Classic cars forum & vehicle restoration.

[Ray White]

Are they all the same? They all work on the same principle and they pretty much all look the same. I guess the volume of air that they have to pass can vary from one engine to another but I would have thought that the amount of wear in an engine would be more a factor than anything else but that has never been a marketing factor for one valve over another.

I have had a suspicion for some time that the XPAG engine is inadequately ventilated and it's
reputation for leaking oil seals in quite recently built engines could be down to this.

I may of course be entirely wrong.!!

Similar to other crankcase ventilation systems of the time, the XPAG has a draught tube from the tappet cover that extends down past the engine.

The theory (for those who have forgotten) is that when motoring, air passing under the car creates a small amount of negative pressure as it passes the end of the tube which has the effect of drawing oil vapour and blow by gasses out of the crank case and into to the environment.

I had an idea that with the supercharger I am fitting, crankcase pressure could be a problem; so I am investigating the fitting of a PVC valve. The problem is that they do not work very well (not well enough) with a supercharger.



One of the more novel ideas that I have read about is taken from the world of light aircraft where crankcase ventilation is achieved through exhaust pressure but the dirty air is not reintroduced to the inlet tract. The use of an exhaust 'evac kit' will work with a supercharger; it requires a "catch can" and eliminates the need for a PCV (although it can be made to work effectively with one) ...but again it produces negative pressure... however, there is the added potential to blow the simple check valve that enables the system to work. They are quite cheap but it is a nuisance all the same.

The kits I have seen are rather expensive for what they are so I plan to assemble my own from cheaply sourced components.


https://www.aircraftspruce.com/catalog/eppages/crankcase05-12429.php

[Churchill Johnson]

Ray, have just read your post i do not agree that the old system of crankcase ventilation was designed to use negative pressure to suck out engine bypass fumes, while i agree that is useful i am pretty sure manufacturers used a tube pointing down because of the amount that was being emitted year's ago one could tell if one was behind a Ford 100e by the trail of fume's, reason's behind my thinking is having spent most of my working life repairing agricultural machines, mostly tractor's Ford later CNH were still making engines that had a metal tube pointing down in 1995 long after it was outlawed in the motor trade and can say that no negative pressure was created due to to it's low speed, next the idea of using exhaust gases to suck out fumes is not much different to the Ford fw series tractor made in 1977 which had a cummins diesel engine and was fitted with a flap valve in the air intake which shut when first starting engine then negative pressure in exhaust sucked flap open in turn sucking and blowing out of silencer any debris that had entered the intake system before it got to air filter..........

[Ray White]

Good evening Churchill. Firstly, let me respond to your dispute regarding the intended use of the "draught tube". I assume it has this term for a reason and while it can expel a great deal of pollution to the environment, I don't expect car makers wanted to do any more than ventilate the crankcase.

MG themselves regarded it as a step change in crankcase ventilation and I presume they (and other car builders) were well aware of the physics behind it.

Until the introduction of the draught tube system, engines were expected to utilise passive ventilation with the result that when by pass pressure built up in the crankcase (to the point where it couldn't escape past the dip stick for example) it found it's way past gaskets and crankshaft rope seals resulting in oil leaks,

While cars with oil dripping from their engines may have been acceptable in the 1920s, by the 'thirties attitudes had begun to change. Before WW2 few people could afford a car of any kind and of those who did own one, very few would have had a smart tarmac driveway in front of their house, so the oil drips didn't notice much.

The filthy pollution emitted by draught tubes on worn engines eventually led to them being made illegal and replaced with the PCV valve. Like most automotive developments, the idea of proper crankcase ventilation originated in North America.

The principle of exhaust evacuation (often found in turbines) became popular in light aircraft. It has subsequently been adopted by hot rodders and classic car owners who want to supercharge their cars. The more modern alternative PCV valve system is not suitable for forced induction engines without a great deal of complication.