Engine breather hose.

[rossbraithwaite]

With reference to the photo in the OP I have a few questions:
Why was the engine breather take off moved to the cam cover from the front of the cylinder head?
Is there a separate breather for the crankcase?
Why are there 2 hoses- is it to get the x-section without using a large pipe?
and more generally:
Does the catch can not require any negative pressure from the inlet, just relying on the positive pressure from the engine?
Does a simple catch can require filling with gauze or similar to help sparation?
What is the benefit of converting the standard setup to a catch can?
Thanks,
Ross

[PeterCrespin]

Not claiming any special insights, but the dry sump system came in with the D-type. That photo shows a dry sump wide angle head E-type.

To keep the sump ‘dry’ you have to pump twice as much on the scavenge side as you do on the feed side (think slurping straws at the end of a milkshake). What gets returned to the oil tank is therefore a mixture of oil and air bubbles, which need separating toute suite by means of a swirl pot and/or a system of weirs and perforated baffles in the upper tank area. The de-foamed oil drops to the bottom for delivery by the feed pump and the air goes out of a generous breather. The engine benefits power-wise from having no oil sloshing in the sump and reliability-wise from having a guaranteed oil feed even in long corners under high G with low oil level ( think endurance racing).

Meanwhile, The D was all about tight packaging and low X-sectional area/drag. The dry sump shortens the tall engine by about three inches and allows it to be tilted to the left - giving room for a longer inlet tract without hitting the wheel arch area. The short tilted engine cuts drag due to reduced frontal area. Deleting the front breather allows the expansion tank to sit closer to the engine and help streamlining. The two cam boxes are joined by a cross pipe and blow-by gasses from the crankcase/timing chain tower or cylinder head can the flow out via the back of the exhaust cam cover where there is more room because that area is lower transversely and fore/aft. As you say, a large bore pipe is harder to manipulate and tuck in than two smaller ones, although the pipe would not have to be huge to flow the same amount. Pumping losses due to inadequate breathing have an effect in race engines that is much lower in road use and does not require recirculation like a road car.

Now, a question of my own to the racing fraternity. I will be making a catch tank for my D at some point. Rumour has it that these days it has to have a minimum capacity of 3 litres to be allowed on some circuits. This is quite large and there’s not much free space under a dry sump bonnet. Before I desgn some kind of flat vertical wedge in front of the short passenger footwell, can anyone confirm the three litre figure?

[rossbraithwaite]

Thanks for the very informative post, Peter. It all makes sense in the context of the D Type dry sumped engine.
Is there any benefit of the catch can in a wet sump road engine? Is there a thought that not putting oil vapour through the combustion cycle will reduce fouling and deposits?

[PeterCrespin]

There shouldn’t be a quantity or quality of breather output bad enough to cause fouling problems, so I use the large bore elbow breatger connection on the plenum backplate. There are plenty of emission aspects I’m happy to comply with.