Hi all, I have an offy dual quad intake and am looking to run a PCV off of the road draft hole on the back of the intake. I've read somewhere in this forum an opinion against running a PCV on the offy intake due to the lack of a splash sheild on the underside. I guess the concern was the potential to suck the top of the engine dry since 409's have the tendency for the oill not to bleed down easily (at least that's what I understood it to mean). In addition I would like to not run valve cover breathers and hopefully get by with just the oil tube vent. What are the opinions on this subject? I don't want to open up the old debate on PCV vs Draft tube:argue: I'd just like to know if I am asking for trouble with this setup. Do I have enough vent? Is there a better way to do it?
Offy intake and PCV setup
- Thread starter dave62
- Start date
Catch can
If you run a hose off the back of an offfy, use a vented catch can like they use for a dry sump system. Jeg's or Summit sell them (JAZ). No Mess! Just vent it, don't suck it into the carbs., catch the mess in the can. Fumes sucked back into the combustion chamber increase the chances of detination on today's cheap gas.
If you run a hose off the back of an offfy, use a vented catch can like they use for a dry sump system. Jeg's or Summit sell them (JAZ). No Mess! Just vent it, don't suck it into the carbs., catch the mess in the can. Fumes sucked back into the combustion chamber increase the chances of detination on today's cheap gas.
you are suggesting to merely replace the road draft tube with a "catch can" that will capture any oil and vent off the fumes. The idea being to avoid having to use a pcv and avoid getting the bottom of the car filthy using a road draft tube right? Sounds like a good solution. Using this method would I still need to use a vented oil cap? Or would I not need one since I would not be running the pcv pulling from the crank case?.....thanks Models.....Opinions anyone?
CDNpontiac409guy
Banned
I've used these intakes a few times. My solution... I make a sheet metal baffle which fits under the intake, at the rear, which is formed to shield the opening from direct oil splashing into it. Simply form the sheet metal to cup around the "pocket" underneath. I drill and tap one hole in the manifold casting, and attach the shield with a 1/4" or 5/16" cap screw.
Nothing new here. GM had the shield in this area specifically for the purpose of isolating the oil from the opening. It HAS to be there.
BTW... in a street vehicle, I always use a PCV system.
Nothing new here. GM had the shield in this area specifically for the purpose of isolating the oil from the opening. It HAS to be there.
BTW... in a street vehicle, I always use a PCV system.
Okay, the uppercase "It HAS to be there" has me a bit worried. As mine is going to be street engine with only the every now and then run, I went with the PCV system on an Offy. I did not install a shield. My logic, I figured the folks at Offy were a lot smarter than I was (sometimes it doesn't take much 
). I guess my question is for Aubrey, what is going to happen without the shield? I hate to open up the whole debate again. I just want to make sure I don't junk my newly overhauled baby.
Is anyone else running an Offy with PCV system? What has been your experience?
). I guess my question is for Aubrey, what is going to happen without the shield? I hate to open up the whole debate again. I just want to make sure I don't junk my newly overhauled baby.Is anyone else running an Offy with PCV system? What has been your experience?
CDNpontiac409guy
Banned
mabeauchamp said:My logic, I figured the folks at Offy were a lot smarter than I was (sometimes it doesn't take much
Smarter
...Hardly !
THEY don't drive 'em:takethat .
In highway cruise conditions ( high manifold vacuum combined with maximum oil flow in the engine ), you will notice unexpectedly high oil consumption. The intake runners will be wet with oil. The spark plugs may get sooty. It could look like you've got bad valve guides or something.
WHY didn't Offenhauser design and supply a shield with each and every one of these intakes
I have NO IDEA
It HAS to be there.
CDNpontiac409guy
Banned
Here's a picture of a new Offy intake that I just used on a customer's 452 inch 409.
The bolt hole I was referring to, gets drilled/tapped from the bottom, through the intake casting, between where you see the bulge for the vent opening and the "F" in the Offenhauser name.
Obviously, this picture was taken before I did it
The bolt hole I was referring to, gets drilled/tapped from the bottom, through the intake casting, between where you see the bulge for the vent opening and the "F" in the Offenhauser name.
Obviously, this picture was taken before I did it
No debate this time. My opinion is already known. I completely agree with models and completely disagree with Aubrey. What has high manifold vacuum got to do with oil flow? How is the oil getting into cyls? Why drill another vent hole in an intake that already has a much larger provision right next to it? Faster the rpm the more the pump delivers. Highway speed is relatively low. Less oil flow than high rpm .I will use the Offy without pcv and without added vent. Only time will tell. I will be using valve cover breathers and vented oil filler cap. To each his own.
CDNpontiac409guy
Banned
Ronnie Russell said:
I think you might have missed something here, Ronnie. The hole I drill, is not a vent. It is a tapped hole that the bolt threads into.
The "bolt", you ask ?
Yes. The one that retains the shield that I install.
High manifold vacuum has nothing to do with oil flow... unless you run a PCV system. OK, call me a tree hugger
... but I like PCV systems on a street driven engine... especially one that gets used frequently. PCV also helps remove acid contaminants from the crankcase... extending oil life... and bearing life. So, what does manifold vacuum do ?
Quite simply, it draws in "pressure" from the crankcase. The higher the vacuum, the greater the difference in pressure between the manifold ( PCV system ) and the crankcase... the greater the flow through the vent at the back of the manifold, the PCV hose, the PCV valve, and the manifold plenum... which of course leads to that high speed flow into each intake port... into each cylinder.
So, you can decide....
Have a baffle/shield in place... and "vent" crankcase gasses into the intake system.
or
Have no baffle/shield in place... and "vent" crankcase oil into the intake system
???
Even if you do not use a PCV, "venting" to atmosphere without a shield at that opening, is going to draw oil out of the crankcase.
Highway speed, for most of us, is around 3000 RPM. Pretty decent amount of oil moving around over an extended period of time.
I won't say I told you so
What we are talking about here is "splash" and vacuum, not blow by ,although blow by has a part. The camshaft gets its oil from splash from the crank. Why do you think we have to break the cam in at 2000 to 2500 rpm? It's to lube the cam with oil that is thrown from the crank! Think about where the hole is at the back of the intake manifold and where the bottom of the intake is in relation to the crank!. Now, where is the PVC hooked to when you remove the road draft tube? Stock 409 and 348 intake manifolds have a "splash" shield attached to the bottom of their intakes. The road draft tube hole is between the splash shield and the top of the intake, without the shield the oil thrown from the crank would find its way to the bottom of the intake manifold and would be drawn or splashed into the road draft tube or the PVC due to vacuum from the tube or the manifold vacuum and the splash from the crank. With all due respect, I think at RPM above idle with a proper road draft tube with sufficient forward motion that would create vacuum through the road draft tube, or with manifold vacuum and a PVC attached to the carb, oil could be drawn from the area between the intake bottom and the lifter valley with enough volume to either coat the floorboards of your chevy or foul the plugs of your W not to mention all that HOT oil being thrown on the bottom of your intake reducing your fuel volume! FYI, try reading through this post and imagine typing it let alone thinking it through after about four Gin and Tonics
Aubrey, you wont have to say " I told you so". If there is oil coming out of the road draft tube, I will stand up and say so. If so, sounds like models has the answer to the problem ( if there is a problem) I will let you know. But I am still several months away from having a car for the motor to go in. Ray,,, You win the prize!! Reading your post made my head hurt!!! I think I agree with most of it, but I will have to re-read several times. It was very impressive writing.
I can and will make my point more clear if needed. I had to read my post more than once to figure out what I was saying 
Aubrey, Thanks for the reply. Looks like I need to either pull my intake and put the shield on or drain it overboard.
I like a closed PCV system for the street, no beathers, seem to work good on my 454"s gaskets don't leak. A vacuum in the cranckcase makes horsepower so it can't hurt.
Just 99cents worth
Ray
Just 99cents worth
Ray