2 Answers 2

Typically it's to catch the blow-by. While the PCV system is designed to vent just air/fumes as Nick C mentions, as an engine wears it'll start to blow some liquid oil past. That's normally redirected to the intake so it'll burn off (as oppposed to just dripping it on the ground like the old cars used to). Car enthusiasts like to install catch cans (that need to be periodically emptied) to catch the oil before it goes in and contaminates the intake. While nice to not get oil all over your intake, it's primarily done on turbocharged cars to prevent the oil from working its way into the intercooler. An intercooler that's collecting oil doesn't work as efficiently...

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Thanks. Apparently, on RX-8s, there is a risk of the oil gumming up the throttle body, and causing the secondary shutter valve to stick, which can cost up to a couple thousand dollars to fix. –  NickC Mar 12 '12 at 17:39
    
This is all true. But along with dirtying everything on the intake side, it also lowers the effective octane of the mixture, which can lead to detonation. –  Nick Feb 10 '13 at 21:49

Most, if not all, internal combustion engines have a breather on the crank case, to allow fumes to escape from the oil pan. In a lot of cars (presumably including the RX-8), this is vented into the inlet manifold to prevent the fumes being released into the atmosphere. Installing a catch can in this breather can prevent atomised oil evaporating up the breather and being burnt in the engine.

The other issue is that Rotary engines (as fitted to the RX-8) are notorious for oil-related problems, which IIRC can lead to the tips wearing away, leading to the engine requiring a rebuild sooner than it might otherwise.

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I think a reason why overfilling a piston engine mightn't be as bad as overfilling a wankel engine is that in a piston engine there is a much larger distance between where crank is whipping up oil and the vents which are often both in the head, where as a rotary engine seems to have much smaller volume (timing gear inside of the rotor) where extra oil can be whipped around with blow-by gasses furiously blowing by. I can't find any good resources on the lubrication system in the wankel, so this is all just speculation on my part. I don't even know where the vents are. –  kahbou Mar 12 '12 at 12:29

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