Engine breathing - sanity check required (not a vented vs sealed debate)

[Smithy77]

A subject discuss a lot before, I know, and from what I've been reading I don't want to entertain the idea of having a sealed engine, so no need to go down that road.

I have a Minister R400 engine with Caterham belltank DS system. The engine and swirl tower are both vented, but from what I've seen/read, my particular breathing plumbing is not quite aligned with the 'norm' with this engine/DS set-up. I have a scuttle mounted catch tank (with vent in the top) and the breather from the swirl tower vents directly to this. I also have a breather from the smaller REAR cam cover outlet which vents directly to the catch tank. The larger FRONT cam cover outlet is blanked.

Most people with this engine/DS system seem to combine the breathing circuits, either a) venting the swirl tower to the rear cam cover outlet, and then venting the front cam cover outlet to catch tank, or b) blanking the rear cam cover outlet and connecting the front cam cover breather into the swirl tower breather with a T-piece, which in turn vents to the catch tank.

So my questions are:

Am I ok not having the circuits combined?

Does it make any difference which of the 2 cam cover outlets are blanked/vented?

[revilla]

Regarding the sanity check ... You're a nutter . 

Regarding the venting ... As you know I'm a wet-sump plenum kind of guy but whilst investigating my oil consumption issues I drew up the following diagrams to show blow-by flow through the PCV hoses to aid my own understanding. In the standard plenum setup the restricted rear orifice vents into the plenum behind the throttle and as I understand it only draws off a small amount of gas e.g. at idle. The crank case is under slight depression in this state and so a little air is drawn in through the larger front vent and circulated through the crank case and out of the rear vent into the plenum to ensure that all blow-by gases are flushed through and burned for emissions purposes. The rear orifice is restricted to prevent air being drawn through bypassing the throttle and upsetting the idle condition. When you put your foot down the manifold pressure rises but the blow-by gases increase to the point where the crank case is above atmospheric pressure. In a plenum system this means you now get gas flow outwards through the larger front hose which is taken to the throttle body just inboard of the air filter to be drawn in and burned again. In your case it is this heavy breathing gas flow you really want routing to the catch tank. So I would say you really want to route the front unrestricted port to the catch tank and blank the rear restricted port, which won't be able to handle the gas flow without pressurising the crank case excessively.

Low Throttle, High Manifold Vacuum, Low Blow-By

High Throttle, Low Manifold Vacuum, High Blow-By

[Smithy77]

Regarding the sanity check ... You're a nutter  [Nuts]

Aren't we all??

So in summary, I'll swap my engine breather from the small rear breather to the larger front. Will leave the oil tank venting straight to catch tank.

[LesG]

I'm running a CC dry sump system on my 1.8 K, but using Jenveys rather than the standard plenum. The bell tank tower breather and rear cam cover breather are both piped into my catch tank on the scuttle (vented to atmosphere), and the other cam cover breather just has a rubber hose to atmosphere too. Seems to work for me?

[Superwhite R283]

Hi Pete

I had the same headache when I installed my dry sump system (lots of different information out there). 

Mine is a little different to yours as I have the duratec oil tank. I ended up as Andrew suggested: vent from the swirl tower to the catch tank along with a separate vent from the front larger breather to the catch tank, with the small rear breather blanked.

 

Cheers Darren

[Smithy77]

Darren - thanks, I've now swapped my engine breather to the front outlet, so our set-ups now sound the same