Small Block Chevy Venting Question.

I know this is an old thread but i have been researching this and i need some input,i am running a wiaend intake 4x2 and i do not have a PCV,only the oil filler up front on my 400 small block chevrolet,but i am blowing so much oil all over my windsheild @ high speeds.After reading everything that people have said i agree it is important to run a PCV,i don't want to drill my covers (but can if needed)and was wondering if anyone has done anything with this sort of set up i have,by the way i am running stromberg 97's,so do i need to drill my intake if i did install one in one of my covers.

 

If you are not running a mechanical fuel pump you could do it there or the 3/8 bolt hole that provides access to the fuel pump pushrod can be rigged to be a PCV or some of the guys running vintage valve covers could use the valve cover spacers for roller rockers/stud girdles and drill and tap one of those as most are 1" tall. Good ventilation is your friend. Hope this helps.

 

Being that a pcv works on vacuum, if you have an aftermarket cam, better get a pcv to match.
A Factory style PCV and breather set up is great for stock to moderate perfomance engines. Beyond that you are in a different territory, remember: an engine is a pump, the higher the RPM and/or displacement the more it pumps, past the rings into the crank case. That means that the size of the breather and the amount of vacuum matters. A PCV opens when the vacuum present on one side (manifold) and the positive pressure on the other (crankcase) push the weighted valve open.

 

also make sure it's baffled, or its just gonna suck oil.

 

This is one that screws into the base of the carb. The flow is towards the threaded end. They also make one that screws into a Corvette fill tube but the flow direction is in the opposite direction. So check the direction of the flow first. Just blow through it to check. On this one the hose goes to a fitting where the old road draft tube connected on my FE Ford engine



This is probably the most asked question on the HAMB. Do a search to learn how they work and what lots of guys use for older engines. You'll get lots of reading material

 

Vent the crank case threw the fuel pump location if you running a electric or mechanical fuel pump

 

Thanks for the info,however i am not running carbs with any vacuum at the base,i am running a 4x2 with 4 97's,would it be wise to drill the intake for vacuum there.

 

Here is my plan ( large journal block) corvette filler tube with twist on cap and fitting in tube for screw in pcv and hose to intake vacuum. Valve covers fitted with Offy breathers with filter material for inlet air. I was considering 3/8" thick spacers under early valve covers to clear roller rockers, but I haven't tried that yet.

 

Just remember that unlike a vac advance there is air flow so that extra air will lean out any cylinder near the connection. That's why most connect into the carb base and not into a separate intake runner to a cylinder.

I only run two 97 carbs so I Tee'd the vac connection to below both carbs. seen here. (bad picture) to try and equalize the added air. My Stude has a PCV valve in valley pan.



If your intake has balance tubes cast into it between the carbs then tap into the balance tube like this one. The extra air gets homogenized into the air fuel mixture.

 

Thanks man,i appreciate it very much,this maybe the way i need to go,with carb risers.The only other question is if i am running progressive linkage,do i need to "T" off to my 2 primaries (idle circuit)or do i need to go to all 4 carbs?

 

I seem to remember that there is mention of a PCV system optional in my 57 Chevrolet shop manual.

 

I wouldn't worry about the secondary carbs. There is no PCV connection on the secondaries of a 4 barrel, just the primaries.

 

So, just to beat this horse some more, if I use a PCV that screws into the back of my carb and drill my intake and run a hose to it, does it need a baffle? Or would the length of hose keep it from sucking much oil.

 

Is that the same type intake I bought from you years ago?

 

Anyone have suggestions for a "push in" valve cover breather that isn't chrome, billet or doesn't have some cheesy-assed design on it? I need to begin searching for something for my passenger side v/c soon that would fit into the stock sized grommet.

Gratis'

 

Yeah

Just buy a cheap chrome one. Then sand it and rattle can it.

 

Whats the solution on a 392 hemi,stock valve covers and vally pan?

 

On my 331 I replaced the steel wool looking stuff in the filler cap with a cut down lawn mower air filter & fit a grommet & PCV valve in the draft tube hole & ran the hose up to the PCV fitting on the Edelbrock carb.

 

I'd say the 2 primarys.

 

Small Block Chevy Crankcase Ventilation Systems and how they work:

All Small Block Chevy V8s were built with one of two basic crankcase ventilation systems. 1956 thru 1967 (except 67 350CID) used a ‘Front to Rear’ type of ventilation while 67 350s and all 1968 thru 1999 SBCs used ‘side to side’ venting. All early small blocks employed a ventilated oil filler cap and tube at the front of the intake manifold and a special hole cast into the back of the engine block positioned behind the intake manifold and just under and to the left of the distributor. The early SBCs had a ‘draft-tube’ attached to this hole that hugged the back of the block and went down to the starter area were gasses were expelled under the car. 1963 and later models had a Positive Crankcase Ventilation System which connected a PCV valve and hose between a manifold vacuum source at the back of the carburetor and a nipple attached to the same hole the old draft tube used to occupy. This PCV nipple was commonly referred to as a ‘TeePee’ since it looks like a little Indian tent with a 3/8 inch hose nipple sticking out the side of it.

The 68 and later model ‘side to side’ crankcase vent system was just that. A PCV valve in one valve cover and a closed vent tube that led to the bottom of the air cleaner in the other valve cover. This was a much more efficient vent system than the earlier type and became universally accepted for most all V8 engines industry wide.

In both systems however, the PCV valve is always connected to a manifold vacuum source and allows the engine’s low manifold pressure to ‘suck’ harmful vapors out of the crankcase and burn them as the engine runs. The problem now is, when you suck dirty air out, you have to allow fresh air in or you run the risk of sucking the crankcase into as much as 25 inches of negative pressure. And that kind of suck can literally pull the valve cover gaskets and those stupid rubber intake gaskets (you know, the ‘China-Wall’ gaskets) into the motor. Which causes big oil leaks. On 63-66 models (except California), fresh air entered the engine through the oil filler cap. On 66 California and 67 Federal Emissions engines, the system was the same but the air flow was reversed. On these systems, the oil cap was sealed on the fill tube and a PCV valve (connected to manifold vacuum) was stuck in the side of the fill tube just below the cap. The ‘TeePee’ was piped to the base of the air cleaner for air entry. This was called a ‘Closed Ventilation System’.

The PCV system on any engine works fine at idle, part throttle and cruising speeds. But, at WOT, or near it, there is no manifold vacuum. There is, however, a hell of a lot of compression, combustion and RPM going on. Even on brand new engines, a certain amount of combustion chamber pressure escapes by the piston rings and accumulates in the crankcase. This pressure has to go somewhere and without vacuum in the intake manifold, the only escape is through the breather cap or tube to the air cleaner. At engine speeds higher than three thousand RPM, the liquid oil in the pan is churned up into a mist and some of it goes with the escaping gasses through the breather cap and ends up all over your engine, firewall and windshield. That’s why drag racing engines have breather caps in both valve covers with smog pump check valves and hoses leading down to the header collectors. This is also why all later model cars with closed ventilation systems always have cleaner engine compartments than early models with just a breather.

If you want to run a SBC engine with ‘no-hole’ valve covers, you have to start with an early small block cylinder block. The 68 & later model small blocks have no hole for the ‘TeePee’ and without that hole, you will never be able to ventilate properly without a PCV valve in one valve cover and a breather or a closed vent tube in the other.

About baffles. All factory GM SBC valve covers have a ‘baffle’ under each vent hole. The baffle is simply there to reduce the discharge of liquid oil out of the breather or into the PCV valve. Using ‘side to side’ ventilation without the baffles allows the PCV valve to suck up liquid oil from the squirt from the rocker arms and make your engine smoke and mysteriously low on oil in a short time. So when choosing aftermarket valve covers, be sure they have baffles built into them. You’d be surprised by the number of cheap valve covers out there that don’t have baffles.

On the early front to rear vent systems that use the ‘TeePee’ or draft tube, be sure there is an oil separating, cylindrical baffle installed at the back of the lifter valley, under the intake manifold. Even if it’s a closed vent, reverse flow 67 small block. Keep in mind that some aftermarket or late model intake manifolds may not clear this cylindrical baffle.

Gentlemen, vent your engines.

 

Okay so I know this is an old thread and this subject has been beat to death but I'm gonna ask anyway because I can't find a definitive answer to my question. Can you over vent a 350? I have a 74 chevy 350 with an old edelbrock dual quad setup with the fill tube and unlike everyone else I have newer style finned aluminum valve covers with holes. The fill tube I have has a breather in it and I was going to run a PCV on the passenger side. The other valve cover is my concern, do I just plug it or add another breather? Will adding another breather "Confuse" (for lack of a better term) the flow of air and create issues or is a second breather okay?

 

I think over-venting will not allow the PCV to function as designed.

 

Vent cap will be fine , no issues "over venting "

 

I think if I had the PCV valve in one rocker cover, I'd put the breather in the opposite cover and run a sealed oil fill cap. To my eye it just "looks" like it would do a better job of venting the crankcase from side to side. Venting the oil fill tube and sealing the LH valve cover would probably work nearly as well. As Devin already mentioned, having two vent breathers might reduce the efficiency a bit, but without doing any real harm. At least you've had the foresight to not just seal everything up airtight and then wonder why the engine tries to push oil out around the gaskets and dipstick tube.

Does the RH valve cover have any baffling built into it to prevent the PCV valve from picking up oil splash?

 

No you can't over vent. Your Pcv will only suck in what it's designed for.

 

i have run with JUST the front oil fill tube...
and usually need s to be rag wrapped to catch blowby
because its ALL vented in ONE spot...

 

I would put a breather in the other valve cover and remove the fill tube/breather and plug it with a core plug. You already have hic-ups on the valve covers; might as well remove the extra one in front.
Probably will be a slow to fill oil due to the baffle; but no big deal. If you want to keep the front tube, use a sealed one with a twist on cap. Either way with the PCV valve in one cover and the breather in the other, it will cross vent like an OEM set up. Many ways to do this; My roadster has late block, PCV in capped filler tube, and Moon breathers on the valve cover; '47 has early block, PCV in line off the rear of the motor, and breather on the fill tube; '35 will have PCV in valve cover and regular front fill tube, unless I find another matching cover with breather holes (or buy a matching set).

 

Yes they both have baffles and thanks I try to think things through (probably too much).

 

Can you over vent the Pcv valve? The answer is no. However that's really not the question.

Too Many holes and the cross ventilation effect is gone. The incoming air will find the path of least resistance to the low pressure source, and if that path doesn't include a route thru the area that you're trying to ventilate it will not ventilate until its pressure raises.

Too many holes and it would be about as effective as leaving the Pcv valve tied on the windshield and just letting the block breath freely.

PCV stands for Positive crankcase ventilation. Positive meaning there is fresh airflow thru the crankcase. Not meaning air into a single breather or 1000 breathers and ending the theory there.

 

If you only have a breather cap on the oil fill tube but no PCV valve supplying vacuum to the crankcase, that's probably why you have to wrap a rag around the cap. The PCV valve should supply enough vacuum to the crankcase to draw outside air in thru the breather and pull the blowby and other vapors back into the intake manifold.

If you do have a PCV valve in the system it may be the wrong one or clogged up and not working properly. Or the rings have fallen off the pistons and are laying in the bottom of the pan...