25 replies to this topic

[IMORAL]

Nice bay Jason

 

Is that a vacuum line running to the catch can?

[gtrboyy]

To have it like that pcv valve would go top no filter.

 

Will work fine as long as engine is not heavy breather.

Edited by gtrboyy, 12 October 2016 - 06:58 PM.

[ozyozyozy]

The catch can should be vented to atmosphere, to be legal for the road needs to vent into air cleaner assembly.

If that is a vacuum line to the catch can, its too big, only need something small or else the engine wont idle properly as its not sucking through the carby.
The reason they fit a vacuum line to the catch tank is to reduce the steam/vapour that occurs in the catch can, its not essential, may just smell a little oily when opening the bonnet.

[gtrboyy]

Sounds kinda wrong man if goes to vac port for carby although usually base of throttle body on efi v8 if google search it

 

Pcv on carby cars normally go to fitting side or base of aircleaner.

 

Shouldn't pressurise as long as engine not got excessive blowby..check/drain it once in a while.

 

Most people just use catchcan with filter no pcv.

 

 

vac port bit usually for charcoal canister suck out fumes...2 types of charcoal cannisters I think.Assuming was done to copy efi setup due to vn heads.

[dron]

May be an older thread but here goes. The way the engin venterlation should work is the pcv draws fumes out of the cank case and the line from the aircleaner supplyes clean air to the crank case. This reduces the chance of fuel vapour building up in the crank case and therfor reducing the risk of a crank case exploshion.

 

If the vack line on the catch can has a pcv on it and is then conected to the rocker cover on the other side of the catch can,  the can should NOT be vented. the OTHER ROCKERCOVER should be vented giving you an air inlet and an outlet for the fumes. The inlet should go to the air cleaner and be drawing filtered air.

Edited by dron, 25 March 2017 - 04:49 PM.

[Punchy]

this is a 50 year old concern.

 

in short. A pcv vale system promotes a 'negative' pressure environment to the crankcase to evacuate it and meet emissions requirements.

Negative pressure means vacuum.  The engine maintains the vacuum by relying on air signal via the port at the carburettor  base plate or spacer plate port, then connected to the PCV valve and to the rocker cover ( generally). The engine attempts to negate the negative vacuum just by normal operation with  gas blow-by via pistons and rings. With the pistons and crank flogging around in there you also get air shift. In high performance engine ( read prostock and similar - in particularly aspirated engines ) creating a negative pressure area diminishes the density of any air in the crank case this reducing windage and promoted oil control. ( Remember the oil Holden Gemini vacuum pumps that were belt driven.. Was all the rage back in the ninety's to naughties onwards to fit one to get cheap high negative pressure in the engine. )

 

Pro's

1. Promote the above negative pressure to the engine and sucks that smelly shit vapour ( burning oil and contaminants and the like into the manifold to be re-burnt - meet emission requirements.

Win !

2. If your gaskets are shit its only going to suck in a bit of air affecting the state of tune and stops the oil hopefully running out your 15 year old ( or older) rear main seal

3. Negative pressure in a gravity environment will help displace oil back to the sump. hence helps with the oil in the top end issue. ( bit of thought for the geeks out there)

 

Cons'

If you have a heavy breathing engine - worn rings or is supercharged,  a pcv valve system most likely with not cope enough ( read not enough air being displaced) to maintain negative pressure and its going to go positive under the loud pedal. And burst at the seems anyways one day so fix the underlying issue of bypass.

 

 

 

In short Seal that filter off and use the PCV for how it was intended - your at best only going to have between 10' to 20 in' of vacuum inside the engine... really healthy standard or mild cam engine - big cams will vary greatly

[koalasprint]

At idle I would hope to see no vapour coming out of a fresh motor. Maybe it isn't run in yet?

[RallyRed]

l looked for a thread on this. Ive just had 355 rebuilt and noticed there is a lot of "vapour" coming out of catch can. Mine is just plumbed from rocker cover to the can. one line from each cover. No PCV etc. (i know all the legalities etc it vents to atmosphere and technically shouldn't) 

 

So two questions how much is too much vapour?

and will the amount vapour change with weather. ie on a cold rainy day will there be a little more than on a warm day?

 

TIA

I am not expert at all, but could offer the following.

We have a newly built LS2 out to 408 ci. Twin Turbo , BIG vent hoses from tappet covers to a temporary can, until we get back to it.

No hoses from can to anything.  No oil comes out of hoses at all, just vapour.

I think it may be different for a boosted motor, as the inlet manifold is pressurised rather than at vacuum.

When recently dyno'd, I commented on the amount of ""vapour"" coming out...the dyno guy who is a specialist in the these engines, said he was totally unconcerned by it. Normal.

He has commented and offered advise on lots of stuff, so I took it as a genuine comment that it was not of concern.....but it does stink.( fuelly)

[76lxhatch]

Without the PCV you don't have much choice, you will have vapour coming out because its the only exit for crankcase pressure. Make sure the lines are big.

[rexy]

My old pro stroke built 355 was nicknamed “puffing billy” when it was run in on the dyno (by someone else).
Went reasonably well until it grenaded a couple of thousand Kay’s later.

[ozyozyozy]

You will get some, the vapour is condensation aswell.
Builds up under the tops of rocker covers and in the breather hoses themselves.
Depends on rpm for breather size.
Most dont run big enough breathers

[Orange SS]

Sorry for the thread steal but I have a question about crank case pressure in my fresh 355.

 

I have a vac u pan system for ventilation connected from the front of both rocker covers(the same as NVR_ANUF's pic above) but the oil dipstick pops out which is dripping oil on the exhaust.

 

my questions:

 

1) Do I require a vent to allow fresh air back into the engine - if so where should I install one? the oil filler? remove one of the vac u pan hoses and install a vent on it?

2) Is there a way to check the vac u pan system is working / check pressure build up in the engine?

[ozyozyozy]

I have played with dry sumps that pull vacuum.
I found and is hard also to find info, under cranking on start up the pumps dont turn fast enough to pull vacuum so crankcase pressure is created.
also if doing a full rpm run then shut the throttle eg gear change or end of drag run, they create pressure.

So the fix, have done this to a few cars, run a one way valve that blows off crankcase pressure, the spring needs to be light to do this, can buy these easy enough.
When the pumps pull vacuum the valve shits to allow it.

[warrenm]

The vacu-pan system is supposed to create a "Vacuum" in the engine, you'll find that the dip stick is pushing out on deceleration(throttle off) Attach a spring from the dip stick to the dip stick tube to prevent the stick pushing out.

[Orange SS]

warrenm - I had thought that putting a spring or similar on the dipstick is just a masking the problem as there shouldn't be pressure build up like that with a vac u pan as that is why its been installed.

My engine has no breather on the rocker covers, only the 2 vac u pan connections at the front and I was tempted to only have 1 of the vac u pan lines and install a breather on the other to allow fresh air to be drawn in when required. Can anyone tell me if this is a good/bad idea?