HEMI Tech: Vacuum, Breathing and crank case pressure

you need to have a small amount of "airflow", otherwise a vacuum will be created in the crankcase eventually causing high oil consumption. Any pcv system has a breather. On modern cars, for instance a 2001 Ford Taurus with 3.0L V6, you will see the "breather" is a tube running from one valve cover to the inlet air tube that feeds the throttle body. On the other valve cover is the pcv plumbed to intake vacuum.

 

The Hemi is little different than any other engine when you are talking about crankcase ventilation. A pcv stuck in the tin valley cover is all that is required and I have never found a 'filter' necessary as the baffle is still there.
Along with other questionable Hemi 'gotta-haves', valve cover mounted breathers came from the blower engines of yesterday because the valley pan mounted fill tube was always removed and some sort of vent was required to relieve pressure (blow-by) created by low tension oil rings. Yes, the breathers look cool, and I have seen plenty that were mounted as dummys.

 

This is interesting info. I was thinking of a PCV in the valley and a 90* breather on each valve cover to get good cross breathing. Without breathers, where will it pull air from? Maybe I'm not understanding how these systems work... 411 please. THNX!

 

On a 427 big block Chevy I built years ago , I used gold Moroso stamped valve covers with a pcv valve coming out of the left cover , the engine was an all fresh 11.5 to 1 L88 , on start up with the engine running at about 2000 rpm's , you could see the the tops of the Moroso covers popping out and sucking in , at about 4000 rpm's it looked like the covers were gunna blow off and stay ballon shaped ,so I built a crossover balance tube with two breathers to bolt onto the front of the face of the covers and never had a problem after that , the blown hemi I am building now is having the same set up , my new Harley has a huge amout of crank case pressure , I'm going to look into that too

 

No engineer here, but I’ve dealt with PCVs for a couple years, so here’s what I can add.
The first engines didn’t have anything except poor seals and a big hole on top. Didn’t last long, but they weren’t expected to.
Next step was a vent on top and a road draft tube. Still didn’t work that well at low speeds, but when driving faster, the draft tube would create a vacuum pulling the bad air out of the engine. This is the same principle as gopher dens.
Next step (a short one) was a PCV valve replacing the draft tube. Still vented on the other side, but under most conditions would pull air from the crankcase.
A word about PCVs here; They are a metered valve with a spring and an anti-blowback valve. They are sized for the engines ability to pump air out (a function of internal pumping and RPM) and metered so the idle isn’t affected too much. They should be connected so that they are pulled equally by all cylinders (even up the vacuum leak and the oil vapors). The spring closes during a backfire to prevent a fire in the crankcase, and will help vary the vacuum leak by resisting the engine’s vacuum.
The next step was the “closed system” this is the one you see on stockers from the early-mid 60s on. The vent is connected to the air cleaner so that when vacuum is very low (hard acceleration), or the engine very worn, the vapors won’t go outside, but enter the top of the carb.
There are some new systems that do without the valve and may even be plumbed to the same spot, but we are talking about smaller engines with tighter specs and better seals. Try not to apply apples to Orange Juice.
The standard and best solution is to have the breather as far as possible from the PCV INTERNALLY. This pulls the vent air through as much of the inside as possible. On a V8, this is usually the 2 valve covers. Having the vent and valve at the front and back of the valley pan will vent only the area above the cam.
Of course, no one wants a PCV in one cover and the vent in the other, so internal baffles should be considered.
BTW, if you run your hemi hard, you may want to increase the breather and PCV size. If you are racing the thing, you really don’t want to blow seals, so you will have even bigger vents. If you can expect major issues during a run (vintage TF anyone?), you want to have big breathers to control the mess and help keep from blowing ever seal and gasket, so you need really big breathers.

Those big racers aren’t worried about blow-by vapors inside the engine either (have you seen the oil on an alky motor after a few passes?) and don’t want to worry about another part, so you guys that pull the motor down every month or less can do without a PCV. If you don’t do a teardown every year, you should run one. It will keep the inside of your engine cleaner and you won’t be leaving a stinky trail everywhere you go.

 

Excellent info and this was exactly the way I understood it as well. Thank you for your input!!!

 

The stock oil fill cap is an open air intake with a (steel wool?) filter in it. I took mine apart & replaced the filter with a cut down green foam lawn mower filter.

 

HAHA! Both my Chryslers are missing the valley cover, so I'll take your word for it!

 

the PCV system is a government-mandated pollution control system.
Early PCV systems drew air INTO the engine via the breather on the oil filler.
Later systems blocked this filler cap/breather off with a sealed cap and employed a breather connected to a tube to a seperate filter in the air cleaner housing.

The optmum place for the PCV valve is in one valve cover with the inlet breather in the other cover. This promotes air flow through the top most portion of the engine and serves to remove moisture along with pollutants.
Removing moisture is critical to preventing sludge build-up because moisture that is allowed to condense on the inside of a valve cover will create sludge.

Now, getting to the hemi configuration and the use of aftermarket breathers on the covers. Looking at the above description of the optimum "design" of the system, it should be pretty evident that, in order to remove moisture, you'll need to prmote circulation in the valve covers themselves. Since the common location for the PCV outlet is usually the old draft tube hole, adding breathers to both valve covers will promote airflow through the covers IF YOU ALSO BLOCK OFF THE OIL FILLER VENT.

Blocking off the oil filler vent (with a non-venting cap) forces the PCV system to draw fresh air in through the breathers and out through the draft tube hole.

If you don't block off the stock filler tube breather, the PCV valve will draw air in through said breather, pass over the lifters then out the draft tube hole. All you'll be doing is removing blow-by but not the moisture which rises....right up into the valve covers.

With the filler tube blocked, you pull the blow-by into the intake AND you remove the sludge causing moisture from the covers at the same time.

FWIW, I've had this type of system on my hemi since the mid seventies with no adverse effects AND I have spotless valve covers (inside).

 

I have a Hot Heads PVC Kit in the road draft hole in the valley cover, and after about 4000 miles I had a slight misting of oil all over the front of my engine.
I removed the original packing in the oil breather, and replaced it with some lawn mower foam, with some K&N Air Filter oil on it, to try this year.
If that doesn't work I've also made up a closed system, from a closed cap and a tube to the LT 1 base of my air cleaner.
What do you think, should this work?
HemiDeuce.

 

How good are your valve guides, especially the exhaust guides?
Is there more than 0.002" clearance?
If so, that's not going to help the crankcase pressure issue.

You don't have bronze guides by any chance.
If so, you'll learn to live with oin consumption, especially if you have a manual transmission behnd your hemi.

FWIW, if you can't evacuate the blowby fast enough, it'll come out the breather.

If you run a PCV valve, you MUST also have a way for fresh air to get back into the crankcase.

 

I shove a shop rag down the oil filler tube then put the cap back on. I pull the rag out to add or change the oil then shove it right back in there.

There's a pair of baffles at the bottom of my filler tube so the rag doesn't go all the way to the bottom of the tube.

 

The engine is a NOS 354 Chrysler, and I replaced all of the gaskets and installed fresh seals on the Exhaust guides, as well as new Moly Rings and valve springs.
I don't have bronze guides and it is a 4 spd. car.
In the 4000 miles I put on it last summer, it used a negible amount of oil.
My thoughts on the closed system were along the lines of the GM system on a small block, where the PVC is in one valve cover, and the other valve cover has a tube to the base of the aircleaner. I have that system on my other car, and it works well.
HemiDeuce.

 

Any info on what size PCV to use with this set up? Part #???

Thanks
Mark

 

The Hot Heads PCV kit did not have a part number on the PCV valve, and I didn't take time to try and indentify the valve for a part number. Sorry.
HemiDeuce.

 

The "closed system" wording was a bit misleading. Sorry.

Yes, that is the preferred system, however, on a hot rod engine it looks a bit modern.

The inlet tube being in the air cleaner helps keep contaminants (dust and dirt) out of the engine. Usually, there's a coarse filter inside the air cleaner to trap the aforementioned contaminants unless the tube is INSIDE the air filter element, itself, in which case the air filter serves as a filter. This system (entering inside the flter element) hasn't been used for quite a while on Detroit iron and I would imagine the reason lies in the fact that, being so close to the airhorn, the airflow would be disrupted by the carburetor intake. The only lace I've seen it recently is on aftermarket air cleaners.

Most manufacturers switched to the coarse filter in the air cleaner rim decades ago.

As for the valve guides, seals on the exhaust guides only prevent the ingestion of oil into the exhaust port. They do NOT prevent exhaust gasses from passing up through the guide and into the valve cover. Sloppy valve guides will allow this to occur.

Sloppy intake guides will allow oil to get sucked into the intake port, especially when manifold vacuum is very high. Very high manifold vacuum occurs when you're decelerating. The high manifold vacuum sucks oil down the valve stem and into the intake runner. With an automatic transmission, the deceleration doesn't raise the manifold vacuum nearly as high as would occur with a manual transmission. Add an aluminum flywheel and the condition gets even worse.

The aerodynamics of the car has a lot to do with it as well. If you're driving a brick and you have the cruise control set for 65 mph, the throttle will never come back to idle going down a steep grade and the manifold vacuum will rise, but not enought to be an issue. The throttle will remain open a bit to overcome the wind resistance at that speed.

However, set the cruise at 50 or 55 and the throttle WILL back off all the way when you go downhill and your manifold vacuum will climb up around 24 inches of mercury (or higher) because you're not fighting as much wind resistance to maintain speed.

If you don't have a hood, you can, simply, watch the throttle arm and see for yourself. If you run a vacuum gauge (as I have for 44 years) you can see, for yourself, what happens when you go down hll with the cruise control set.

The rocker arm geometry of a hemi engine creates a treendous amount of side loading on the intake valves. If you have a stock cam, it's not too bad. However, if you gave a high lift cam, the side loads are enough to tear out a set of precision-fit valve guides in 10,000 miles or so. Once the guide gets egg shaped, it wears out pretty quickly. By the time you hit 30,000 miles, it's time for new guides.

 

Thanks desoto, I appreciate your insite into this.
I did replace the original cam, with a 272 Chris Neilson cam with .450 lift so I will remember the intake wear advise, and watch for it.
HemiDeuce.

 

I've got my 354 truck motor running. After reading the last three pages it seems that I've not thought about how to properly ventilate the crankcase. I can hear air hissing in and out of the spark plug tube rubber seals when I crank it.

Mods include a HHH valley cover with just the filler opening in the front and a Weiand dual quad intake. There is no vented filler cap or draft tube to equalize the pressure.

I'm considering putting some Moon or Donovan style breathers on each of the valve covers. I like the HHH set up with the PCV valve in one of the breathers but it's pricey and I'm still not clear on the benefit of a PCV valve. I can understand it's better for the environment but how does it benefit the motor? Two breathers will run me ~$70. The HHH set up is an extra $100. Can you justify the added expense?

 

Pressure has to bleed off, has to come in & get out, right now it can't except by going around the tube seals. I've seen Fords that have blown the rear main seal out, & I blew 3 qts very rapidly out the dipstick tube when the PCV hose kinked on a Mustang at speed. The OEM set up just has air blown in through the fill cap that sticks up in the wind. The puke tube out the back is suppose to use the syphon effect to help pull moisture, gas fumes, ect out of the crankcase so your oil stays good longer, besides allowing the crankcase to relieve pressure. If you run up the revs the pressure will increase & you risk blowing some gasket out not to mention probably oil too. Your best bet was the OEM valley cover & oil fill cap with a PCV replacing the puke tube. As you are now you probably should set up a filtered inlet on the front of one valve cover & an outlet on the rear of the other, unless you can fit a PCV valve in the HH oil fill cap & put a breather on the rear of one or both valve covers.

 

Thanks for the response, George. So if I understand this breathers shouldn't be thought of as a pressure outlet, that's part of the PCV job that used to be accomplished by the draft tube. The breather is properly a fresh air intake in a well ventilated crank case. Putting breathers on both valve covers without the PCV is really just a half ass set up?

 

If you put a pcv in the valley cover, you really need to baffle it underneath to prevent it pulling oil out of the engine.

 

Exactly, you want the PCV valve to suck fresh air the length of the engine. On most engines it from valve cover to valve cover. The OEM valley cover has a metal & rubber barrier to block the mid point of the valley to force air down into the crankcase for at least a little distance. The PCV system was good at cutting down on sludged up engines.

 

I recently put pcv in a finned alloy valley cover using donovan valve covers with functioning breathers. All good, seems to evactuating all crankcase fumes, BUT I am pulling the vacuum from the rear of one log manifold [6 carb logs] and I also get a large puff of smoke on start up sometimes. It also has fouled a spark plug at the rear of the left bank occasionally. I have made a canister to plug the pcv into with an oil trap inside. This has helped a little. Looking inside another block I have here, I think it is throwing oil off the camshaft straight up thru a casting hole directly at the pcv. I am going to have to bite the bullet and pull the intake and fit a baffle under the valley cover. BTW I did freshen the engine whe I built the car, [2000 miles ago]

 

"The rocker arm geometry of a hemi engine creates a treendous amount of side loading on the intake valves"

This doesn't make any sense. There is some side loading on the rocker arms but it all is placed against the rocker stand, not the valve itself.

 

as the rocker arm rotates on the shaft, it drags the surface of the tip across the intake valve resulting in a pulling of the valve stem towards the spark plug. The higher the lift, the worse it gets.

Measure the wear in an old guide sometime. They're oval shaped towards the spark plug.

 

It doesn't do that if the rocker is arced properly and positioned properly. That's why rockers have an arc on their face. It's also why why roller tip rockers are popular, other than looking sexy. They tend to reduce the need for proper valvetrain geometry.

But if there were in fact tremendous side loading on the valve, you wouldn't see early Hemi's lasting very long at all. If you were rocking the valve with every cycle with tremendous force, they would be worn out very quickly.

I'd give you that on any engine with rocker arms there will be 'some' side loading due to normal production tolerances being not perfect. But the early Hemi is no better or worse than any other engine in this regard.

 

Although this topic is in the wrong section I'll offer my 2¢.

I'll go with desoto on part of this. The stock Hemi geometry does not like the high valve lifts that many folks think they need (based entirely on their shiverlay experience...)

IMHO, as lifts approach, and exceed, the 0.480 mark, this does indeed create sideways pressure at the tip and the desire/need for roller tipped rockers. We rarely even entertain requests for lifts over 0.495 unless other work is included in the conversation.

"They tend to reduce the need for proper valvetrain geometry."

Really? You don't see a need for proper geometry?

.

 

Well, you can have your opinion....

"They tend to reduce the need for proper valvetrain geometry."
That's exactly what I mean. A roller tip can compensate, to a degree, for improper valvetrain geometry. If the arc the rocker travels is wrong due to improper fulcrum placement (in certain planes), the roller tip will allow for this by not scrubbing across the face of the valve tip as much as a pad would. It's not ideal but it's there.

 

Opinion or experience, either way you look at it, after 49 years of driving the same car with a hemi running a 0.480" lift cam, I can attest to the fact that intake valve guides don't last much more than 30,000 miles before they're severely oval shaped.

 

19" of vacuum! How much compression and what is the cam-200 degrees?