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Old 01-07-2019, 12:50 PM   #2
Scooby Newbie
Member#: 422673
Join Date: Jun 2015
Chapter/Region: MWSOC
Location: Rochester, MN
2003 Subaru Imp. WRX
World Rally Blue


Nice addition to the original post and quality pictures! I'd been pondering the plumbing of the breather hoses myself as a thought experiment but have yet to get in the guts of my engine bay and verify the hose routing. From the original post combined with your input the assumption I made was the following:

1) Crank case breather is not plumbed into the valve cover breather lines and is completely separate, hence the two turbo inlets.
1.1) On boost both crank case and valve covers have approximately equal vacuum from turbo inlet source (I understand vacuum is split between valve covers so overall vacuum is greater at crank case).

1.2) Off boost crank case receives greater vacuum from intake manifold while valve covers receive minor vacuum from turbo inlet due to throttle being nearly/completely closed. Also turbo inlet for crank case breather may be providing some air directly to intake manifold bypassing the throttle body, but idle air valve may offset this extra input or the air bypassing throttle body is negligible?

Proposed Solution: Similar to yours with a slight modification which covers both on boost/off boost oil catching:

1) T the crank case and combined valve cover plumbing together and run it to the catch can. This ensures all breathers run through the catch can first before entering the intake plumbing. Also yes, the crank case does not T immediatly with one output running to the PCV and the other outlet to the catch can, the crank case runs directly to the catch can first.

2) The catch can outlet is T'd with one outlet running to the turbo inlet and the other to the PCV.

Potential issues:
1) Reduced crank case ventilation during off boost as it doesn't run directly to the PCV and is combined with the valve cover plumbing reducing the overall cross sectional volume of the hoses running from the breather T to the catch can and beyond.

2) On boost vacuum overall is reduced due to combining all three breathers together.

Potential solution to issues: Increase catch can outlet diameter to PCV/turbo inlet T and match T size to new catch can outlet. Reduce hose size from catch can outlet T to PCV but maintain hose size from catch can outlet T to turbo inlet. The hose running from the catch can outlet T should then be T'd before reaching the turbo inlet, connecting both ends of the T to the two turbo inlet nipples that used to connect the valve cover breathers and crank case breather respectively.

This would increase the vacuum force (amount is negligible as any increase is beneficial at this point) during on boost and off boost scenarios.

Diagram for reference of final solution:

Thoughts? Suggestions?
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