My current street/strip engine build will have a monster cam making very little vacuum and a race carb with no vacuum ports in the base plate. What is the best way to do crankcase ventilation on a setup like this? I figure my options are:

1) PCV in one valve cover connected to a port in a carb spacer; open breather in the other cover.
2) Two open breathers, one in each valve cover
3) Connect each cover to a small oil tank with breather (road race style)
4) Run lines from the valve covers to the header collectors (drag-race style)

My only requirements are that I don't want oil mist all over my engine bay and I don't want engine sealing issues due to crankcase pressure.

Thanks. -Neil

I'm to lazy to type it out but here (http://www.dragstuff.com/techarticles/vacuum-pumps.html) is some really good information on tunning a pump. We have used some of these guidelines and so far this information is accurate.

I'm to lazy to type it out but here (http://www.dragstuff.com/techarticles/vacuum-pumps.html) is some really good information on tunning a pump. We have used some of these guidelines and so far this information is accurate.

Thanks, that's a good link. It's interesting that the vacuum pump came about when the drag cars starting running mufflers. This leads me to believe that the header-collector evac systems don't work with closed exhaust systems. I really don't want to run a vacuum pump unless I have to, but it looks like I'll need a least 4" vacuum to prevent leaks.

You're right, collector evac systems don't work with mufflers (at least well, and if they do work at all it the exception not the norm).

If you don't want to run a pump (if you run lots of street miles) you can always run a couple of breathers into a puke tank.

I don't know if a PVC in one cover and a breather in the other will work for you, but that is the setup I prefer for a street car. Seems to work well for me and my engine only pulls about 8" at idle in gear.

They biggest issue with running a pump like this is getting vacuum set correctly. To much and you can suck the valve cover gaskets in. (usually the first gasket to fail) We haven't used any of the electric pumps just belt driven so I cant comment on those. just make sure you use a guage and a bleeder set properly and it works great. And they will make more power its not a myth. But we haven't tried this on a 300to 400 hp engine. I have seen it on the dyno on big inch 500+ 800hp and up engines. Anywhere from 10 to 20hp gains. I haven't tried to keep track of why more on some than others because there are so many variables.

You're right, collector evac systems don't work with mufflers (at least well, and if they do work at all it the exception not the norm).

If you don't want to run a pump (if you run lots of street miles) you can always run a couple of breathers into a puke tank.

I don't know if a PVC in one cover and a breather in the other will work for you, but that is the setup I prefer for a street car. Seems to work well for me and my engine only pulls about 8" at idle in gear.

Is there a point where a traditional PCV valve is no longer functional? With my 268/274@.050 cam, I'm expecting 5-6" vacuum at idle.

Another consideration, often over looked, is that the oil you suck out of you crankcase via the PCV system is ultimately burnt in the combustion chamber. As we all know oil in the combustion chamber is a contributing factor in detonation, something we generally attempt to eliminate.

I plumbed my valve cover PCV ports into the air pump on my car (vents to 4th gen Headman Headers with air pump fittings) over a year ago, but it never occurred to me till now that it might not be still working. Thanks for the info, I will now check crankcase vacuum and probably upgrade my system If as predicted the system is trashed.

Larger Dave

I'm sure there is a point where the PVC quits working, but I haven't found it yet. If there is one I bet you are getting aweful close Neil . . .

Dave has a point too, but I will say though that if you have baffles in your valve covers you won't suck much if any oil. At 11.3:1 static compression I didn't have any detonation issues on 93 octane.

I really don't want to run a vacuum pump unless I have to, but it looks like I'll need a least 4" vacuum to prevent leaks.

To prevent leaks, you only need a small negative pressure in the crankcase as leaks don't flow from low pressure to high pressure.
We run a GZ vacuum pump on our road racer and it works very well. We set the relief valve to get 8-10" in the crankcase. We also use casidiam coated wrist pins to keep any potential galling problems at a minimum.
If you're going to run a vacuum pump, make sure the engine is vacuum tested before you install it. I have a vacuum pump that I use to test for vacuum leaks while the engine is on the stand.

Use just one PCV, or put a PCV in each valve cover with no breather. Your crankcase will run a higher vacuum and you'll make (slightly) more power.
That's what I do..

I run one PCV in my SB Vega with a 125 shot of nitrous and have no problems what so ever.
On my new Camaro I plan on running an electric one so that I can shut it off when not needed in order to get oil to the pins.

Thanks guys. I will start with a traditional PCV valve setup and see how that works. I'll have to fab a baffle in the valve cover that clears the shaft rockers.

The more I think about it, the old DZ302 combos with the -140 cam used a regular PCV and that's a pretty big 'stick.

Good point about the DZ motor -- I wonder if different PCV's have different set points . . . if that is the case the old DZ valve would work great in a fairly lumpy street motor.

Let us know how it works out -- I'm interested to know how "low you can go" with manifold vacuum and a PCV

different pcv valves do have different set points.

Jim is right diffevert valves have different "values" I have been thinking about going to a vacuum pump, but after reading up on them, I am not sure that is the best option for a street driven car. The whole possible lack of wrist pin oiling is scary stuff.

OK, now what PCVs have what set points guys ???? Is there a PCV cross-reference chart out there somewhere like we have for spark plugs ???

If someone has please post -- that would be the "post of the week" IMO

OK, now what PCVs have what set points guys ???? Is there a PCV cross-reference chart out there somewhere like we have for spark plugs ???

If someone has please post -- that would be the "post of the week" IMO
I just "knew" you were gonna ask that Eric, and I have no clue... other than that the part numbers are different depending on application. a Z28 does not use the same part # as a 307 2bbl.

Bet JohnZ knows?

Here's some interesting reading....

http://www.qualitymag.com/CDA/Articles/Supplement/63b3995196c38010VgnVCM100000f932a8c0____

This article states that the PCV actually closes at a vacuum level ranging from 1-20" depending on application. Based on this, it follows that the valve is always open under WOT/zero vacuum situations. It would also be open at idle if the engine doesn't produce enough vacuum to close it. In a properly functioning PCV system, is the valve supposed to be open or closed at idle? Does it matter?

You could probably test if the PCV is open at idle by checking to see if it 'rattles' while connected to full manifold vacuum at idle.

-- I wonder if different PCV's have different set points . . . if that is the case the old DZ valve would work great in a fairly lumpy street motor.


PCV valves are picked for their initial opening value (vacuum signal) as well as their flow ratings.
As a reference you could try the old Solid-Cam PCV used in both the 302 & 350's.
It's a A/C-Delco CV-746-C or also sold as a #PV-746C under Fram and other names.
GM's HP Hydraulic Cammed engines often used an AC-736-C

PCV valves are picked for their initial opening value (vacuum signal) as well as their flow ratings.
As a reference you could try the old Solid-Cam PCV used in both the 302 & 350's.
It's a A/C-Delco CV-746-C or also sold as a #PV-746C under Fram and other names.
GM's HP Hydraulic Cammed engines often used an AC-736-C


According to the link I posted above, the valve closes as vacuum increases. Is this correct? So the 746-C valve would close at a lower vacuum reading than 736-C?

In a properly functioning PCV system, is the valve supposed to be open or closed at idle? Does it matter?


I did some more digging and now I understand that, at idle, when the vacuum is high and the force of the vacuum overcomes the force of a spring inside the PCV Valve, it is closed down to allow a flow of about three cubic feet per minute. So, if the PCV stays 'open' at idle, this contributes to rough idle conditions.

When the throttle is opened and the intake manifold vacuum is reduced (closer to atmospheric) then the spring in the PCV Valve forces the valve to open allowing up to six cubic feet per minute of flow to occur.

So, in a radical-cammed application, it seems you would want to make sure the PCV is closed at idle to protect what little engine vacuum you have.

So, in a radical-cammed application, it seems you would want to make sure the PCV is closed at idle to protect what little engine vacuum you have.

:thumbsup:

And the flow is sufficent to allow crankcase pressure to remain neutral to low...

Good info guys :D Looks like the old DZ 302 PCV would be about right for my new motor. I think both motors will pull 8-9" vacuum at idle . . .

Just for reference, I run the small GZ Motorsports vacuum pump on the street (set for 12") and got nearly 6000 street miles using the smallest pump pulley they offer before it needed a rebuild. I now have their new setup that has a positive oil feedback loop and I also run their standard size pulley. I expect to get much more mileage between rebuilds now. The pumps work as advertised.

Edmund,
Thanks for the info. I will look into what they have to offer.