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Old 01-04-2011, 12:22 AM   #1
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Default cam cover blowby flow and crankcase pressure, tiny hole modification

I noticed slight weeping of oil into my compressor outlet when I drive the car hard. 10 psi GT2560 setup. I checked the crankcase pressure by connecting a Magnehelic pressure gauge to my dipstick tube with a hose - I get 12"-15" H20 at full power, even though the blowby flow out the cam cover breather was well within acceptable limits, by using a flow gauge.

2001 Engine is freshly rebuilt and broken in, and compression and leakdown are all good.

So something is restricting the flow, causing higher than acceptable crankcase pressure. I think I saw somewhere that it should be below 10". Perhaps too my turbo is a wee bit weepy and prefers to have less crankcase pressure. This crankcase pressure appears as a backpressure to the oil drain line, and thus drainage isn't good during hi boost operation, causing the weeping.

I examined the blowby baffling under the cam cover and saw that during boosted operation, all the blowby flows through a 1/4" hole. I plan to enlarge this hole, and possibly bore out the breather hole and enlarge the barb and hose to 1/2", up from 3/8". Ditto for the port it connects to between the air cleaner and the compressor inlet. FWIW on my E36 M3, the breather hose is at least 1/2", for a 240 hp factory rated motor. (see here: https://www.miataturbo.net/showpost....&postcount=125)

I had already done this mod (enlarging the 1/4" hole) before on my tired old 99 engine and it reduced the crankcase pressure during full power operation.

Last edited by JasonC SBB; 01-04-2011 at 12:55 AM.
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Old 01-04-2011, 12:26 AM   #2
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See photo of underside of cam cover. See 3 baffle chambers. Intake side, exhaust side, and center. I had already removed the screws holding the cover of the middle chamber.

The covers are held down with RTV. It helped to slit it where I could, before prying the covers up. Be careful to minimize the distortion of the covers, because you will have to flatten them else they may leak.

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Old 01-04-2011, 12:27 AM   #3
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See diagram of blowby flow during boost. The PCV valve will be closed.

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Old 01-04-2011, 12:32 AM   #4
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Here is the flow during cruise.

Note that any oil caught in the center baffle / chamber during high power operation, can drain back out the inlet of the middle chamber. (it's an inlet during high power operation)

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Old 01-04-2011, 12:34 AM   #5
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Here is the closeup of the 1/4" hole through which the blowby is forced.

Note that any oil trapped in the exhaust side chamber has to be slurped through this hole, sucked back into the middle chamber, for it to drain back into the cam box. This hole comes down to the "floor" of the exhaust side chamber, so it's at a low point. The breather is at a high point so liquid oil that floods here will not go out there.

I will take this to a machine shop and have the hole enlarged to at least 3/8", and possibly have a 2nd hole added. The angle of the drill is awkward due to the shape of the chambers, such that I can't do it with a drill. The machine shop will have to hold the cam cover somehow and have the drill bit come in at an angle.

Because the slurping effect will be reduced, I will have to add plastic kitchen scrubbers in the center chamber to reduce the amount of oil that gets to the exhaust side chamber.


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Last edited by JasonC SBB; 01-04-2011 at 12:45 AM.
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Old 01-04-2011, 12:34 AM   #6
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I am amazed at how much of a geek you are (in a good way)
You have an ability to measure anything anywhere anytime.

Sorry for the off topic post
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Old 01-04-2011, 12:40 AM   #7
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Here's the intake side chamber, cover removed.
Note it does not have a direct connection to the middle chamber.
It has 3 baffles, and oil trapped there will drain out the inlet.
I plan to add plastic scrubbers in here.

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Old 01-04-2011, 12:50 AM   #8
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Here is a pic of all 3 chambers with the covers off.

Weird that the middle chamber has no baffles. I plan to put scrubbers in there too.


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Old 01-04-2011, 03:20 AM   #9
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very interesting!! subscribe!
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Old 01-04-2011, 03:21 AM   #10
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Are you absolutely sure the pcv is not 'passing' boost pressure?
Have you disconnected it during a 'high-boost' run and checked the crankcase pressure?
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Old 01-04-2011, 12:34 PM   #11
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Jason, did you see Julian's attempt at scrubbers in the catch can thread? IIRC it didn't go well.
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Old 01-04-2011, 12:41 PM   #12
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Quote:
Originally Posted by RattleTrap View Post
Are you absolutely sure the pcv is not 'passing' boost pressure?
Have you disconnected it during a 'high-boost' run and checked the crankcase pressure?
This is a good point, I don't have a pcv at all, but would keep it on a street car. I run both of those ports to a catch can, seemed so solve my problems.
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Old 01-04-2011, 12:47 PM   #13
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Quote:
Originally Posted by RattleTrap View Post
Are you absolutely sure the pcv is not 'passing' boost pressure?
Yes.

Quote:
Have you disconnected it during a 'high-boost' run and checked the crankcase pressure?
I clamped the hose shut with a small vise grip.
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Old 01-04-2011, 12:47 PM   #14
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Quote:
Originally Posted by Savington View Post
Jason, did you see Julian's attempt at scrubbers in the catch can thread? IIRC it didn't go well.
Found it - excellent pics:
https://www.miataturbo.net/showpost....0&postcount=85

And his reply to himself:
https://www.miataturbo.net/showpost....&postcount=102
Quote:
I made a newer version with much less scrubber material which is away from the out ports - seems to be working. Catch can was almost empty after a day of racing. Also don't make any drain holes in the baffle covers regardless what some say.
I think I made pictures - will have to find those and post later.

P.S. Another thing is to make sure the baffle covers are well sealed with some gasket maker but I assume most already know that.
I found that out too, you need to use less scrubber and keep it away from the ports (PCV and breather). Also, if packed tightly near the ports the scrubbers will greatly increase the flow resistance.

Last edited by JasonC SBB; 01-04-2011 at 12:58 PM.
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Old 01-04-2011, 12:56 PM   #15
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BTW one of these sizes is my blowby flow gauge. *The* best way to judge ring seal.

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Old 01-04-2011, 01:34 PM   #16
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Thanks for the pics of the VVT cover. I too have a VVT head and have to do something about my venting system.

I am thinking about reaming out the PCV and breather pipes and welding on -12 fittings to connect up to a VTA catch can with a manual drain. The 1/4" slurp hole will be reamed to 3/4", maybe an inch? No scrubbers.
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Old 01-05-2011, 07:46 PM   #17
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Sweet thread. Thanks for all the yummy info!
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Old 01-06-2011, 10:40 PM   #18
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Here's the hole enlarged to 0.4", from 0.25"



The i.d. of the exhaust side breather is only 0.32". I think I'll bore it out to take a 1/2" barb.
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Old 01-10-2011, 12:20 PM   #19
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Here are my scrubbers

PCV valve in upper right:


exh side breather in upper left, inlet in bottom right:


Enlarged hole in bottom left, inlet in cover on right, not shown:
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cam cover blowby flow and crankcase pressure, tiny hole modification-cimg_2011-01-08-170316.jpg   cam cover blowby flow and crankcase pressure, tiny hole modification-cimg_2011-01-08-202030.jpg   cam cover blowby flow and crankcase pressure, tiny hole modification-cimg_2011-01-08-204932.jpg  
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Old 01-10-2011, 12:22 PM   #20
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Here is my new breather port on the valve cover. The hole is 1/4th NPT, I have an elbow with a 1/2" pipe compression fitting. I used a short length of 1/2" copper pipe. Ignore the original 3/8" breather hose lying there under it:
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cam cover blowby flow and crankcase pressure, tiny hole modification-cimg_2011-01-09-154254.jpg  
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