[s-cars] What constitutes a leak?

[Trevor Frank]

I went through this as well, what is a leak, I vaguely remember some
rule of thumb..I think it was around 5psi every 30sec, I don't know what
the cumulative brain trust might say?  As well not to prusurize more
than like 15psi.  I found that the turbo compressor housing leaks on a
lot of the aftermarket turbo's.  Also it's very possible to blow air
thru the valve stem seals.  The cam's though, well the valves should be
pretty leak tight, you should get air past the pistons, but I don't
think that there is any overlap in our cams, so I would suspect that you
wouldn't have a path into and out of the cylinders.  But if you let it
sit for a while you will start to compress your crankcase and you could
start to test all of your engine seals as well, don't know if this is
such a good idea.  Typically there is a path out of the motor into the
inlet tube before the turbo, but since I assume you are pressurizing
after the maf and before the turbo your directly pressurizing the crank
case thru the vent as well.  If this is the case it really is a closed
system, from what I understand.

   Oh ya and my number above was also on a car without the vent from the
crankcase hooked up to the inlet, so it includes the leak down true the
piston rings and past the valve stem seals, since it isn't a closed
system, as well with 7a cams that I think do have some overlap. 

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] On Behalf Of djdawson2 at aol.com
Sent: Wednesday, August 17, 2005 11:18 AM
To: T.Guttmann at ms.unimelb.edu.au; s-car-list at audifans.com
Subject: Re: [s-cars] What constitutes a leak?


Tony,
Your expectations for a "leak proof" system are unrealistic.  You are
expecting to pressurize the intake tract, and have zero leaks... is what
it sounds like from your description.  When you put a plastic bag over
the exhaust pipe, you were on the right track, but drawing the wrong
conclusion.

When you pressurize the intake, where does that air go?  Well, first
through the turbo compressor housing, then the plumbing (BPV, and IC),
and finally the intake manifold.  At that point, the pressure goes down
each intake runner.  If that particular cylinder's intake valve is open,
the cylinder will fill.  BUT, if it isn't open, that cylinder will NOT
fill... or fill very slowly.

Suffice it to say that air under pressure WILL (not yelling, just
declaring the certainty on this fact) go into some cylinders.  Likewise,
some cylinders WILL have exhaust valves open, or partially open.  As a
result, you WILL certainly create a path for the air used for
pressurizing the intake to eventually escape through the exhaust system.
This is not a fault, but simply a reality of testing in this fashion.
You could be super anal and remove both the intake and exhaust
camshafts, thereby closing all valves, and reducing (not eliminating)
all possible paths for air to "leak" past the cylinder head.  OR, you
could remove your downpipe, and seal the output flange on the turbine
housing.  Then you would  be able to expect zero, or very nearly zero
pressure loss.  Bottom line... if the space shuttle were powered by a
reciprocating engine, the test results would be the same... it would
"leak"... or more accurately, simply have a path for pressure to escape
the system.

Your questions....
Do you have a leak?  My answer... If you can't hear it, you don't have a
leak subsantial enough to cause mixture issues.  Further, if you can't
hear it and trace it to a point between your turbo compressor outlet and
the point at which the intake manifold bolts up to the cylinder head,
you do not have an issue.  Your issue is elsewhere.  I would give you
the same answer to your first 3 questions.

Another behavior you can expect from even minor leaks... the engine will
typically die out when you lift throttle and allow it to return to idle.
Is it doing this?

I'm assuming you have a means to measure mixture (wideband?).  If you
do, I would suggest that you pull your ECU and install an unmodded box
that is "known good."  You will need to swap the stock injectors back in
as well.  See how it behaves.  This will effectively eliminate the
software and injectors as a possible source of your woes.

If there is no improvement... I would move on to the fuel side.  Check
fuel pressure to insure that the FPR is working properly.  To much
pressure = rich mixture.

IMHO, you have adequatley tested the intake system, and it's time to
move on to other possibilities. Best of luck, Dave


-----Original Message-----
From: Tony Guttmann <T.Guttmann at ms.unimelb.edu.au>
To: s-car-list at audifans.com
Sent: Wed, 17 Aug 2005 22:46:40 +1000
Subject: [s-cars] What constitutes a leak?


Inflated system to 20 psi. As I watch pressure gauge, it slowly drops,
over a minute or so to 4-5 psi. But surely, I think, some leakage
through the turbo must
occur?

I check by tying plastic bags around exhaust pipe outlet, and sure
enough, they slowly inflate as the pressure drops. So it slowly leaks
out through the (new) RS2 turbo.

My question, ladies and gentlemen, is do I have a leak? My mechanic, who
was doing all the work, claims that (a) expecting a turbo to be leak
proof is unrealistic (he put it more graphically), and that (b) such a
slow leakage would not significantly affect air/fuel ratios.

I told him that my impression, from the collective wisdom of this august
group, is that under the conditions I have described, 20 psi should
hold. Bullshit claims he, it's not the @#$%^ space shuttle! I shall ask
the guru-hood I reply. So here's the question:

What constitutes leak-free? I've already spent more than 12 months of my
time and my tuner's time (by remote control--we are 20,000km apart)
trying to get a decent chip set for my installation, thus far without
success. Am I, or more importantly (on this occasion) is my car leak
free under the above conditions?

And yes I have replaced the OX sensor, the ISV is leak free (100%), the
plugs and plug boots are new, and the whole car has only done 55,000
miles (new cam belt, RS2 6-speed gearbox, Koni sport shocks, blah, blah,
blah).

I eagerly await the collective wisdom. My thanks to anyone who deigns to

respond.

Tony Guttmann
'94 UrS4, '94 80 2.6 quattro (the daughter's)
'96 328i convertible (the wife's)
2004 Kona single speed and Giant OCR Compact (the son's) + 1 dead Volvo
244

_______________________________________________
S-CAR-List mailing list
S-CAR-List at audifans.com
http://www.audifans.com/mailman/listinfo/s-car-list
_______________________________________________
S-CAR-List mailing list
S-CAR-List at audifans.com
http://www.audifans.com/mailman/listinfo/s-car-list

=======
Notice: This e-mail message, together with any attachments, contains
information of Symyx Technologies, Inc. that may be confidential,
proprietary, copyrighted, privileged and/or protected work product,
and is meant solely for the intended recipient. If you are not the
intended recipient, and have received this message in error, please
contact the sender immediately, permanently delete the original and
any copies of this email and any attachments thereto.