Oil Catch Can Splatter

[SLR No.77]

If there's a feed back to the inlet manifold it is vented, the manifold is not a sealed area.  If a catch tank is fitted without the ability to vent somewhere, a breathing cap, or a feed to under the car or back to the intake, at some point something will need to give and a hose will be blown off, or worse something more significant such as a crank end oil seal.

[BrettJ]

With a dry sump is the catch can pressure created from the blow-by or the oil pump cavitating?

[SLR No.77]

With a dry sump it's the dry sump tank that is usually vented to the catch tank rather than the engine itself. Even with blow by gases the engine will run at a negative pressure due to the scavenge pump trying to create a vacuum, this scavenge then fed into the dry sump tank creates a slight positive pressure in the dry sump tank which is a combination of oil, blow by gases and air. It's this tank that then needs a catch tank.

[MattB]

I think there's some confusion over the terminology.

A vent is a small opening that allows gases, vapours and oil mist to escape from a container of some sort.  It doesn't have to escape to atmosphere to be a vent.

In the case of my 420 the rocker cover (and hence the crankcase) is vented, and vents to the top of the dry sump oil tank. The tank is in turn vented, and vents to the catch can.  The catch can is in its turn vented, and vents (in my case) to atmosphere, but it could easily vent to the intake manifold.

If the catch can doesn't have a vent path - to atmosphere, the intake manifold, or anywhere else - the pressure in the crankcase, the oil tank and the catch can will simply rise until the pressure is the same throughout. There will be no flow of gas/vapour/liquid from the engine (as there's no pressure differential to create a flow) and so the catch can becomes useless.

Note that there are issues with connecting the catch can vent line to the inlet manifold, particularly the need to ensure that the flow can't reverse (especially relevant with fi engines). The one-way valves fitted for this reason can block up, and are often the reason that the pcv system fails altogether.

MattB

[GulfSeven]

The odd thing is that on factory cars the can has a vented cap and an outlet hose to the inlet manifold. Why the need for holes in the cap at all?

[SLR No.77]

#55  That might just be a throwback to catch tanks (or washer bottles as Caterham have been using) being vented for dry sump installs.

Stu.

[aerobod - near CYYC]

Venting to the inlet manifold without a PCV or oil separator with vacuum diaphragm will cause a number of problems:

- carbon buildup in the intake

- poor mixture control under high inlet vacuum conditions

- high oil consumption under high inlet vacuum conditions

A sealed catch can could fulfil the role of an oil separator with vacuum diaphragm if a PCV is used at the head vent as well.

[MattB]

#55 - With that layout you'd have a fairly large vacuum leak downstream of the throttle body where unmetered air would be drawn in through the holes in the catch tank cap, down the catch tank vent hose and into the inlet valve tracts, weakening the mixture. Which doesn't sound like a good thing ...?

MattB

[GulfSeven]

#58 Well that's how Caterham supply the cars, so I'm assuming there's a reason 

[ScottR400D]

#59 I wouldn't bet on that one!! At least not on there being a good reason.......

 

 

[Dan R]

Any advice or recommendation on which piping to use for this ? 

[MattB]

Suggest using an oil and petrol resistant hose.

unless you're venting to the intake manifold the hoses won't see vacuum or much pressure, so you just need something that can deal with being under the bonnet, and with the oil.

MattB

[GulfSeven]

Right, so while I was talking to Lee at Caterham about something else, I quizzed him on the mystery of the vented cap in addition to hose to the inlet manifold on the factory can setup. He said that even with the hose to the inlet manifold the vented cap was still required to stop over pressurisation of the crankcase. I've no idea why a sealed can is fine on a Fiesta ST with what appears to be the same plumbing, but there you go - from the horse's mouth.

[Graham Hutton]

Thank Tim.  Bizzare, although there would be no problem if they provided something better than a 50p plastic catch can. If it were baffled there'd be no splatter. In other news I can confirm that my little breather filter solution has worked perfectly and despite a 400mile blast on the Taffia run, there is no oil misting in the engine bay at all :-)