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FD Oil Catch Can Plumbing Alternative?

Old 06-15-16, 12:26 PM
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FD Oil Catch Can Plumbing Alternative?

For stock twin turbo configuration, believe the conventional wisdom is to put the catch can between the oil fill neck and the primary turbo inlet and cap the PCV line from the fill neck to the UIM. This works, but maybe too good in that the OCC will fill rather rapidly from primary turbo pulling large vacuum (and thus oil), esp under boost.

Instead, has anyone done the opposite and put the OCC on the PCV line and cap the line to the primary turbo. This will keep oil from getting into the turbo and IC, however crankcase gases won't be scavenged during boost since the only route for crankcase gases would be the PCV (since the stock primary turbo route is capped and during boost the PCV is closed). That said, the boost duration is probably minimal even in track situations.

Has anyone tried this or have any thoughts about this configuration?

Last edited by TomU; 06-15-16 at 02:17 PM.
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Old 06-16-16, 03:43 PM
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Originally Posted by TomU View Post
This works, but maybe too good in that the OCC will fill rather rapidly from primary turbo pulling large vacuum (and thus oil), esp under boost.
The primary turbos aren't creating enough vacuum to pull oil out of the motor and into the catch can. I believe The oil is sloshed up and pumped out when the oil covers the vent lines. Then the crank case pressure pumps it out. I also believe that happens when in boost when exiting a corner.

Originally Posted by TomU View Post
Instead, has anyone done the opposite and put the OCC on the PCV line and cap the line to the primary turbo. This will keep oil from getting into the turbo and IC, however crankcase gases won't be scavenged during boost since the only route for crankcase gases would be the PCV (since the stock primary turbo route is capped and during boost the PCV is closed). That said, the boost duration is probably minimal even in track situations.

Has anyone tried this or have any thoughts about this configuration?
I would think you would catch similar amounts of stuff no matter which line goes to the catch can. In Fritz's setup where he leaves the PCV line in place I think the oil is going back into the intake but since you disconnect the primary turbo line it's just spared from being pushed thru the intercooler and hoses and instead goes straight to the UIM. perhaps that's the lesser of evils though?

As for boost duration I believe it's not minimal and the cause of the issue. If done right you should be I'm boost for half or more of the corner. My home track has 2 tight and long right handed corners so this is a regular issue for me and it's annoying as crap.
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Old 06-16-16, 03:51 PM
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Yeah, it's a little counter intuitive—but if you think about it, with a 2.5" inlet to draw air from, the primary turbo isn't going to suck much vacuum on a little 1/4" line. I was talking with Ari once on this.

Conversely, a big line with lots of vent off the filler neck means less pressure built up in the motor—think about what would happen if you pinch a garden hose or release it—the pressure and velocity of the water would increase if you squeeze.

I tried just about every setup imaginable. I run a -10 line off a custom filler neck to 1-quart jaz catch can with a big 2" filter on it. I don't run anything to the turbo or the UIM.

Last edited by ptrhahn; 06-16-16 at 03:55 PM.
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Old 06-16-16, 07:29 PM
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Originally Posted by Smokey The Talon View Post
The primary turbos aren't creating enough vacuum to pull oil out of the motor and into the catch can. I believe The oil is sloshed up and pumped out when the oil covers the vent lines. Then the crank case pressure pumps it out. I also believe that happens when in boost when exiting a corner.
Interesting. So there's enough gas pressure to push the oil out vs it being sucked in by the turbo. That makes sense then to disconnect the oil fill neck completely ala ptrhahn, but it would need to be vented, otherwise there's no route for the crankcase gases

Originally Posted by Smokey The Talon View Post
In Fritz's setup where he leaves the PCV line in place I think the oil is going back into the intake but since you disconnect the primary turbo line it's just spared from being pushed thru the intercooler and hoses and instead goes straight to the UIM. perhaps that's the lesser of evils though?
If you don't vent, you will need to plumb the OCC to some source where gases can be purged. To me, the UIM is the lesser of the evils because it keeps the oil out of the intake track and hooking the catch can to the UIM simply provides a little more "premix" if you will. Only problem here again is a PCV is needed to keep the boost pressure from entering the OCC, so there's no path for crankcase gases during boost. If that's a significant duration as you say, that could be problem
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