Crankcase breathing

[daver clasper]

Hi

Currently Vitesse is breathing straight out from the outlet stub on rocker cover. Bit fumey.

Have bought original PCV valve and hoses to fit to between R/C and inlet manifold.

A couple of questions please.

Some oil leaks from R/C and oil filler cap.

Will  oil leaks affect carb tuning when I fit PCV  as maybe will stop it being a sealed system.

Also will fitting the valve affect carb tuning generally from the fee flowing as is now.

Any help, great please.

Cheers, Dave

[JohnD]

You could elect to direct the outlet into a catch tank, as long as you check and empty the tank as required.

[esxefi]

going further on johns post,

if you are not concerned about originality the best you can do is fit the biggest oil separator tank you can plumbed in to a naturally aspirated system.

in detail;- connect the r/c breather and ideally if you can a fabricated crankcase breather to a large separator tank,the vapour outlet can be directed to the inlet of the airbox  if you have one.
here the key is the bigger the better as a large diameter hose and tank will reduce gas speed so enabling oil mist to drop out of the air stream and condense back into liquid from which you can either drain off regularly or as I have done
connected to the dipstick tube for a permanent drain.

these engines are old tech and can burn a lot of oil once worn.
in these times of 'greenness' anything we can do to make them 'cleaner' is a good thing,..ho..hum.

[Clive]

Not sire if I am correct, but would have thought the valve would enable some vacuum to the rocker box breather, and that is helpful. It "pulls" the fumes and helps reduce the pressure build up.
Or as John has sad, feeding the breather into the airbox helps (but can result in oily air filters, that no longer let much air through)

In your case I would be fitting the valve you have purchased, see how it goes. Make sure it is complete and correctly fitted, otherwise you will get an air leak into the inlet manifold which will affect idle. (absolutely fine when correctly done though)

[daver clasper]

Hi and thanks

Finally got around to fitting the PCV valve.

Before, when it was breathing directly out, I did the healthy/unhealthy top end check, by blocking the R/C breather stub with my finger, removing the oil filler cap and putting a surgical glove over it. It blew the glove up. Normal I thought, as the engine is worn I think.

Now, with the valve fitted I tried the glove test, expecting the same. It took me surprise and sucked the glove in straight away, almost could have got mangled in the rockers.

Don't understand the vacuum principles well.

Does this vacuum seem excessive now please?.

Cheers, Dave

[esxefi]

your logic is sound.it is because the valve connects to the positive vacuum side of the inlet manifold so will open at idle under high vacuum and suck volume from the crankcase.

I've never used or had one so not too sure how it operates at part/full throttle but to me it seems that you would want maximum engine breathing at these revs rather than idle.

[daver clasper]

Thanks

Any offers on whether this vacuum is excessive. I can't find much info online, though suggests it shouldn't be too high and can cause excessive oil consumption.

Car runs pretty much the same as before I fitted it.

Any help great please.

Cheers, Dave

[RobPearce]

Sorry but esxefi is wrong. The point of the valve is that it SHUTS at idle, when there's too much vacuum. It's intended to hold a small depression in the rocker cover, which would probably be enough to pull in a surgical glove but not much more.

[daver clasper]

Hi and thanks.

As there is now vacuum at idle (there was none when it breathed openly in fact a mechanic once said there should some?), does that mean the valve does not shut completely at idle please?.

Cheers, Dave

[Nick Jones]

The valve is designed to maintain a constant, moderate vacuum within the crankcase.  At idle and light throttle cruise the pressure will be higher in the crankcase than it is in the inlet manifold.  Under these conditions full manifold vacuum would be too low for the crankcase leading to inward air leaks through the crank seals and possibly excessive vaporising of the lighter fractions of the oil.  As throttle opens /load increases and the vacuum in the manifold reduces (ie pressure rises) the valve will open to try to main the vacuum level in the crankcase. Under wide open throttle conditions there will be no vacuum in the crankcase and thus none in the crankcase but the valve will be fully open to draw as much fumes as possible through.

It's a perfectly decent system when working properly and better than the simple open pipe arrangement than came later - presumably due to cost cutting.

Nick

[daver clasper]

Thanks.

Still can't my head around it. Maybe have to look more into what causes vacuum.

Also. When I said, now there is vacuum at idle. I should have specified, that I mean't In the rocker area.

Cheers, Dave

[RobPearce]

As a physicist I find the following explanation painful to write but in "layman's terms":

The vacuum is caused by the engine pumping air out of the manifold - by drawing it into the cylinders to use it - against the restriction presented by the throttle plate.

The breather system uses some of this vacuum to pull blow-by gasses out of the crankcase.

The valve senses that vacuum and shuts down (not completely closed but nearly so) when the manifold has a lot of vacuum. This means the flow from the rocker cover to the manifold is restricted so that leaks "from" (vacuum leaking out, if you insist on dealing with vacuum as if it's a real thing, rather than leaks of air into) the crank case balance that flow.

The manifold vacuum can reach nearly 25"Hg but the crank breather only wants a couple. And yes, I know those are crazy American units but we're measuring 'vacuum' and an old-fashioned manometer is the only tool that gives 'vacuum' measurement. If I were being a physicist I'd measure in kPa absolute, which is NOT vacuum, it's absolute pressure, but then you need to understand that there's no such thing as vacuum sucking stuff 'cos it's all down to less pressure pushing back against the atmosphere...  

[JohnD]

You are so right, Rob!

And anyway, a "vacuum" is " a space entirely devoid of matter", which even Deep Space as in inter-Galactic doesn't satisfy, but the word has been corrupted, principally by the 'vacuum cleaner', to mean anything that contains less than atmospheric pressure.

What we have is a space with less in it, air or pressure, and the Physics Trueism that "nature abhors [detests] a vacuum".
Just as water runs downhill and will "find its own level", so gas in containers with different pressures will flow to fill them with the same pressure, if connected.   Gas flows out of a pressure cylinder, because atmospheric is less, or into an inlet manifold because atmospheric is more.

And just as the physicists' battle to contradict the popular belief in "centrifugal force" is doomed to failure, so is their fight to stop people thinking of a vacuum as having positive qualities.
But keep on fighting!
JOhn

[Jonny-Jimbo]

Quoted from JohnD

doomed to failure, so is their fight to stop people thinking of a vacuum as having positive qualities.
But keep on fighting!
JOhn

Just like when my Mum said 'Why can't you get a radiator that works in reverse so it makes the room colder?'

'Because cold isn't something different to hot, it's just a lack of heat energy.'
'That doesn't explain why though.'

The other one was she doesn't believe me that walls push back when you push on them, and the image in a mirror is the same distance behind as it is in front.

[daver clasper]

Fitted PCV and no more fume smells in cabin, also rocker cover has stopped leaking now some vacuum in there.

I also seemed to have improved my fuel mixture settings. Before, if I set up carbs to give some brown (along with sooty)colour plugs for city driving they would be very white on a run and car seemed to lack power. Now I have at least a grey colour after a 15 mile run up the motorway at 3000 revs and car seemed to pull well, so will live with that and hope its not running a bit weak.
Could this be related to how it's breathing now with valve fitted please?.

Cheers, Dave

[dinger]

Absolutely