[urq] Too much Crankcase ventilation? - More

Ben Swann benswann at comcast.net
Wed Sep 5 20:02:44 EDT 2007

OK - looks like I have some closed-system options and a myriad of catch can
and vent options.  I'd prefer to keep the system closed, and am  posed with
basically two choices - EITHER #1 use valve/restriction directly into manifold
- eg. PVC valve at Port on back of head or post throttle port.  This is pretty
much the way it is shown in most diagrams I've seen for EFI/turbo systems.  OR
#2 use pre-throttle venture-effect via hole and fitting installed in the
intake hose pre-turbo.  It would not be a big-deal to install a fitting in the
rubber hose in the air intake tract, but I'm not sure I like the idea of
sucking  blowby from crankcase and head as it will likely just get trapped in
the intercooler and cause problems down the line.  This is essentially the way
the earlier CIS car did it, but just does not give me a warm fuzzy.  


The other way is to use the PVC  valve located at a port in back of the head,
or just after throttle-body.  I found the original restrictor.  It is a
fitting that is inserted in the hose.  The diameter of the fitting is about 2
cm and the hole is about 1 mm. and must restrict the flow of CC gases
sufficiently to have done the job for so long.  


I could even install a catch can inline, but it seems to me it is best to just
get any unburned hydrocarbons, condensation, and oil into the combustion
process where it just adds to the fuel content, keeps pollution down.  I would
think this is not enough to effect combustion process unless there is a lot of
oil - in which case there is some other problem.


I'm just playing cautious at this point as it seems like the large draw of
vapors from the system might be still too much, but after reading the paper
Scott turned us onto, I have to presume that most PVC valves will prevent too
much vacuum in the crankcase.  Am I correct in thinking that the tiny hole
will proportionally drop the pressure differential (vacuum) down, or will this
just reduce flow.  If it were electronics, then I'd call it a resister and say
the voltage would be lowered or current dropped, but how does the small orfice
at 20 inches of mercury effect the pressure in the larger chamber basically
combined crankcase and valve-cover volume?  Do PVC valves really work as well
as described - basically to keep the draw fairly uniform at all load levels
except for backfire or positive pressure where the valve get forced shut?  I'm


Anyway, I'll be hooking something up tonight that uses a PVC valve which
ought to be better than having no restriction which is the case presently.





From: QSHIPQ at aol.com [mailto:QSHIPQ at aol.com] 
Sent: Wednesday, September 05, 2007 4:55 PM
To: benswann at comcast.net; Djdawson2 at aol.com; vegener at post7.tele.dk;
quattro at audifans.com; urq at audifans.com
Subject: Re: Too much Crankcase ventilation? - More



I'll address this to you since you started it, but Claus and Dave's responses
are in here as well.


First, a look at how the 20vt works.  Indeed there is PCV at the valve cover
Dave, at least on a 95.5 S6, I just looked.  It's the little hose below the
turbo intake breather hose connection at the back of the valve cover.  It goes
down to the vacuum valve at the crankcase, where crankcase, valve cover and
intake feed all converge at the crankcase '8 ball'.  According to TST 218,
during high vacuum (idle) the PCV is drawn into the intake via the ck valve at
the intake manifold.  During low vacuum or boost, the PCV is drawn into the
inlet side of the turbo via the pressure control valve.


This means that a larger turbo with the same size inlet pipe will have more
vacuum pulling on the pressure control valve.  It also means that oil sloshing
in the head and oil sloshing in the crankcase, has a really good chance of
getting sucked up at idle, just off idle, launches and at a higher than stock
boost level.  Not sure why audi mounted all this so low on the block, it's
really conducive to sloshing issues of both the block and the head (guessing
the head causes more oil issues than the block, since that 8ball crankcase
design on the block has been around a long time without issue).


WRT what to use for a modded/improved PCV, I would highly suggest the supra
twin turbo app (don't forget the gasket) 1993-1998.  It operates per the
website I provided, and it has a really good turbo shutoff action on it.  And,
discounted, it's less than 10 bucks.  I noticed that the Mustang supercharger
boys use this valve as the 'standard' as well.  


For hookup on your 10vt Ben, I would consider putting this valve in place of
the stock urq pcv valve, and mount it above the valve cover somehow.  I would
think you could machine up a simple block that would tie all this together
with the stock system.  Since the supra twin turbo uses a really high
compression ratio, I'd bet the match to the orifice of that PCV valve would be
really good.


What may screw all this up, is the amount of vacuum present at the turbo
inlet.  That's pretty key to making sure the breather is acutally breathing
air vs fighting an alternative vacuum source.  Claus, wrt octane vs oil
vapor... In a properly designed PCV system, there shouldn't be enough blowby
to make this significant.  And if you read the site I posted, if 70% of that
vapor is unburnt HC, you aren't affecting octane much at all.  IME, running
back to back PCV connected vs not, there appears to be no significant
difference in dyno output (read: just another run can result in the same


Martin, PCV is one of my interests along with the black mysteries of Torsen
operation.  Call me PCV boy if you want, but I really believe there are very
few that really understand how it works, more specifically, how to work it.  I
don't see closed loop PCV as a bad thing, and both Eaton and Magnuson advocate
running PCV closed loop to carbon treat the blades of the SC and put a coating
of deposits in the plenum to screw clearance.  I don't see why this theory
also wouldn't hold true for a turbo as well.


Scott Justusson

PCV Boy formerly known as Torsen Boy

'94 Landcruiser Supercharged with modified PCV 








In a message dated 9/5/2007 3:19:48 P.M. Central Daylight Time,
benswann at comcast.net writes:

Thanks Scott,

This is helpful in furthering my understanding.  I thought I knew what a PVC
valve did and how the system functions - or is supposed to.  I think by the
end of this I'll really understand.  Until the next time that is.

So any recommendation on which valve to use - if you go to FLAPs there are at
least 50 different versions of valves and even more variation when you get
into model, port size and orientation.  Some are simple check valves and some
are apparently much more.

We have to presume that I no longer have the proper remnents of the UrQ setup,
and that the restriction would be significantly different.  I checked out the
early TQ implementation and it works because there is just a slight venture
vac. Used inside the metering box.  With speed density, no such port.




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