Trying to get my head around the breather hoses and connections we are all using, and could do with some help, advice and explanations. Apologies for the length of all this, but you should see where I am going by the end of it.


On my original 1300 single carb engine, I used to have one of those soppy little breather filters near the oil filler and after a decent run the engine was often caked in oil, and it wasn`t until someone suggested to me that this filter should be removed and the pipe connected to the air filter that it would stop my engine getting so messy. It was an easy conversion to do and it worked like magic. It has left me understanding that the breather hose is very important, but I`m still not entirely sure exactly what this breather hose actually does, which is one of my first questions.


My next question is how it attaches to the air filter itself, or more specifically, "where" it attaches on the air filter. I know that we are dealing with relatively low volumes and low pressures of air here, but just go with me on this for a minute, and you may understand my confusion. When most of you attach the hose to the filter, it often involves drilling a hole in top plate and effectively pointing the hose directly down the venturi, and there are several screw in clamps and connectors to facilitate this. Surely the effect of this would mean that you either have negative pressure in there from the carb suction that due to its proximity and alignment with the hose, would mean that the breather hose would be also under suction, or the other scenario would be that you have positive pressure from the breather hose forcing air directly into the venturi and causing the mixture to be incorrect. Most of the cheapo pancake filters I have seen all have a breather hose attachment on the bottom plate facing upwards, in the same angular plane as the venturi and not firing into it. So we know that the pipe has to attach to the filter, but can the actual direction and alignment of the attachment hose cause more harm than good?


What got me thinking about this was a recent comment by Andy Robinson saying that connecting a breather to one of his carbs had resulted in rough running. The picture looked like the breather had gone into the top of the carb, pointing down the venturi, so were his troubles caused by a mixture fault which appeared to have even more of an in-balance effect as he only connected it to one carb ? If that was the case, should the breather hose be connected to both carb filters via a "T-Piece" to ensure that they are both running the same pressures? Many of the original OEM air boxes when used with twin carbs have a large plenum chamber that usually covers both carb inlets, with a single common air inlet, and any breather pipes are simply plugged into this massively large box thus ensuring that everything that reaches any carb is of equal pressure.


See my thinking here?  Could be complete twoddle of course, but my real predicament is this:


My new 2.0 litre CU Type 4 engine with twin carbs has a small vertical breather pipe coming out of the top of the crankcase, and given the rubbish I have just spouted, I`m not entirely sure where best to connect it, nor what to do with it. Originally it would have plugged into a large plastic air box that went from one side of the engine to the other, encompassing both carbs, but that has obviously long gone.


Any thoughts?   

[So may be we need to compare the amount of blow coming from different engines]

Going to be a short reply as only on iPad.

Firstly, I'm certainly not the most mechanically experienced here so very open to others knowledge.

Yes my set up was as you described Dave, and so yes for sure the air was going only into one carb filter and right above a stack on one of the chokes. So yes highly likely to mess things up. With those carb air filters it was the only place you could feed a pipe in.

However, I'm pretty sure Dan had the same issues with his single carb set up?

I know for sure though that for many the breather pipe to carb method works well.

My query would be how much air pressure is coming from the breather? My engine amazed me how much it blows out even when idling and running well. Ok it's only a weak blow but it's present constantly. Certainly a lot more than I remember the 1600 type 3 engine did on the old GP. Just for reference my engine is (well was now) an all new 1776 from Heritage SSP and has now done 6500miles since new last year. The same type Krug has. I've seen some other engines blow just as much and some blow a little less. But in my case certainly enough to mess with the fuel/air mix.

So may be we need to compare the amount of blow coming from different engines? Then am sure you will see those not blowing as much be fine with the breather pipe (balanced too for dual carbs) to carb air filter.

To recap from other threads, mine at present was a temp solution (not ideal) I have a pipe from breather to an empi breather box (already had it), the rocker covers are not vented (currently have my clip ons), the other two ports (intended for the rocker venting) on the breather box have the little air filters on too to "fill" those holes, though the box is naturally vented by the manufactured gap in the lid. I do inspect and clean the box regularly.

On my engine I modified an intake pipe to the throttle body to connect my breather pipe too So I maintained a vacuumed to my crank case. Works very well for me.

Hi Dave, the trouble with connecting an engine breather into the air filter or air filter box; is that when an engine gets old, it starts to chuck a lot of oil vapour out. This will muck your air filter up sooner or later.

As for your type 4 engine, leave it as is; it is not chucking oil out of the breather pipe at present  If it does in the future, just run a longer bit of pipe on it which can drip on the floor rather than blow an oily mist over your engine.

My new type 4 engine I built to go in my buggy has vent holes in both rocker box covers as well, I have put no pipes on these either. As yet I have not had problems with oil vapour coming out of these exits!. However I have found out that it will blow the oil dipstick out when you get to 6,000 rev's. Luckily it caught on the engine case so I didn't loose it. So even with a new build, you do get quite a bit of pressure coming out of the crankcase when the engine is working hard!

Rob

I know its going to be a while before that engine chucks anything out Rob    But I am just not overly keen on open ended pipes that don`t go anywhere and I really want to avoid dropping any oil on my nice new garage floor either now or in the future.

I also understand that the breathers can be important and just wanted to make sure that connecting that pipe was made to the best possible place, in the best possible way.

I`m guessing an oil catchment tank with the air fed into the filters to burn off any vapour is the sensible way to go.

OK, I'll have to confess to still running a dump pipe on my buggy engine (improvements are on the 'to do' list), but I've been having crankcase pressure problems with one of my motorcycles this summer, and have worked on crankcase ventilation on other car engines in the past, so do have a bit of experience in this field.

In my humble opinion, the VW crank vent system is inadequate at best.  It relies on that tiny little hole in the breather tower / pedestal to pass air into the air filter, or out to the road through an open pipe.  Now consider the engine – forget blowby and all that, just the pistons moving in and out.  The engine's a boxer – the pistons move in and out together.  That means there is over a litre of air moving into the crankcase every time the pistons move in towards the crank.  All of this air has to rush out through the tiny breather tower hole.  Then as the pistons move up, it all has to rush in again.  So essentially, the engine is using horsepower acting as a pump, forcing air in and out of an orifice.  As it's a tough job to force all of that air out, some remains in the crankcase and causes an increase in pressure, potentially forcing oil out past the crank seals, rocker gaskets etc.

To stop this air movement and pressurisation, you simply need to take the air out of the crankcase – create a vacuum (or at least a partial one).  This means the pistons will not have any air to shift, allowing the engine to turn easier.  It also gives the additional benefit of better piston ring sealing, as the rings are pulled against the grooves in the piston by the crankcase vacuum.  The way I fixed this problem on my motocycle was to make a reed valve which allowed pressure to pass out of the cases, but sprung shut as soon as the air flow reversed. So on the first kick of the motor, the air was forced past the valve, but not allowed in again.  This actually made a noticeable difference to the starting and tractability of the engine, as well as eliminating the oil leakage.  So a reed valve fitted into a hose connected to the crankcase vent, and exiting to atmosphere or the air filter would do the job for a VW aircooled, I reckon.  A solution I've used in the past (on a SAAB V4, a Citroen 2CV and a Russian bike) is the KrankVent, a self-contained reed valve in a case, which connects into the engine's breather pipe.  They're expensive and only easily obtainable from the States.  However, what I did look at recently was a valve designed for the Citroen 2CV, and sold by ECAS.  It looks to me to be a neat solution, fairly reasonably priced and is what I shall be using on my buggy, and another of my bikes, when I get around to it. 

Can't get the hyperlinks to work.

KrankVent...   http://www.haydensm6.com/kvp.htm
2CV valve...   http://www.ecas2cvparts.co.uk/reed-valve-crankcase-pressure-dump-valve-this-instead-breather-includes-clips-p-1401.html

Thanks for that explanation - has certainly answered a few questions 


Interesting what you say about the reed valve, and from memory, there used to be a little plastic valve type thing on the CU engine near where that breather vent comes out of the block

Looked something like this

http://www.ebay.co.uk/itm/271268289565?ru=http%3A%2F%2Fwww.ebay.co.uk%2Fsch%2Fi.html%3F_from%3DR40%26_sacat%3D0%26_nkw%3D271268289565%26_rdc%3D1


So, it it worth connecting something like that as well as the oil catchment / breather setup idea ?   

I am not sure there is that much pumping effect, I thought for every piston going up another is coming down so the air is moving around within the case, there will be massive turbulence and some pumping in and out of air but t he volume should not be enormous, however as Dave pointed out over a run that small amount ever revolution adds up to a lot of oil vapour over a run.
I believed that the case had to be vented to not only deal with that pumping effect but also the expansion and contraction of air, oil and the metal that would otherwise subject it to pressure and positive pressure would encourage oil past the seals and the rings.
Feeding the pipe to the air box results in a slight negative pressure in the case as well as burning off the ol vapour.

the problems we introduce are partly caused by twin carbs, we are trying to get 2 cylinders, instead of 4, burning the vapour as well as halving the air flow it gets mixed with, that is probably why it doesnt work too well on twin carbs but works fine on a bog standard Beetle with original air cleaner.

Mine vents to a breather box that gives the vapour some chance of settling out and vents the rocker covers as well allowing oil to easily flow back after cornering (also lets it flow up there easily during cornering so extended sump stops starvation - but thats another topic)

IMHO a good setup should have a catch tank, much bigger that the little breather boxes we tend to use, to catch the vapour by slowing the air from the pipe right down possible with internal vanes to further trap the particles, this must not be open to atmosphere. Then a pipe from this via a T to the air cleaners would provide the slight negative pressure needed to reduce engine oil leaks and take away the pre cleaned oil air.  I stress, this is an opinion only based on nothing other than what I think seems logical. It could vent to atmosphere via a filter but then you dont get negative pressure (but far better than positive)

I think Dave Dream uses a catch tank to collect the oily waste.

Ah, yes, good point - still got my vee-twin head on 

I really want to avoid dropping any oil on my nice new garage floor either now or in the future.

you own an aircooled vw!! 

I really want to avoid dropping any oil on my nice new garage floor either now or in the future.

you own an aircooled vw!! 

That's what cardboard is for

I have always found that you don't need negative pressure to stop breathing problems.

Personally I don't think chucking oil vapour straight into a carb is a good idea, VW didn't do it, they let majority of the oil separate from the vapour before dumping the oil through 1 of these:

https://www.vwheritage.com/vw_spares_Rubber-valve-oil-pipe-253260180_act_shop.product_pID_45843.htm?crumbStartPage=4&crumbStartRow=96

on to the floor but only at high RPM/crankcase pressure.  The best solution is probably to use 1 of these:

https://www.vwheritage.com/vw_spares_Oil-fill-funnel-dynamo-no-cap-113115451D_act_shop.product_pID_37357.htm?crumbStartPage=4&crumbStartRow=96

as per VW and vent it to both carbs,  But it's Fugly so by using 1 of these:

https://www.vwheritage.com/vw_spares_Breather-tower-Oil-Filler-CB-AC1151771_act_shop.product_pID_183604.htm?crumbStartPage=4&crumbStartRow=96

You achieve the aim of separating the oil from the vapour before venting it either to the carbs or to atmosphere through a flap valve.

If you have a type 3 or 4 engine 1 of these to sort your problems:

https://www.vwheritage.com/vw_spares_Engine-SSP-All-New-1776-No-Exchange-needed-211100037_act_shop.product_pID_74811283.htm?crumbStartPage=1

Some very interesting comments coming in guys - thanks   

What is very evident though is that rather than connecting a breather directly to an air filter, is that there needs to be something in place to separate and catch the oil from any vapour that exits the crankcase, and if possible, return the oil back to where it came from.

The theory of the reed valve mentioned seems sound, and if I understand correctly, if something like this were used



then as the vapour hit the internal flap it would automatically separate with the oil remaining on the flap and eventually running back down the pipe, whilst the air would then exit via both air filters to burn off the unwanted gasses.

Have I understood that correctly ? 


And No, Flags, I`m not buying yet another engine 



I know that there are easier ways to do this, hose to drip on the floor via a valve or venting into the atmosphere, and its not that I am going all environmentally friendly and all that, its just that I would like to keep the oil inside the engine on this one, as I now have transferred my shares from castrol to the garage floor company instead, and need to protect my investment

A standard beetle oil bath filter would probably collect the majority of amy oil mist from the breather due to the directional change of air flow above the oil bath meaning the heavier oil mist would go straight on into the oil.

A catch tank would be the way to go to keep everything clean.