Crankcase evacuation system question

[JacobM03]

I am thinking of setting up a crankcase evacuation system that dumps into exhaust for my blown 5.0

I am thinking of routing one valve cover to a catch can and then down to the exhaust longtube header collector with an angled pipe and a checkvalve, would it work with only 1 valve cover routed and the other one sealed off, this more for aesthetics and simplicity. So basically would it work with only 1 breather to evac on one valve cover in a sealed system to create abit of vacuum? My exhaust would be pretty much free flow other than X pipe straight through muffler.

 

[Grog6]

What you're describing is the factory system, the air comes in the driver's vc, thru the system, out the pcv into the intake. It uses metered air from the breather tube, and burns the crap coming out the pcv; that line is where the catch can goes. Go for at least a quart, lol.

 

[XR7-4.6]

What you're describing is the factory system, the air comes in the driver's vc, thru the system, out the pcv into the intake. It uses metered air from the breather tube, and burns the crap coming out the pcv; that line is where the catch can goes. Go for at least a quart, lol.

No, that’s a factory PCV system, there would be no connection to the intake with this, which is more like an old pre-emissions era road draft tube, only connected to the exhaust to provide a low pressure area and act as a simple crankcase evac.

I know drag racers do this but I thought it only worked as an open header thing where there’s a low pressure area at at the collector, on a full exhaust I’m not sure this would work as intended

 

[Zep5.0]

Don't catch cans allow for air to separate from the oil and then escape? If so, I think you'd lose your vacuum there. Wouldn't routing 2 lines to the catch can do what's needed?

 

[Grog6]

Yeah, without plumbing it to the intake, there's no constant vacuum. You want constant vacuum, it keeps the rings seated at odd moments, but the big thing is it prevents ring flutter. That is really bad. Not as bad as what happens after the pieces of the rings leave, or during, for that matter, but bad nonetheless.Zep, the oil sep is a pass thru to the intake vacuum. Mine flows out the pass side pcv valve,into the seperator,out of the sep into the manifold.The air goes into the engine from the inlet tube to the drivers vc. a leak of air there shows up as a vacuum leak; it's unmetered air.

 

[XR7-4.6]

PCV isn’t under constant vacuum either, WOT has no vacuum, plus OP has boost which if not for the check valve can actually put more positive pressure in the crankcase. PCV isn’t the ideal solution for performance, the crankcase is essentially closed off when manifold vacuum drops and crankcase pressure intern increases,PCV is its most effective during cruising conditions with high intake vacuum, and since that’s where most street cars live it’s perfectly adequate. Dealing with crankcase pressure for high performance however brings in the relevance of open air breathers and better yet EVAC systems(a true constant vacuum source) that could be anything from a whole dry sump oil system, a vacuum pump, or the aeformentioned exhaust evac using the low pressure area of header collectors - again I’m not sure this can work with a full exhaust and am not sure it would be a constant source at anything but WOT(sort of an inverted PCV).

 

[dDUBb]

Even if a pressure differential between the exhaust and crank case exists under certain conditons, it would be substantially less effective than a system that draws through vacuum.

It would function as an excess crank case pressure relief and vaporized oil may not fill up the Catch can nearly as fast as it would on a vacuum drawn setup.

The purpose of the catch can is to seperate and contain oil vapor that has been saturated with blow by exhaust contamination / unburnt fuel - this oil tends to form sludge and should be removed from the crank case efficiently.

 

[Grog6]

PCV is close to being constant; the plenum under the intake is sized to prevent total loss, before the next cylinder opens. Adding a vacuum bottle helps some, but a race system would have an air pump.

 

[XR7-4.6]

It’s close to being constant because a street car only sees WOT for a brief period of time before you’re back in high vacuum cruise conditions, but vacuum absolutely does drop to zero with heavy to wide open throttle positions, a Vacuum gauge will show it in real time. For the vast majority of applications factory PCV + a catch can is better than the alternatives. But since the OP is asking about exhaust evac the conditions where PCV is effectively non functional (when you’re at sustained WOT like drag racing) where crankcase pressures are at their highest, that’s where the open header evac becomes relevant, just as a means of getting some crankcase pressure evacuation in those specific conditions and better than just open air breathers on the valve covers. A vacuum pump is the step above this concept, and dry sump is the best of all worlds.

With exhaust based EVAC you wouldn’t be using a catch can anyway, whole point of the separator is so oil vapor pulled from the crankcase via PCV doesn’t make its way through the intake tract.