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Crankcase Pressure, Pcv's And Oil Catch Cans On The Rb25

Mike_

By Mike_,
in General Maintenance

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Mike_ Rising Star

Mike_

Posted
Posted (edited)

I'm not the most technically minded person when it comes to these things, so hear me out.

Just been wondering lately a couple of things;

As far as I'm aware the RB25 is a relatively high compression engine. I would imagine that this would result in a relatively high crankcase pressure - and naturally I assume that the engine would have been designed to handle that. However how have people had it perform in this regard when modified? Naturally a good oil cooler and oil catch can would be imperative for race applications on any engine, but my specific query is towards the design of the RB25's (& silvertop RB20's, and I'm sure other engine's) standard Pressure Control Valve setup: What appears to be two PCV's, one on each rocker cover, plumbed together.

If I am correct, then I would assume that the PCV's would be at optimal performance if each valve's release was independent of the other, but I haven't ever seen this to be the case. As it stands there is surely some restriction and/or turbulence in the current system, but is flow even important for this type of pressure release? If so, each valve could run into their own oil catch can before being plumbed into the intake or released to the atmo. Wouldn't this be ideal? Or am I not fully understanding how these things work... Plumbed together seems somewhat like a bandaid fix...

My reasoning is that if enough air pressure was released from valve one, it would impact on the rate of air pressure released from valve two as the air was forced past it.

Would there then be further benefit in drilling out the existing PCV location and installing a higher flowing PCV?

Cheers

- Mike

Edited by Mike_
TurboX Mentor

TurboX

Posted
Posted

"As far as I'm aware the RB25 is a relatively high compression engine. I would imagine that this would result in a relatively high crankcase pressure"

Ummmm... no, the compression has nothing to do with the crankcase and a turbo engine by definition is usually moderately low compression anyway.

The crankcase gasses are the product of blowby - what escapes around the piston rings and is usually in a fit low mileage engine, very little.

So the pcv only needs to vent freely in a way that is not to atmosphere without being filtered or back into the engine via the intake before the airfilter preferably. In this instance you dont even need the PCV one way valve.

The PCV is only needed when piped directly into the intake manifold.

Mike_ Rising Star

Mike_

Posted
  • Author
Posted (edited)
"As far as I'm aware the RB25 is a relatively high compression engine. I would imagine that this would result in a relatively high crankcase pressure"

Ummmm... no, the compression has nothing to do with the crankcase and a turbo engine by definition is usually moderately low compression anyway.

Noted. Cheers :(

The crankcase gasses are the product of blowby - what escapes around the piston rings and is usually in a fit low mileage engine, very little.

Noted

So the pcv only needs to vent freely in a way that is not to atmosphere without being filtered or back into the engine via the intake before the airfilter preferably. In this instance you dont even need the PCV one way valve.

The PCV is only needed when piped directly into the intake manifold.

Surely the PCV would be needed in all applications though? no release valve would mean too much pressure buildup when the engine is under load. I may be wrong.

My question is really in regards to the twin PCV setup on the RB25 specifically - and whether they would be more efficient if they worked independently

Edited by Mike_
blind_elk Grand Master

blind_elk

Posted
Posted

There is only 1 PCV valve - located in the front of the plenum. The 2 fittings you are referring to are simply pieces of pipe connected by the rubber hose to join each rocker cover to the PCV valve. The other end (passenger side) is an open feed back to the inlet side of the turbo.

The PCV valve is not a pressure release valve. It is a one-way valve, that opens under the vacuum in the plenum chamber, most particularly when throttle is closed. It is designed to cause the engine to ingest, and burn, excess oil vapour, rather than vent the vapour to atmosphere.

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