Crankcase pressure evacuation

The problem of oil leaking past the crankshaft seals is something that seems to plague XPAG engines so I have endeavoured to do something about it.  From the outset, my rebuild of TC 10030 has been driven by a desire to have a TC with fewer of the original poor design choices that, in my opinion, detract from what is otherwise a delightful car.  

We are all familiar with the draught tube ventilation system that is designed to draw out engine fumes and expel them to the atmosphere. This is not exactly environmentally friendly; it’s also inefficient. The draught tube system can only work when the car is moving so that an atmospheric depression is created at the end of the tube. This, combined with the internal pressure is designed to evacuate crankcase gasses.  When the car is idling or in slow traffic ‘blow by’ can build up pressure in the crank case as it cannot escape fast enough. Fumes from the crank case are drawn out the faster the car moves… but here’s the rub… the harder a car is driven, the more ‘blow by’ it will create; more in fact than the draught tube can remove and excessive pressure in the crank case will force oil past the crankshaft seals.  Obviously, the more worn the pistons/cylinders are; the worse the blow by. 

Positive Crankcase Ventilation is an emission control feature of modern cars.  The PCV valve recycles crankcase gasses to the combustion chambers where they are burned and expelled through the exhaust. Much more environmentally friendly!  I could have incorporated this system into my XPAG engine rebuild but there was a problem.  My engine will be supercharged and I had read that PCV valves are designed to work with normally aspirated engines; they do not work well with the pressure of superchargers.  I needed a different approach so when I read about how one enterprising TF owner in America had adapted an exhaust-controlled crankcase evacuation system used in light aircraft, I decided to investigate the possibility of fitting one to my TC. 

The method of crankcase ventilation utilising exhaust vacuum was my preferred option for several reasons: 

Firstly, this method will work with my supercharger. 

Secondly, it fits in with my ethos of not making any irreversible changes to the car. 

Third, this would be an environmentally friendly alternative to the draught tube. 

A fourth reason is that it will protect the new crankshaft oil seals and eliminate leaks.

The fifth reason was that I could do it myself at a reasonable price.

At the heart of the system is a “scavenger” unit which is fitted at an angle to the exhaust pipe between the manifold and the silencer. Using a piece of scrap pipe on which to practise, it became apparent that the scavenger unit I had bought could not possibly work without some modification.  I also had a tricky time making a 5/8” hole in the pipe as my drill only had a 1/2” chuck!

With the scavenger unit inserted so that its slot located with the edge of the pipe – necessary to establish the correct angle – it became evident that it couldn’t be tightened.  The reason was that the separate wedge-shaped securing tube was threaded – which clearly was wrong.  Another problem was that it had a flat rather than curved end.  Ideally it should be the same profile as the exhaust pipe.

I mounted the wedge-shaped ‘securing tube’ section in the lathe and stripped out the threads.  I then filed the end to match the profile of the exhaust pipe.  The angle was also wrong and required careful filing to match the angle of the scavenger unit in the exhaust.  Eventually I was able to tighten the unit securely to the pipe with the locking nut.    YESS!!!

This photo shows a 22mm to 15mm reducer with a short length of copper tubing on the end of the scavenger unit.

There was originally a 1” UNC threaded end which I found impossible to match to any suitable reducer so I again mounted it in the lathe and carefully stripped the threads down to a point where – using the vice – I could press on a 22mm ‘Yorkshire’ I was now ready to install the scavenger unit into the exhaust proper.  There was a moment of trepidation before actually drilling a hole in my brand new stainless-steel exhaust pipe but the scavenger unit came with a blanking plug should it all go pear shaped.  Fortunately, the fitting went well.  The expected vacuum should be in the region of 3Hg however there is some adjustment to prevent excessive oil drag.  I have a vacuum gauge which remains to be fitted.

Location of fitting of scavenger unit.

One view of the catch can location.

Another view of the catch can location (see text below)

Other important components of a system like this are the provision of a non-return valve (for safety reasons) and a catch can to prevent oil being carried to the exhaust system. I have improved the catch can with additional baffles. It has a dip stick and can be easily emptied.  It will be interesting to see how much oil is collected!  The take-off is from the breather in the rocker cover. Here I have another 22mm to 15mm reducer to take the rubber hose.   I have also fitted an adjustable brass vent in place of the draught tube.

I think, on balance, it is quite an attractive installation.  I just hope it is fit for purpose when in service. 

Ray White

The adjustable brass vent in the tappet chest, replacing the pipe.

6 thoughts on “Crankcase pressure evacuation

  1. Geoff P says:

    Interesting. While I do not have a supercharger, I do have a positive crankcase ventilation based upon the later MGB system.

    Modifications involved (a) fit the late Austin/Morris 1800 outlet to the tappet cover, (b) modify the TC SU carburettors (see MGB) and place a 9/64″ restrictor in the air cleaner to rocker cover inlet hose.

    Though I have never felt the need, all the above modifcations are reversable.

    • irelandp says:

      I am planning to fit a similar system to my TC this spring. I have a couple of worries:
      1) Air leaks into the engine from the dipstick and filler cover. Any air getting into the engine would weaken the mixture.
      2) Excessive pressure in the sump. When running at full throttle, there will be a) no vacuum on the inlet and b) more leakage of gases past the rings. With a sealed system these could increase the pressure in the sump.

      Perhaps we could have a chat. If you contact John, he will give you my telephone number so we can speak.

      Thanks Paul

      • Geoff P says:

        Australia to UK phone can be expensive.

        Re Crankcase breather. The MGB has crankshaft seals but no attempt has been made to seal the dipstick.

        I used the later MGB system because it is simple (there is no actual PCV) it works and examples can be studied at any MG meet. I have a friend who has also adopted the system to his TF, claims it has reduced oil loss from the rear crank scroll.

  2. Sherwood Parker says:

    Will this system help with the leaks with the old fashioned “reverse screw’ system still in place?

    Thanks for sharing this with us.

    • RayWhite says:

      I would think any crankcase pressure evacuation system would work with the original set up. Personally, with a normally aspirated engine, I would fit a standard PCV valve . You only need an exhaust operated system with a supercharger fitted.
      I have yet to finish the restoration so I am keeping my fingers crossed that it actually works!

  3. Ray White says:

    One aspect of my installation that appears unconventional is that the vacuum (created by the exhaust scavenger unit) is attempting to draw blow by fumes out of the crankcase from the top of the engine. By using the “fresh air” vent in the rocker cover I am in effect reversing the original air flow with fresh air being taken in through the adjustable vent on the valve cover that has replaced the original draft tube.

    I have yet to establish if the system will work as it is, or if it will need reworking. There is a risk that crankcase pressure will be reduced – with the elimination of oil leaks – yet insufficient venting could result in acidic pollutants being retained, where they will have a detrimental effect on engine life.

    My thanks to Paul Ireland for his helpful advice in this often misunderstood area .

    There is more to crankcase ventilation than I had realised.!

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