The Big End - Over Pressure - Part II

On the A-Series engine sump oil pressure is, as I said in last months article, a problem that needs special attention and not quick fix remedies.

An engine when working has a normal by product called "blow-by", or a slight leakage of combustion gases that by-pass the piston rings, especially the oil control rings passing pressure into the sump. This pressure agitates the oil in the sump and causes oil vapour. It is very desirable to release this pressurised vapour as it will cause oil leaks as oil is forced past seals. Any gaskets and seals can also become damaged and engines can become gunked up. Oil vapour can condense inside an engine and cause coke deposits throughout the engine, eventually causing blockages and damage by lack of oil feed. A further problem is that excess pressure causes a drag on the engine's performance. Years ago pressure was reduced by venting it direct to atmosphere, then as vehicles advanced as with the Morris it was vented from the rocker cover and an oil separator tank on the timing chain cover, venting into the carburettor through the air filter. It was never really successful and the A-series engine gained a bad name for oil leaks.

On later more modern vehicles, it is forcibly removed using the engine vacuum and the vapours burnt in the engine, thus reducing the environmental problems.

The device for this job is called a "flame trap" or "flame guard" fitted near the inlet manifold, also incorporated is an oil separator fitted above the sump oil level incorporating an oil return to the sump, hoses and a special take off fitting on the inlet manifold.

In operation sump pressure is directed through the "flame trap" system during normal driving as follows, engine vacuum taken from the inlet manifold is passed through the top of the "flame trap" through the oil separator ("breather box") and into the sump. When the vacuum exists pressure will be drawn from the sump back through the system and the oil vapours passed to the combustion chambers and burnt. The gases in the sump are highly combustible so it is very important to prevent igniting them with a backfire. If this did occur you would probably need to replace all the gaskets and seals and you could also need a new sump. This is prevented by the "flame trap" which must be maintained for efficiency. The trap in working must absorb any thermal energy from any backfire and quench any advancing flame front. In addition to the special vacuum system extra filtered breathers may be required to, if possible, keep the sump pressure from going above atmospheric pressure.

Note: It is very essential when fitting any type of device that draws oil vapour directly into the inlet manifold that a flame trap is fitted.

The function of the oil separator is to allow oil vapour and droplets some time and space to coalesce into larger drops and scavenge back to the oil in the sump.

Several different designs of venting systems are around and can be adapted to work with the A-series engines without too much trouble. One of these is an Evacusump pressure reducing system.

The system works as follows:

With the engine running at idle rpm, an inlet manifold take off, vacuum control valve draws vapour pressure out of the sump. When under power the vacuum is lost due to the carburettor butterfly's being open. In this condition suction from the exhaust system is used as a secondary stage evacuation process.

As the exhaust gasses are taken away through the exhaust pipe the passage of these gases will cause a suction which is used to draw the vapour pressure out of the sump flashback prevented by a special anti flash valve fitted into the takeoff on the exhaust.

Again incorporated is an oil separator unit in situ in this system. This is a very simple but efficient system and one that has been used on many high performance A-series powered vehicles.

For extra information on this type of pressure reducing system, read David Vizards book Tuning The A-Series Engine.

What ever you do to elevate this very important problem area, you must ensure that the engine is in top running order and not worn out, as it's no good trying to shut the gate after the horse has bolted.

Also regularly change the oil with a good grade make. Variants of oils are today on the market shelves but an mineral 10-50 oil is preferable to any synthetic oil, and if the engine shows signs of clogging drain the existing oil and flush through the system with a good flushing oil, finally fitting a new filter and the good brand oil.

The are no palliatives I can advise, only that the stating point is the engine assembly which must be accurate and of a high standard, regular engine maintenance and careful monitoring of any leaks should be made. If and when a leak occurs and it's caused by pressure don't try and just seal it off, fit a better vent system it will be worth it in the long run.

© Chris Street, November 2001