Crankcase Ventillation

[joehonee]

I am looking for some advice on the best way to ventillate the crankcase on my 351W based 393 stroker motor. I am running a pretty mild 9.2-1 compression ratio, with a 670 Holley street avenger carb and Edelbrock performer intake. The aluminum "cobra" valve covers offer a push in style breather access on one side and the twist on oil cap on the other. I have heard many different opinions on how to set up a PCV valve setup on one side and or both sides. There also seems to be a difference of opinion on where to connect them on the vacuum side of things. Some say the vacuum port on the intake directly below the carb while others say directly to the large vacuum port on the side of the carb. Any advice would be apprechiated...I want to get this thing fired up!

[narve00]

To run a PCV system:

Run the PCV valve from the valve cover or galley (EFI guys) to a vacuum source.

Run a vent on the opposite valve cover.

Not sure which port on the potato to use (I'm an EFI guy).

[CraigS]

Agree w/ Narve. I ran the pcv hose to the big vac fitting in the base of the carb.The other VC had a breather/filler cap that had holes in the bottom and coarse screen/steel wool looking stuff inside to filter the air since this air is going to be passing through your engine.
Mine looked like this
Spectre Performance 4281 - Spectre Performance Twist-On Valve Cover Breathers - Overview - SummitRacing.com
This isn't the correct part but has a good pic to illustrate the holes underneath.

[Someday I Suppose]

I used these guys from Billet Specialties

On the passanger side this one has a PCV in it, and comes apart to replace it if ever needed.
Billet Specialties 21120 Billet Specialties Valve Cover Breathers

and just the breather on the driver side

Billet Specialties 20120 Billet Specialties Valve Cover Breathers

(check the hole sizes though)

On the PCV one, I then just ran a hose to a vac port on the back of the manifold instead of the base of the carb.

-Scott

[hotmansweep]

pvc on pass side,breather on driverside,and DITCH the 670 street avenger carb,I had one and never ran right,swapped out to a 650 dp and wow what a difference,screwed around with the 670 for 4 years,i actually threw it in the trash

[Eldrik]

What about straight hoses? This isn't an answer, its a question.
Driver-side to carb vac port. Passenger-side to under side of air filter?
Would that be too much suction coming out of both valve covers? Is the trick to have air going into one valve cover and out the other?

[joehonee]

All good ideas. The problem I am now running into is finding a matching set of breathers to fit on the push in tube (passenger side) and twist on (drivers side).

Hotmansweep...bummer about the 670. What kind of problems were you running into. Prior to throwing it in the trash of course!

[SJDave]

Originally Posted by Eldrik

What about straight hoses? This isn't an answer, its a question.
Driver-side to carb vac port. Passenger-side to under side of air filter?
Would that be too much suction coming out of both valve covers? Is the trick to have air going into one valve cover and out the other?

That's the setup that is required in California for SB100 registration. There isn't much suction inside the air cleaner if the filter is clean compared to the vacuum port on the carb below the throttle plates. So "clean" air is drawn through the passenger side valve cover from the air cleaner and through the crankcase and out the other valve cover to the carb base or intake manifold fitting....except at low rpm where the carb vacuum is too low to open the PCV valve.

[MikeKelly]

Originally Posted by Eldrik

What about straight hoses? This isn't an answer, its a question. <snip> Is the trick to have air going into one valve cover and out the other?

Straight hoses would be like creating a large vacuum leak.

From the standpoint of the engine, all you're trying to do is get the blow by out of the crankcase without blowing a gasket or blowing oil out a seal. All engines are going to have some blowby when cylinder pressures get high (WOT).

From an emissions standpoint, you'd like to burn that vapor.

The PCV valve also acts as an orifice to limit the size of the vacuum leak you're creating. It's called "positive crankcase ventilation", but you don't really want to be sucking air in through the breather cap (that's what you'll be doing at idle, if you're not careful where you tap vacuum), and you don't really want to be blowing off the vapor (but that's what you're doing at WOT because you don't have any vacuum then anyway, that's when you might want to dump it in the air cleaner)...

It's supposed to be a balance at highway speeds.

If you ever notice your dipstick (or PCV) is no longer seated after "enthusiastic" application of the throttle, you probably need some more ventilation (or you've got a broken ring, etc). Else you're liable to cause a leak, most likely at a valve cover or the front or back of the intake (PITA).

HTH,

Mike

[joehonee]

Mike...what your saying makes a lot of sense. Could you enlighten me on how you would recommend setting the system up with those points in mind?

[Horizon200]

I'm running two filter type push in breather caps on my car and am experiencing a lot of mist and recently drips of oil onto the header tube during spirited driving. I am looking to get a catch can with a breather on top and will replace the breather caps with the PCV type. Then run lines to the can.

[Todd Buttrick]

I've run many setups over the years and the best solution I've found is to run the PVC off a neck as shown or elevated far enough away that it doesn't vacuum the oil off the valve cover baffle. Run it to the large spicket on the rear base of the carb and use unmetered vacuum to the dizzy. I would NOT recommend the valve cover breather with integrated PCV. It acts as a catch can and will unload the accumulated oil from inside the breather when you get on it.