36 replies to this topic

[Dr Terry]

Nothing wrong with venting the catch can. Early PCV systems (pre-73) had a vented oil cap & the later ones vent to the air cleaner. It's the PCV valve itself that restricts the total flow.

 

Also when adding a PCV valve to a system it's only the idle speed & mixture which is effected, so they just need resetting. Mixture @ 2,000 or 3,000 RPM won't be altered.

 

Dr Terry

[RallyRed]

Interesting topic this one.

I don't know much about it ( as usual), but I thought the whole PCV / Catch can thing went something like this -

 

1. Back in the day cars had the hockey stick thing that dumped the rocker cover gasses/ oil mist onto the road. tut tut.

2. ADRs or whatever then mandated that the said gases/mist should be instead, plumbed to the inlet manifold, so they were introduced to the combustion process and theoretically burnt off.

3.For race/revving applications, these gases/mist were plumbed from the rocker cover, to a catch can and vented to atmosphere. i.e. not to the inlet.

Further, one of the main reasons for a race/revving catch can was to catch the excess oil in the rocker cover, that may accumulate due to high oil pumping versus poor drainback to sump ( i.e. the rocker cover fills with oil). Thus, is that case, the fittings for the rocker cover hoses are in the Side of the rocker cover, not the top.  Further enhancement of this last system involves a drain from the catch can, back to the sump. This manages both the tappet cover flooding issue, the automatic draining of the tank and also equalises any pressure differential from top to bottom of motor. Thus helping drainback.

 

I guess it depends on what application you have for the catch can in the first place?

[Gatti]

Thanks.

 

I have had a fair bit of experience balancing the carbs now so that won't be an issue to sort.

 

I imagine I'd probably have to slightly richen up the carb that that gets the PCV plumbed into the runner.

 

Hopefully the PCV is delivered today so I can fit it all over the long weekend. 

 

I mainly want to get the vapour issue sorted as it's a bit embarrassing sometimes when I pull into a car meet and it's got a haze coming from under the bonnet

Edited by Gatti, 06 April 2023 - 09:17 AM.

[Gatti]

Interesting topic this one.

I don't know much about it ( as usual), but I thought the whole PCV / Catch can thing went something like this -

 

1. Back in the day cars had the hockey stick thing that dumped the rocker cover gasses/ oil mist onto the road. tut tut.

2. ADRs or whatever then mandated that the said gases/mist should be instead, plumbed to the inlet manifold, so they were introduced to the combustion process and theoretically burnt off.

3.For race/revving applications, these gases/mist were plumbed from the rocker cover, to a catch can and vented to atmosphere. i.e. not to the inlet.

Further, one of the main reasons for a race/revving catch can was to catch the excess oil in the rocker cover, that may accumulate due to high oil pumping versus poor drainback to sump ( i.e. the rocker cover fills with oil). Thus, is that case, the fittings for the rocker cover hoses are in the Side of the rocker cover, not the top.  Further enhancement of this last system involves a drain from the catch can, back to the sump. This manages both the tappet cover flooding issue, the automatic draining of the tank and also equalises any pressure differential from top to bottom of motor. Thus helping drainback.

 

I guess it depends on what application you have for the catch can in the first place?

 

That's my understanding too.

 

My car didn't have a PCV on it when I bought it and it just had a hose sitting on the the inner guard with a bit of stocking over it.

 

I fitted the catch can to dress it up and make it a bit nicer to look at 

[Gatti]

One last question, do most PCV's open and close at roughly the same amount of vacuum?

 

As there is no 'inline' type that lists as suitable for an old red motor I have just ordered two different ones to try.

 

One from Fuelmiser and the other is an Ebay special.

[claysummers]

Think the only thing missing in Col’s description is to emphasise the importance of creating flow of the crankcase gasses. Will reduce gunk and varnishing and all the factory systems provide it. The so called walking stick is properly named “road draught tube” for the obvious reason. It hangs down in the breeze and sucks the gasses out when the car is moving.

The ideal location for the air inlet (your filter) would probably be the crankcase, followed by the rocker cover. I would be concerned that manifold vacuum will draw air from the catch can filter rather than open the pcv valve, but then they do open pretty easily.


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[Gatti]

Think the only thing missing in Col’s description is to emphasise the importance of creating flow of the crankcase gasses. Will reduce gunk and varnishing and all the factory systems provide it. The so called walking stick is properly named “road draught tube” for the obvious reason. It hangs down in the breeze and sucks the gasses out when the car is moving.

The ideal location for the air inlet (your filter) would probably be the crankcase, followed by the rocker cover. I would be concerned that manifold vacuum will draw air from the catch can filter rather than open the pcv valve, but then they do open pretty easily.


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I had seen the name 'road draught tube' and had wondered what it meant.

 

I have bought a cap so I can remove the filter and cover the hole so it's a sealed system as well 

[Gatti]

That worked out well.

The PCV fitted perfectly in the AN12 fitting at the manifold




Then all the adaptors fitted and so did the cap for the catch can so it’s now sealed



I haven’t started the car out of respect for the neighbours as it’s late but I’ll fire it up in the morning and see how it runs


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[Gatti]

Fired it up this morning and re-synced the carbs. (they were still pretty close)

 

It seems happy and no more vapour!

 

https://youtu.be/fyhKYc3_8nc

[claysummers]

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[Bruiser]

One last question, do most PCV's open and close at roughly the same amount of vacuum?
 
As there is no 'inline' type that lists as suitable for an old red motor I have just ordered two different ones to try.
 
One from Fuelmiser and the other is an Ebay special.

I’ve just come across an issue here at my end about this too, so your problem has sent me off
researching for myself
With my bigger head and cam upgrade and it’s new lower vacuum issues, my Pcv seemed to stay open
at idle, and there was a lot of oil in the manifold. The engine isn’t a real heavy breather, either.
I discovered that the rattly pea is not closed at rest, but the vacuum has to defeat the spring pressure
acting against it and pull it up to close it. Low vac leaves the thing open and pulls oil all the time.
Never really thought hard about it before
Like Adam theredmotorfiend said, big cams don’t like standard pcv’s
Loads of discussion about on this on yankee forums, some suggesting fixed orifice types
with a hole around 2mm (this is what I am trying out currently)
Or pricey types that are adjustable to suit lower vac situations

Anyway, I am glad you’ve got your steamy business sorted out
Cheers and beers

[76lxhatch]

... some suggesting fixed orifice types
with a hole around 2mm (this is what I am trying out currently)
Or pricey types that are adjustable to suit lower vac situations

There is an in-between, you can get factory and aftermarket PCV valves with heavier springs, e.g. Corvette ones are common for mild engines. The fixed orifice setup will work but least effective and usually only necessary with a very wild cam.

Ideally you want something that is closed (or mostly closed) at idle so it's not a vacuum leak, but still opens up at cruise to generate some crankcase vacuum.