Keeping oil in an XPAG – Positive Crankcase Ventilation

In my last article on “Keeping oil in an XPAG”, I described how replacing the original cork side cover gasket with a steel one would stop the gasket distorting and blocking the breather holes. The article also described the system used in modern cars. Here, there is a connection between the crankcase and inlet manifold to help reduce the pressure in the sump. This is called Positive Crankcase Ventilation.

In this article I look at how Positive Crankcase Ventilation works and show the results of some tests using my TC. I would like to acknowledge Jim Blackwood’s article in BritishV8 on Positive Crankcase Ventilation and David Heath, Bruce Morgan and Ray White for their articles on how to fit such a system to an XPAG.

No matter how well the piston rings seal with the bore of an internal combustion engine, unburned petrol and combustion gases will escape past the rings into the sump. This is called “blow-by”. Unless removed, these gases will contaminate the lubricating oil and pressurise the sump.

The combustion gases consist mainly of nitrogen, carbon dioxide and water vapour. These combine with the crankcase oil to form a white water/oil emulsion, acids, sludge and other harmful contaminants.  In addition, the escaping gases will pressurise the sump. This will blow the oil out of the engine past even the best lip seals. Less obvious is that pressure in the sump can reduce power output. The power generated by each combustion stroke depends on the pressure difference between the top and bottom of the piston. The higher the pressure in the sump, the less this pressure difference and the less power each stroke will produce.

Crankcase ventilation removes the blow-by gases to prevent these problems.

Despite first appearances, the XPAG has quite a sophisticated crankcase ventilation system, sometimes referred to as a Draught or Road Tube. This consists of two parts. The first, and most obvious is the large diameter tube that runs from the side cover down the left (or nearside) of the engine, disappearing under the bulkhead. This is the road tube.

If you examine this carefully, you will see the bottom end is bent upwards. The reason, to better angle it into the airflow underneath the car. When travelling down the road, the air passing the end of the tube creates a partial vacuum helping to “suck” the combustion gases out of the engine. It is worth checking this tube is not restricted or blocked by a build-up of oily deposits. I have seen some articles suggesting a bottle is fixed to the bottom of the road tube to collect oil. Should you do this, it will stop the airflow creating a partial vacuum.

The second part of this system is the link between the top of the rocker cover and air filter. This allows filtered, fresh air to enter the engine to flush out the combustion gases. Check the channel in the rocker box cover and hole in the air filter are clear. For those cars that have been fitted with pancake or alternative filters, another means must be provided to allow filtered air into the engine.

Pic illustrating the link between the channel (vent) in the rocker cover (red circle) and the air filter as referred to in the text above.

This breather is very important. Without it the sump would fill with blow-by gases. While the road tube will prevent the build-up of excessive pressure, without the inflow of clean air these gases will not be flushed out of the sump.

While the Road Tube system is reasonably effective, it is far from environmentally friendly. As well as the combustion gases, unburned hydrocarbons and any oil carried down the road tube are vented into the atmosphere or dropped onto the road surface.

The Positive Crankcase Ventilation system addresses this problem by recycling and burning these pollutants. Interestingly, it was first introduced during WW2, not to prevent pollution but to allow vehicles to drive through deep water without flooding the engine.

There are two different forms of Positive Crankcase ventilation system. The first recycles the gases from the sump back into the inlet manifold. Here they mix with the petrol and air and are burned in the cylinder.  This is the system normally fitted to modern cars. David Heath and Bruce Morgan describe how this arrangement can be fitted to an XPAG. I refer to this as the “re-cycling system”.

The second method is to insert a small tube into the exhaust down pipe called a scavenger. This has an angled end similar to that of the Road Tube. Ray White has written an article about fitting such a system to an XPAG. I refer to this as the “exhaust pipe system”. With this system, the unburned hydrocarbons and oil are burned in the high temperature exhaust.

Ed’s note: Ray’s article was in Issue 64 (February 2021) (please see comment on this later on).

Components of the Positive Crankcase Ventilation system

(Please refer to the diagram when reading the following explanations).                                        

Fresh air intake – This is normally situated on the rocker box or camshaft cover at the top of the engine. It is usually fed from the air filter or inlet manifold. Airflow is restricted by a small hole, 3/8” (375 thou) or less in diameter. 1/8” is typical. This hole restricts the fresh air flowing into the engine. The reason for this is discussed below. The hole in the TC air filter is 3/8”.                               

Side chamber vent This is the connection to the crankcase. Typically, about half way down the block. On the XPAG, the existing road tube connection is ideal.

Oil catch can – When an engine is running, the lubricating oil is being sprayed out from the big end bearings to lubricate the cylinder walls and in the case of the XPAG, the camshaft. This creates a fine mist of oil droplets which can be carried into the crankcase ventilation system along with the blow-by gases and fresh air. In the crankcase ventilation system, they can mix with the water vapour in the blow-by gases forming an emulsion that blocks the flame trap or PCV valve. While not necessary, an oil catch can avoids this problem. Additionally, some oil catch cans can act as a flame trap removing the need for a separate trap.

Flame trap There is the danger that a backfire could cause the oil mist in the sump to explode. The flame trap consists of an expansion chamber filled with wire wool to prevent this happening. It is important one is fitted and that it is regularly inspected to check it has not become blocked.

PCV valve This is the component that controls the flow of gases from the crankcase into the inlet manifold or exhaust. It is a flow control valve that allows full flow at low pressure differentials across the valve and a metered restriction above that level. Sounds very complicated. In practice, a very simple component. It consists of a spring-loaded tapered pin that controls the gas flow through an orifice. As the pressure difference across the valve increases, the tapered pin is pushed into the orifice to reduce the gas flow. PCV valves have a non-return mechanism fitted to prevent blow back increasing pressure in the sump.

Fresh air intake & PCV valve – Working in conjunction they serve two purposes:

  1. The PCV valve and Fresh air intake work together to control the depression, or reduction in pressure below atmospheric in the sump.
  2. The Fresh air intake controls the volume of un-metered air entering the inlet manifold.

The volume of air that can flow through the Fresh air intake hole depends on the pressure difference across it. The greater the pressure difference, the greater the volume of air. In contrast, there is a fixed volume of gases flowing through the PCV valve, independent of pressure. Hence, the pressure in the sump drops to the point where the volume of air flowing in through the hole in the fresh air intake, plus the volume of the blow-by gases is the same as the volume of gases flowing through the PCV valve.

The smaller the hole, the lower the pressure in the sump. The engine is not designed to withstand a low pressure in the sump. For example, on the XPAG, a drop in pressure of 1 lb/in2 (about 6% of atmospheric pressure) corresponds to a large person standing on the side of the cast aluminium sump!

The second purpose of the Fresh air intake is to restrict the volume of air entering the engine. This is important for the re-cycling system. The carburettor accurately measures the volume of air flowing through it, adding a controlled volume of petrol. Air entering the inlet manifold from the positive crankcase ventilation system weakens this mixture by an uncontrolled amount. The less volume of blow-by gasses (which have no oxygen in them), the more air can flow in through the Fresh air intake and the greater the weakening of the mixture.

Modern engines are “air tight”. The oil filler cap and dip stick have seals, bearings have lip seals. The only fresh air flowing into the engine is through the Fresh air intake. This is not the case with the XPAG where air can leak in through the filler cap, dipstick and the front and rear crankshaft seals; a factor that needs to be considered.

Re-Cycling System

With a normally aspirated engine the outlet of the PCV valve is connected to the inlet manifold between the engine and throttle butterfly. When running at part throttle, there is a partial vacuum in the inlet manifold. This draws air and blow-by gases through the PCV valve.

There are two problems with this arrangement:

  1. Un-metered air from the fresh air vent is being drawn into the inlet, weakening the mixture. The larger the hole in the Fresh air vent, the worse this problem.
  2. Under full throttle, there will be no vacuum in the inlet manifold. This is when blow-by will be at its maximum. In this case the sump becomes pressurised and the blow-by gases are vented through both the PCV valve and Fresh air intake.

With a “blown” engine, the outlet of the PCV valve is connected to a point between the carburettor and blower where the blower creates a partial vacuum. With these installations, an oil catch can is important to prevent oil/water emulsion damaging the turbo or supercharger.

This system is not suitable for cars that are raced. In this case the engine is running on full throttle for a lot of the time.

Exhaust Pipe System

With this arrangement the outlet of the PCV valve is connected to the scavenger that is fitted into the exhaust pipe between the manifold and the silencer. The scavenger is basically a pipe with an angled end that protrudes into the exhaust pipe. As the exhaust gases flow past the end of the scavenger pipe, this creates a partial vacuum. Like the Recycling system, this vacuum draws air and blow-by gases through the PCV valve.

The main advantage of this system is that the volume of exhaust gases and vacuum increase with increasing throttle settings. Additionally, the fresh air flows into the exhaust and does not weaken the mixture.

The greatest disadvantage is the risk that the very hot exhaust gases could ignite the oil vapour in the blow-by gases creating an “explosion” in the sump.

Does the “Recycling System” Work on an XPAG?

Before fitting a Recycling system, I decided to test how effective it was at stopping oil leaks on my TC.

I have already fitted a vacuum advance and I have two 1/8” tubes connected to spacer plates between the inlet manifold and the carburettors. These provided the vacuum connection for the test. All that was then needed was an adapter to “plug” into the vent on the side cover after removing the road tube.

A separate tube provided a connection to a pressure gauge to allow the depression in the sump to be measured.

For the first set of tests, I fitted a restrictor between the air filter and rocker box cover. This had a very small 1/32” Fresh air intake hole. This was removed for the second set of tests. The next challenge was to measure how much oil was leaking. For this, I drove a 20 mile route over country roads and a section of 60 MPH dual carriage way. On return, the car was parked over a clean piece of cardboard. The number and size of the oil stains gave an indication of how effective the system had been.

Pic showing the test rig connected to the engine side cover with the pressure gauge, the Flame trap and PCV valve in position and the tube leading to the inlet side of the engine.


The temporary fitting suffered from two problems:

  1. The two tubes connecting the PCV valve to the inlet manifold were only 1/8” diameter. Ideally the PCV valve should be connected with a 3/8” tube which has 4 times the area for the gases to flow through. This means these tubes as well as the PCV valve were restricting the flow of the blow-by gases.
  2. I do not think the connection to the side plate was 100% gas tight, allowing air to leak into the PCV system without passing through the engine.

Both these factors reduce the possible crankcase depression.

The depression in the sump was measured by running at fast tick over (i.e. maximum vacuum in the inlet manifold), and measuring the pressure. This gave values below atmospheric of:

  1. 1/32” Fresh air intake hole – 1.75 to 2lb/sq
  2. 3/8” Fresh air intake hole – 1lb/sq in.

Had the engine been airtight, this large difference in the diameter of the Fresh air intake hole should have given a greater difference. One possible explanation was that there was a high level of blow-by. However, cylinder compression tests did not suggest this was a factor.

The cardboard “leak tests” are shown below. They look like the ink-blot tests used by psychiatrists and need some explanation.

When I stop my TC, I normally get three areas of dropped oil. Two smaller patches, one from the front seal and one from the rear seal and a larger area in the middle where the oil runs down the sump and drops off the bottom.

This is shown on the No PCV picture.

With the 1/32” Fresh air intake hole, the oil leakages from the front and rear seals appear to have gone. However, there were signs of oil leakage past the dip stick which had run down the sump and a large patch from the centre of the sump.

The probable reason for this was that the high level of depression in the sump had stopped the leakage from the front and rear seals. However, at higher throttle settings, the Fresh air intake hole and diameter of the pipes connecting to the inlet manifold were too small to allow the blow-by gases to escape. This caused the sump to pressurise, blowing oil out of the rear bearing seal and dip stick.

The second test with the 3/8” hole only showed a few drops from the front and rear seals. None from the centre of the sump. With less depression in the sump, oil is still leaking at a lower rate from the two seals. However, the larger Fresh air intake hole was able to better vent the blow-by gases at high throttle settings.


These tests suggest that a PCV system fitted to an XPAG is one way of keeping the oil in – particularly for engines fitted with lip seals.

For anybody considering such a modification, the steel side gasket sold by John James is recommended. There is a risk that the additional pressure difference across the original cork gasket may cause it to distort and block the breather holes.

If a Re-Cycling System is fitted, thought should be given to the size of the Fresh air intake hole. Too small and it may not be able to cope with the blow-by gases at high throttle settings or provide sufficient fresh air to flush out the blow-by gases. Too large it may not produce an adequate depression in the sump. It may also make the mixture too weak.

The ideal size of the Fresh air intake hole will depend on the “airtightness” of the engine.

The size of the Fresh air intake hole is less of a problem for anybody considering the Exhaust Pipe System.

Thought also needs to be given to sealing the oil filler cap and dipstick. If not properly sealed, these will allow un-filtered air and dirt to enter the sump. For those engines that are not fitted with lip seals on the crankshaft, there is a small risk that in-flowing air could carry dirt into the seals causing excessive wear.

For the record, these tests were run with a FV237 PCV valve on an XPAG that is not fitted with lip seals.

Paul Ireland

Ed’s note: Reference was made earlier to Ray White’s article in Issue 64, which described the “exhaust pipe system”. Ray has since posted the following comment to his article, which may not have been seen by many of you. It has since been edited by me (without altering the sense of the original comment):

I have to say I had something of a quiet panic when I read Paul’s article (having received advance notice of it) because Paul has described how he believes the rocker box to air filter pipe connection works.  He says that “fresh air” is drawn into the engine from the air filter.  I have always understood the system worked the other way around; in that the carburettor sucks out the fumes from the top of the engine and recycles them with fresh air via the filter – into the combustion chamber and out of the exhaust.  This would assist in crankcase ventilation when the vehicle is stationary and the draught tube is inactive. 

My point is that if the system works as Paul describes then my crankcase ventilation system (which relies on the exhaust drawing the fumes out of the top of the engine through this pipe) may not work.  Having said that, however, this system is essentially the same as one fitted to a TF that has been working fine in the USA for some time.

Ed’s further note: Paul has commented as follows:

In practice what Ray says about “I have always understood the system worked the other way round;” is “sort of” true.

I did not bring this point out in the article as it, a) is only a minor effect and, b) complicated to describe.

No matter how the crankcase is ventilated (Road tube or PCV), there are two “holes” through which the blow-by gases can escape. The fresh air intake in the air filter and the vent tube.

Which route the gases take is only dependent on pressure difference. If there is a lower pressure in the road tube (or PCV) than in the engine, this will draw the blow-by gases out of the engine. If the “residual” pressure in the engine is, then less than that in the air filter. Air will flow from the air filter into the engine. This would be the “normal” operation when driving along the road, unless there is an excessively high blow-by due to wear.

If the residual pressure in the sump is greater than in the air filter, the blow-by gases will flow into the air filter.

The clue that this is not happening is that normally the free air intake is free of excessive oil. This suggests air is flowing INTO the engine rather than OUT OF in which case it would carry the oil from the rocker box.

The next question is “what is the normal pressure in the air filter”? This is a difficult question because it depends on the efficiency of the filter and the air flow through it (or throttle setting). At low throttle settings it will be at or very nearly at atmospheric pressure. This means that when the car is stopped at tick over, with a PCV system, there is a low pressure on the vent tube and nearly atmospheric on the air filter so all the blow-by gases will vent via the vent tube. With a road tube system, I suspect the majority of blow-by gases will vent via the vent tube (it is atmospheric pressure with a large diameter “hole”) and yes some will vent via the air filter as it is at a slightly lower pressure but only connected via a small hole.

At high throttle or full throttle, the picture is different. There is a large airflow through the air filter and there will be a pressure drop, although this is small. With a “recycling” PVC system, the vent tube will be at virtually the same pressure as the air filter (in practice it will be slightly lower due to air friction in the carburettor, etc. So, the situation will be very similar to that described for the road tube at tick over. For the road tube, the vent will be at a lower pressure due to the movement of the car and the original comment will apply where the flow depends on the residual pressure difference.

The American cars use a slightly different system to manage the case of full throttle running, however, I will not go into that here.

As I said, I did not include this in the article as I feel it makes it very complicated. Please let me know if you think I should include more detail.

Ed: I think we will leave it there for the time being!

One thought on “Keeping oil in an XPAG – Positive Crankcase Ventilation

  1. CASTEL says:

    Hi Paul,
    On the same topic, interesting study about the venting of on MGA :
    If the discussions can be boring sometimes, don’t miss the last experience at the end of the thread.
    For the TD, if the air flows from the air cleaner to the tappet cover while driving, it is unexplained why the output of the air cleaner for engine venting is located on the unfiltered side. Note also that the draft tube could have been longer and lower than the cross member.
    Second point, the article focuses on the still air pressure, but there are also great air flows inside the cranckase itself when pistons get up and down. This may result in local high pressure areas even if the mean presure is a bit lower than atmosphere. Leaks…

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