Crankcase Breathers

Here’s a question for the motor philosophers.

My '38 SS has a breather tube on the left side. These are often called a road draft tube, the theory being that road drafts suck the crankcase vapors out this tube as you drive. But where does the air come in? Is it just the blow-by from the pistons?

My '50 Mark V has the same breather tube on the left side, and also a breather hole in the rocker cover, which connects to the air cleaner body.
Which way does the air flow? Does the air flow into the crankcase from the air cleaner to the rocker cover and out through the side tube? Or does the air flow up the side tube and out the rocker cover into the air cleaner, and thus into the carbs?
Anybody ever run a test to see?

G’day Rob,
I may be mistaken, but the two breathers you ask about are two separate systems. The breather on the rocker cover recirculates unburnt fuel/oil vapours back into the carbs via the air silencer (it’s not actually a filter - it would only filter out large insects and small birds) - to be burnt in the cylinders. The breather on the side of the motor reduces the air pressure in the block. The angled cut (open section) on the bottom of the pipe should face the rear of the car so that when you are moving in a forward direction, the air flowing past the pipe creates a venturi effect and sucks out any high pressure air in the block. By doing this it also reduces the oil being forced out past the rear bearing and into the bell housing by the high pressure in the block. The pictures in your post seem to show the open section facing the front of the car which is the wrong way around and would actually forced more air into the block. When I replaced my crankcase breather pipe, I made it an inch or so longer so that the opening was sitting just below the bottom of the sump and therefore in “clean” air to give a better venturi effect. Hope this helps.

I have just found out how to attach photos so I’ve attached a photo of my crankcase breather for your info.

Both my tubes are the original tubes with the angle cut to the rear, identical, apparently unchanged from '38 to '50.
There is a large space around each push rod where the oil drains back to the sump, so the whole block and head are one air volume where the blow-by vapor would collect and build pressure if it were not relieved.
I’m curious to know why Jaguar added this connection to the air cleaner. Which way does the air flow in a Mark V?

On my Mark V, prior to rebuilding the engine when there was a fair bit of smoke in the exhaust, the vapor flow through the rocker cover was into the silencer. The evidence for this was oil buildup in the silencer, one could drain oil out of the silencer upon removal. The oil collecting in the silencer was eliminated when the rocker cover opening was sealed.

The vapor flow at the vent pipe was to the atmosphere. This could be felt with the hand below the outlet and also could be seen by eye with smoke tracer provided by running engine.

So air was going out both ways, but probably because you had bad rings?