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MG Midget and Sprite Technical - Catch tank

I am thinking of making a catch tank as my car is beginning to blow just a bit too much blue smoke out the back. Of course if this is derived from the valves / valve stems, then a catch tank won't help. But I probably want one anyway!

So, am I right in my thinking that the tank needs to be fully sealed to the atmosphere - so that it doesn't leak air in and reduce the suction on the crankcase.
And then both the in and out pipes need to be short so that the open ends inside the tank don't get submerged in any residue oil.
And, should the tank contain any wire wool or similar to aid in condensing any oil in vapour?

Not the tanks I have made and used mate.

I simply use the tank to catch any residue emitted by the engine breathers, sealing the tank would reduce the benefit of the breathers VENTING into the catch tank.

So a simply tank well vented to atmosphere with pipe entries for the breathers.

Some refined tanks have little level glasses on the side to make the user aware of when emptying is required. :)

However with the breathing problems I seem to be having at the mo I have considered sealing the tank and finding some way of evacuating the tank, maybe something like the US spec air pump.
Robert (Bob) Midget Turbo

Bob, my system does not simply vent to atmosphere. It actively draws fumes from the crankcase by suction, the depression being produced by the carb manifold.

In my thinking, the tank needs to be part of this sealed suction system, but effectively with a sump to catch and store the oil between emptying, rather than delivering it to the manifold and hence out of the exhaust as blue smoke!


The reason why catch tanks are used especially by racers is that venting oil mist into the inlet manifold reduces the octane rating of the fuel and will cause pinking on engines pushing the envelope of compression

thus the answer is to seal the manifold and simply allow breathers to vent into a tank with no suction via the manifold.
Robert (Bob) Midget Turbo

Yes I understand about the effect of oil on octane rating. I have a CR of 10.5:1 and this is part of the reason I have for adding the catch tank. But I am also reliant on the manifold depression to control the amount of leakage from the rear seal.

My thinking is that designing in a baffle system and maybe a pan scourer into the tank will precipitate the oil into the tank and reduce the oil vapour continuing on to the manifold.

Then you are building a large oil seperator and not a catch tank not a bad thing.

As I said I was thinking of something similar with a seperator and suction pulling fumes through it.My problem is that I do not have manifold depression so my solution has to be via a seperate vacuum device, you could use the manifold.
Robert (Bob) Midget Turbo

My favourite topic :-)

I did exactly what you are describing with my first efforts at a catch tank, a few years ago now.

My 'catch tank' was a Yorkie chocolate metal can, same size as a beer can, with a removable top. 2 pipes went into the can, the inlet protruded about 3" into the can, the outlet only 10mm or so (from memory). I then wrapped one of those s/s scourers around the inlet, so any oily air which was being sucked out had to pass through it first and hence the oil mist was trapped, then dripped down into the can.

Did it work? Most definitely! I had a bit of a breathing problem, and it didn't take long to get it half full, at which point I had to empty it manually.

Wouldn't take much to adapt those rectangular tanks you can get (230592901011 - I've bought one, very nice albeit quite heavy for ally) to make the inlet protrude further in. They also have a drain on the side which if you were inclined, you could run a pipe down to the sump so it would self drain.

I will be venting to atmosphere as I have a weber on, which is why I will be using 2 tanks, I have 3 breather points on the engine (fuel pump blank, rocker cover, and dissy hole) along with a tiny one on the front cam belt cover. We will see if this along with the rear seal kit will be enough!

Hope this helps

Thanks Matt. That clarifies things.
I do want to keep the manifold extraction system drawing pressure from the crankcase as its wot keeps the oil in! And since I am using an SU this works well.

I do think that a "vacuum extraction" system and a "breath to atmospheric" system are often confused. The vacuum system requires just the right amount of restricted breathers to maintain the suction, whereas the breath to atmosphere needs as many ventilation breather pipes as is practical. And the latter does little to actively help retain the oil in the sump!

Adding extra breathers to the vacuum extraction version quickly renders it useless. As I think, from reading earlier threads on this, Bob has found out!

I take it that your Yorkie can was hermetically sealed then?

"hermetically sealed"
well, it was until I scoffed all the chocolate :)

I wouldn't have thought that the number of breathers would have an effect on the total suction, as long as the system is closed, then the airspeed in the breathers may be lower, but total airflow would not change.

<<I wouldn't have thought that the number of breathers would have an effect on the total suction>>

It does. On a manifold extraction system the idea is to sustain the pressure inside the crankcase below normal atmospheric. There is then no positive pressure to force oil out along the crankshaft scroll. Too much air feed into the crankcase and the manifold depression cannot keep up. You end up with the same pressure inside the crankcase as outside. Only higher because of piston movement and blow- by and the excess then reverses the flow out of the other breathers.

You still need some air feed in, so as to create a through current of air and take the oil fumes out. But the amount is fairly critical. Air feed in is usually at the top of the engine via a vented oil filler cap.

"The vacuum system requires just the right amount of restricted breathers to maintain the suction"

I meant that it doesn't matter if you have 1 breather going from the engine to the carb, or 4, as long as there is no outside influence it should be fine. Of course, why someone would put extra breathers on then combine them to feed into the carb is beyond me. But do you see what I am getting at?

OK, got you now. That makes sense. What one mustn't do with his sort of system is go adding extra breathers open to atmosphere. It just defeats the vacuum extraction system.

Now, where can I find a Yorkie bar tin....

Sounds good just would add.

If you had one of the pipes entering the bottom of the tank and this was above the engine connection then the tank would never require emptying as it would naturally drain condensed oil back into the engine.
Robert (Bob) Midget Turbo

I was embarrased a bit when showing people my engine, but at least it worked... I'll find some pics to post later.

I made this for my land rover. It's significantly reduced the leak onto the clutch and it uses much less oil than before. mpg has gone up too.

not exactly advanced but it does the job.

get eating your jam, and find some BNC bulkhead connectors and you have one made :)

you can even tell if it leaks, as the vacuum seal poppy lid pops in when the engine is running :)

the stuff that collects in it I would not want back in the engine, it looks like nasty brown mayo stuff. horrid. rest of fluids are OK, it's just the stuff in the jar that's nasty.

Rob Armstrong

Im actually building a 2nd oil seperator/catch tank at the moment out of some schedual 40 pvc plumbing pipe, and similar to the lucas tuming cover design an alot like what guy is decribing only the lower hose will go up furthur into the cly, and the oil will collect further down and hopefully drain back into the engine, least thats the idea by way of a one way pcv valve,,, so once the engine is off, the oil runs back into the engine ,,, I havent figured out if such a pvc valve exist yet,,,, to many irons in the fire

Ill snap some photos tomarrow so you can see what. Im working on, along with some other fun little cool evac mods


I had understood that the crankcase fumes contained a cocktail of combustion gasses, water vapour, acids etc as well as the oil vapour, and that this was highly corrosive. Not good to just recycle into the dump oil. I prefer Rob's idea of chucking it out.
Not sure what a BNC connector is. Who would sell those? Hydraulic pipe specialist?


Just googled bnc. Type of co-ax cable connector. I presume that is what you mean, and that you are just removing its innards.


Is this what I need?


Yup that's what I used, these ones:

are significantly cheaper, I used 8mm fuel pipe wedged over it, the little bayonet things help keep it on.

I drilled out the inner plastic stuff and inner conductor, and sealed the connectors to the lid with silicone instant gasket stuff.

certainly I don;t want the stuff in my jam jar anywhere near the engine working bits.

that's only after 100 miles. It smells HORRID :)

Rob Armstrong

well I guess its back to the drawing board, im pretty sure NOW I dont want to recycle back that mess into the engine also, thanks for the heads up

here is a photo of what im making,

I was going to add some kind of pass thur PCV valve at the bottom and run a small hose back to the timing cover, my thinking was with a Pcv valve it would reverse the vac and suck the oil from the oil speriator back into the timing cover... notsure if that would really work, but it seemed like a good idea at the time.


here it is taken apart... what I like about this is the concave inside ... Im not sure what these plumbing parts are called or used for but it would be easy to find out as I know several plumbers


here is my intake manifold intake for the vac hoses... its for the timing cover hose, the valve cover and the vacume gauge

okay try the photo agian for the above


here is an idea Im now kicking around after reading your above post about not using the used collected oil

its an oil seperator with a catch tank

the oil seperator is the vertical tube and the catch tank is the lower horizontial... the used collected oil has a line that can be attached at the front of the car inside the fender with a shut off valve so you can turn it to drain into a bottle for disposiale or just on the ground if your that type of person.

anyway just an idea... hopefully its readable... it would be made from shedule 40 pvc plumbing pipe and is based on what ive already built... btw the connections on the concave end pieces are threaded



awe... a bonus photo.. here is a better view of my oil seperator taken apart


Some good ideas Prop. I like the use of ABS plastic plumbing parts - whatever they are designed for? As usual, its a question of using what comes available.
Of course I could always just buy one:
Quite smart, but I couldn't use one with that name engraved on the top!


Thats a great price,,,

I was looking at some simlar morosoes,,,, CHA CHING!!!, mine is costing about $8 so far, I figure ill wrap it in aluminum tape for appearance,,,,anything to save a dollar at this point.


Catch tank all done and fitted up. Not tested yet, that happens tomorrow. Then I need to find a proper test run for this weekend. Now, where shall I go?

Here are the parts. Illy coffee tin, two bnc connectors and a scouring pad.


And two small plastic plant pots, used to hold the scouring pad in position.

One goes upside down to hold the scourer 2/3rds way up the tin. The second springs into place hold the thing down in position. I think the oil feed should have a extension to take it below the scourer, but I will try it first and add that later if needed


Finished catch tank


And installed in the car.

Just held with cable ties for testing at the moment, but I need to devise some proper straps for the longer term


Looking good, looks a lot more pro than mine!

Mine's still cable tied on...
Rob Armstrong

Yep looks OK Guy I think at sometime in the future I will end up sealing my catch tank, fitting a vacuum pump (no manifold vacuum available) and begin a new life of no oil leaks. :)
Robert (Bob) Midget Turbo

Decaffee :-)
a.o. arnold

I like that, vary cool,

How stiff is the coffee can, im a little taken back by how much suction there is from the intake manifold, it keeps collapsing the containers in my prior attempts.

Interesting point Prop. It hasn't collapsed yet, but then has only been for a short drive in it today, about 10 miles, and I doubt if I exceeded 5000rpm. I must admit I didn't even consider that!

This thread was discussed between 07/06/2011 and 10/06/2011

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