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Old 01-15-2015, 06:49 PM
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Tom M'Guinn
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Default Catch cans and crank pressure

Under repeated 20psi+ boost runs, I sometimes get enough crankcase pressure to push up the dipstick. Probably blow-by I'm stuck with. Today, oil was leaking out both the top and bottom of the dip stick tube (where it plugs into the pan) and started smoking on the exhaust. I'll shore up the dipstick mounting for sure, but would like to find some way of venting the crankcase better.

For those who have installed catch cans (open or closed), does it help with crankcase pressure at all or does it just keep the oil out of the motor? Does drilling the AOS hole bigger help relieve crankcase pressure with a closed catch can?
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Old 01-15-2015, 07:44 PM
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Bill
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Tom,

Typically excessive crankcase pressure comes from blow by the rings. You might want to perform a compression or a leak down check. Rigging a catch can may only be putting a bandaid on the real issue.

I run 22psi and while my adrenaline is rushing....my dipstick remains limp.
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Old 01-15-2015, 08:46 PM
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Tom M'Guinn
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Originally Posted by Bill View Post
Tom,

Typically excessive crankcase pressure comes from blow by the rings. You might want to perform a compression or a leak down check. Rigging a catch can may only be putting a bandaid on the real issue.

I run 22psi and while my adrenaline is rushing....my dipstick remains limp.
Thanks, I understand...Working on another "next level" motor, but in the meantime enjoying this one as is...
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Old 01-15-2015, 11:23 PM
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Compression/leakdown is a good idea, but highly boosted turbo engines are always going to exhibit blow-by to some degree.

I'm not sure how your catch can is routed, but a "closed" system like the factory version but with a catch can in-line is a good idea. All of Porsche's factory Turbo breather systems (early 930 to present day) are designed to maintain a vacuum in the crankcase.

When the crankcase is vented to the inlet side of the turbo (J-boot), it maintains vacuum in the crankcase, which helps ring sealing and frees up a small amount of power. This is why you see some high-HP drag racing V8s with engine-driven mechanical vacuum pumps connected to the crankcase. The amount of power that is freed up in these super high compression engines easily overcomes the parasitic drag of the pump.

Of course, the downside of the "closed" breather system is that any blow-by will introduce oil into the intake tract, which not only makes a mess but can dilute the intake charge and lower the effective octane rating of the fuel. This is were oil separation comes in, so hopefully your catch can employs some form of mesh screen and/or labyrinth to separate the oil droplets from the air.

This is where many of the over-complicated newer factory BMW/Mini/Audi turbo breather designs fail. An excessive amount of oil vapors are floating around in the intake manifold, and this is how the DFI intake valves form carbon buildup that eventually results in misfires. At my shop, we clean BMW/Audi DFI intake valves often. Porsche's DFI breather systems are much better, and I have never seen one with enough buildup to warrant repair.
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Old 01-15-2015, 11:50 PM
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Originally Posted by Droops83 View Post
Compression/leakdown is a good idea, but highly boosted turbo engines are always going to exhibit blow-by to some degree.

I'm not sure how your catch can is routed, but a "closed" system like the factory version but with a catch can in-line is a good idea. All of Porsche's factory Turbo breather systems (early 930 to present day) are designed to maintain a vacuum in the crankcase.

When the crankcase is vented to the inlet side of the turbo (J-boot), it maintains vacuum in the crankcase, which helps ring sealing and frees up a small amount of power. This is why you see some high-HP drag racing V8s with engine-driven mechanical vacuum pumps connected to the crankcase. The amount of power that is freed up in these super high compression engines easily overcomes the parasitic drag of the pump.

Of course, the downside of the "closed" breather system is that any blow-by will introduce oil into the intake tract, which not only makes a mess but can dilute the intake charge and lower the effective octane rating of the fuel. This is were oil separation comes in, so hopefully your catch can employs some form of mesh screen and/or labyrinth to separate the oil droplets from the air.

This is where many of the over-complicated newer factory BMW/Mini/Audi turbo breather designs fail. An excessive amount of oil vapors are floating around in the intake manifold, and this is how the DFI intake valves form carbon buildup that eventually results in misfires. At my shop, we clean BMW/Audi DFI intake valves often. Porsche's DFI breather systems are much better, and I have never seen one with enough buildup to warrant repair.
I currently have no catch can. If I put one in, will it help reduce crankcase pressure under boost.
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Old 01-15-2015, 11:57 PM
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I've pondered and farted around with this whole crankcase vent issue; and over time I've come up with a simple solution.
Replace the oil filler cap with a cut piece of foam to fit into the oil filler opening. Cap off the port from the AOS to the turbo compressor. You now have an independant crankcase vent to atmosphere, where you leave the oil filler cap off, and the crankcase vents through a huge opening (the oil filler neck), but the foam traps the oil vapour. Any crankcase pressure escapes instantly and the intake track, including IC, remains squeeky clean (catch cans still let oil through, somewhat)
I'm all about function before form.
Is it redneck; yes.
Is it effective; very
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Old 01-16-2015, 12:54 AM
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...but then don't you get oil vapor all over your engine bay?
and in the land of spontaneous road-side smog-checks that might not fly so well.

have you considered one of those vacuum pumps?
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Old 01-16-2015, 01:36 AM
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I found the link to the one everybody raved about back in 2007.

http://www.saikoumichi.com/951_page.htm
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Old 01-16-2015, 02:04 AM
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Old 01-16-2015, 03:03 AM
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Originally Posted by Tom M'Guinn View Post
I currently have no catch can. If I put one in, will it help reduce crankcase pressure under boost.
Answer to this is no. All a catch can does is catch the oil going back into the intake giving a cleaner combustion. I have a catch can on a freshly rebuilt engine and i only get condensation in the catchcan.
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Old 01-16-2015, 06:11 AM
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Originally Posted by Paulyy View Post
Answer to this is no. All a catch can does is catch the oil going back into the intake giving a cleaner combustion. I have a catch can on a freshly rebuilt engine and i only get condensation in the catchcan.
Same for me, only slightly oil contaminated water on my expensive ultra exclusive coca cola catch can. Hooking the AOS line to the intake will make vacuum in the engine and increase the sealing of piston rings.

If you are using e85 be aware there may be need for more breathing than running normal gasoline. (This I have been told by several guys running E85, don't remember the technical explanation for why)
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Old 01-16-2015, 06:56 AM
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A lot of cars employ electric crankcase vacuum pump, maybe that would work.
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Old 01-16-2015, 09:28 AM
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Tom a catchcan won't help.
Like pauly I have one just for condensation.
You should inspect your AOS.
A tiny crack in my AOS caused a similar situation to yours.
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Old 01-16-2015, 10:52 AM
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Originally Posted by Tom M'Guinn View Post
Under repeated 20psi+ boost runs, I sometimes get enough crankcase pressure to push up the dipstick. Probably blow-by I'm stuck with. Today, oil was leaking out both the top and bottom of the dip stick tube (where it plugs into the pan) and started smoking on the exhaust. I'll shore up the dipstick mounting for sure, but would like to find some way of venting the crankcase better.

For those who have installed catch cans (open or closed), does it help with crankcase pressure at all or does it just keep the oil out of the motor? Does drilling the AOS hole bigger help relieve crankcase pressure with a closed catch can?
Hey Tom,
I suspected that I was pressurizing my crankcase as well. I took one of the allen plugs on the cam tower out and rigged up a hose connection to the open port using some spare fittings. I ran the hose into the cab and connected it to a gauge so I can monitor pressure/vacuum as I drove. At idle the gauge was at zero but as I increased in boost, so did the pressure in the crankcase. Crankcase pressure got up to about 8 PSI before I lifted the throttle. I opened the hood, pulled the dipstick out and drove it again. At idle the crankcase gauge read zero and under boost it reached 1 or 2 PSI. This test told me that I had to 'free up' the air flow out of the block.

What complicates this is that the MAF does not provide any vacuum to the crankcase (at least mine doesn't) through the line that connects to the AOS. The factory J-boot is more restrictive and a better vacuum source for the crankcase as well as helping drain the oil through the turbo. In my setup I must apply a small amount of vacuum, from the intake to the crankcase, so that the turbo won't smoke at idle. The line from the intake to crank has 2 one way check valves inline to insure that no boost enters the crankcase.

Having that line from intake to crank gives me about -3psi or 80KPA in the crankcase at idle. Does anyone know how much vacuum should be in the crankcase?

You could run a similar test using the vacuum/pressure gauge connected to the crank. See what it reads and pull dipstick or oil filler cap and test again. Seeing as though your dipstick tube leaks it might be safer to remove the filler cap instead.
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Old 01-16-2015, 11:58 AM
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Howabout running your "vac source" line into the exhaust piping with a check valve like the old-timey racers?
Venturi effect of rushing exhaust would pull the crankcase right down.
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