First of all, in case anyone's reading this who doesn't know what a PCV system is and wants to know, let me provide a brief explanation. PCV stands for Positive Crankcase Ventilation, and is the system used on modern engines to deal with combustion gases that blow-by piston rings (or in our case, rotor side seals) and into the oil system (i.e. sump) of the engine. The accumulation of these gases is undesirable because it not only pressurises the sump, valve cover, etc. but also adversely affects the oil itself. Previously, manufacturers used to simply have the crankcase ventilate to atmosphere, but this not only excessively pollutes, but also dripped oil all over roads. So, the solution was to re-circulate these combustion gases and oil vapours back into the engine intake.
In the RX-8, the PCV system is exceedingly simple. As you can see below, there is simply a hose going from the oil filler neck to the intake, just before the throttle. There isn't even a one-way proportioning valve!
(A note here regarding '06+ models and also those that have had the "breather mod" or "ventilation kit" retro-fitted by dealers. This changed the PCV system so that there are two paths from the oil filler neck: one to the intake as before, but another to a pair of ports directly on the intake manifold, as below.)
An oil catch can/tank is commonly used as a supplement to the PCV system, in order to trap or 'catch' any oil liquid or vapour in the ventilated gases and prevent it from going back into the intake, whilst leaving other gases to pass through. This is particularly desirable on turbocharged engines to stop oil vapour coating the inside of an intercooler (which reduces its efficiency) and on track vehicles because oil may be forced uncontrollably by increased G-forces into the PCV system.
So, if not primarily for the aforementioned reasons, why would you want one on your RX-8? There are at least a few reasons:
- It is an unfortunate flaw of the RX-8 that due to its exceedingly straightforward PCV path even the slightest oil over-fill will inevitably end up being regurgitated straight into the intake, making a mess and possibly ruining air filters.
- Even though the MAF sensor is 'upwind' of the PCV inlet, oil vapour seems to still be able to contaminate the sensor, leading to poor running. Also, excess oil can contribute to the throttle plate getting dirty and sticking, also causing poor running.
- Excessive amounts of oil dumped into the intake causes smoke from the exhaust, which although not damaging per-se, doesn't do anyone any favours.
- It is postulated that excess oil being drawn into the intake doesn't do the SSV any good long-term, and may be a contributor to a sticking SSV when oil build-up sludges or carbonises over time.
- It looks shiny in the engine bay.
- Should have some kind of baffling and/or filtering inside, which input must pass through before returning. Any empty vessel doesn't do much!
- Must not vent to atmosphere (even if through a filter) as this is not only illegal for on-road use, but also obviates the entire PCV system. (Note: I can't find any specific reference or regulation for this, but people do fail MOTs for this.)
- Should preferably be mounted below the level of the feed from the valve cover (or rather, in our case, the oil filler neck outlet), so that oil vapours that condense in the hoses naturally run down into the tank via gravity.
- Is preferably made of metal, as composite tanks don't seem to be as durable and might leak (sorry carbon-fibre fetishists!).
- Has a level indicator so that you can easily tell when it needs emptying (not absolutely necessary, though).