Is it really necessary?

[Headrope]

Is the pcv valve and hose running from the valve cover to the carb really necessary?
Emissions is not an issue on my '66, so I'm thinking that basically I'm just robbing myself of extra power by venting oil (read: poor fuel) into the carb.
I've got a '68 383 in a '66 Charger.
Thoughts?

[Ghoste]

Interesting question.  I don't know how much power loss there actually is there but since the crankcase has to be evacuated somehow, the alternative is to just dump it into the atmosphere like they did prior to pcv.  Is the power loss sufficient to make it worthwhile to plumb it to air?

[Headrope]

Plumbing it into the air would seem to be easy: just remove the hose. The pcv valve would then become another vent, and would function similar to the vented oil filler cap on the opposite end of the valve cover.
The only consequence would seem to be a little oil on the underside of the hood. I don't show my car ... heck, it'd probably be a great way to measure how much blowby there is. And if I decided to show my car, I could just wipe off the oil and put the hose back on.

[Chryco Psycho]

it will make the oil rings work harder to seal the oil out

[Ghoste]

Why is that Neil?

[Jon Smith]

leave it on you're not losing any power it shouldn't be sucking any significant amount of oil, if anything it will help by lowering crankcase pressure and helping ring seal as Neil said

[Just 6T9 CHGR]

A PCV is a controlled vacuum leak to help the rings no?

[Ghoste]

How does it help ring seal by being controlled though?  Wouldn't a vented crankcase like the older engines have provide the same thing?  I always thought the pcv was just to redirect those vapors into the induction system and provide the crankcase ventilation without releasing the pollutants into the air?

[andy74]

ghoste,i am pretty sure you are correct,but the vaccum created by the pcv not only reduces emissions,it helps seal the rings,by not having atmospheric pressure in the crank case.  in other words,the sucking of the un burned gases lets the rings do a better job of oil control!

[Ghoste]

Well, I see that in the fact that many drag cars have an evacuation line plumbed into the headers but I just wondered if it made a great difference with the average street car from the 60's?

[andy74]

a lot of the dragsters have the plumbing to the exhaust because they use low tension oil rings,those require a greater amount of vaccum to work properly.on the average street car,i dont think we will find much of a power difference,but the pcv system will help the enegine last longer,by burning the polutants out of it,and not leaving shit in the oil,and also by keeping the moisture form the air out of the crank case

[firefighter3931]

PCV is just a form emission control....nothing more. I never run a pcv valve because it puts oil vapor into the combustion chamber and impedes the combustion process, reducing power. Before pcv valves became mandatory the breathers just vented to the atmosphere via hoses aimed at the ground. I just run dual breathers on my engines. There are long stack breathers that will keep oil inside the valvecover instead of spreading it around the engine bay.    You could also run breathers and hoses into a vented catch can and accomplish the same thing.   

Ron

PS. here's a pic of the breather i use. These are going to be mounted on my "old school" M/T valvecovers.  

[Tom_440]

You could also run a clear fuel filter in your PVC hose to trap the oil if you wanted to keep the system...

[Ghoste]

Ron, how much power do you think they cost?

[Headrope]

a lot of the dragsters have the plumbing to the exhaust because they use low tension oil rings,those require a greater amount of vaccum to work properly.on the average street car,i dont think we will find much of a power difference,but the pcv system will help the enegine last longer,by burning the polutants out of it,and not leaving shit in the oil,and also by keeping the moisture form the air out of the crank case

So the crankcase doesn't vent itself? I'm not trying to be argumentative, just seems it would have to vent itself to the valve covers, since the pvc hose only runs to the valve cover.

[Chryco Psycho]

I can tell you this without a pcv or header evac & spilt a 440 inatke valley pan in 2 making 7000 rpm runs at the drag strip , I quickly added a pan evac system to the headers & never bent or split a second intake pan

[is_it_EVER_done?]

PCV is just a form emission control....nothing more.

Actually that's not true. Long before emissions requirements came into effect, cars used "road draft tubes" because they helped keep the fuel and exhaust charges blowby past the rings from contaminating the oil. The road draft tubes were angle cut, and hung down into the airstream so that a vacuum was created as the air moved around the tube (exactly as a race header evacuation system works today). A negative crankcase pressure also helps reduce ring flutter caused by the blowby which builds up in a crankcase that does not have it's internall pressure controlled. This was done for power and longevity long before any emissions requirements were mandated.

As for the question of how much power does it cost? I would like to see someone do a back to back dyno test, as the results would be quite interesting, but I would be willing to bet anything that a PVC system would show a power increase over a non vacuumed system, plus It will live a lot longer, with less maintenance, also.

[firefighter3931]

PCV is just a form emission control....nothing more.

Actually that's not true. Long before emissions requirements came into effect, cars used "road draft tubes" because they helped keep the fuel and exhaust charges blowby past the rings from contaminating the oil. The road draft tubes were angle cut, and hung down into the airstream so that a vacuum was created as the air moved around the tube (exactly as a race header evacuation system works today). A negative crankcase pressure also helps reduce ring flutter caused by the blowby which builds up in a crankcase that does not have it's internall pressure controlled. This was done for power and longevity long before any emissions requirements were mandated.

As for the question of how much power does it cost? I would like to see someone do a back to back dyno test, as the results would be quite interesting, but I would be willing to bet anything that a PVC system would show a power increase over a non vacuumed system, plus It will live a lot longer, with less maintenance, also.

The road draft tubes you're talking about were nothing more than hoses pointed towards the ground. There would be minimal venturi effect from air passing over the end of the tube pulling out the spent gases/fumes. Even the header evac systems pull low vacuum at wot....maybe 2-3 in of vacuum.  

If the engine is properly built, there won't be any ring flutter or blowby. Personally, i refuse to contaminate the combustion chamber with oil vapour. If the valvecovers are properly vented with high flow breathers there won't be any problems. Crankcase needs to vent one way or the other : whether it get's released directly into the atmosphere or recycled back into the engine is inconsequential as long as it gets done. The problem with a pcv setup on a real hot combo is that it has a hard time keeping up at high rpm. Think about it....trying to vent a 440 at 6500 rpm through a 3/8in hose isn't exactly efficient.

Walk around the pits of any dragstrip and look under the hood to see what people are running. I'm pretty sure you won't see too many pcv valve setups on serious street/strip cars.

As for a street setup, ya they work but you're polluting the incoming air/fuel mix with oil vapour. Something to think about.  

Ron

[Ghoste]

I agree with you Ron, we never had much road draft to pull fumes from the vent tube of our old farm tractors.  I believe that at speed a venturi effect would help but not much.  As for needing vacuum to vent the crankcase, I thought the whole problem was high pressure in the crankcase, hence the term "positive carnkcase ventilation".  Is it less a case of the induction pulling vapors into the carb than it is the bottom end of the engine pushing them there?

[Headrope]

Wow. This might be a DC.com first: Ron and Neil disagree on something.

I think Ghoste is on the key question: Is vacuum needed to vent the crankcase, or just to suck the vented crankcase air out of the valve cover and into the carb?

[Steve P.]

Many good points are made here. There are many considerations to go with these points. Mainly, once again it depends on your particular build. Street. Strip. Street/strip. Blower. Emission controlled environment. Show.

I think the drag tube thing went out with vacuum wipers. Higher milage motors without a PCV will leave their mark all over your driveway.. Not to mention stink up the car when the oil starts cooking on the headers..  I guess what I am trying to say is that there is no cookie cutter answer for this question in my humble opinion...

[firefighter3931]

Wow. This might be a DC.com first: Ron and Neil disagree on something.

Actually, if you read Neil's response it states that he went to a header evac system to solve the crankcase pressure issues he was having. I think that the header evac is a better option than a pcv on a hot 440 and i'm sure Neil would agree. The one caution i would make on running this type of system is that they are designed for open headers and not really for the street. The evac system will fill the exhaust pipes with oil vapour and if you're running a chambered muffler, oil will pool in the muffler case. This has been known to cause muffler explosions when the unburnt oil reaches a certain temperature (flashpoint) due to hot exhaust gases. This isn't an issue with a true flow thru design muffler like a Dynomax Ultraflow. I wouldn't run it with a chambered flowmaster, however.

Opinion are allways mixed on this subject and many still swear by the old standby PCV valve. I use one on my daily driver beater truck but then again, that engine rarely sees the high side of 4000 rpm.

Ron

[firefighter3931]

Wow. This might be a DC.com first: Ron and Neil disagree on something.

I think Ghoste is on the key question: Is vacuum needed to vent the crankcase, or just to suck the vented crankcase air out of the valvecover and into the carb ?

You answered your own question: " to suck the vented crankcase air out of the valve cover and into the carb? "

Ron

[Ghoste]

But is it being sucked or pushed into the intake?  And does it really matter anyway?

[Headrope]

Air is sucked from the valve cover into the carb.
If you pull the PVC hose off the carb while the engine is running, and put your thumb over the hole you can feel the vacuum.