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A blow-by valve installation procedure (encore)
Hello, virtual club!
As a result of a recent discussion on Samco hoses and my mentioning of a
blow-by valve, I was flooded with the questions as to *What is it?*, *What
for?*, and *How?*.
It looks like I've posted the original procedure almost a year ago, and since
then a lot of new people, interested in installing the said valve, have
appeared on the list. I feel that these circumstances warrant a repost from my
archive, along with the nessesary amendments and corrections of that goofy
ASCII artwork of mine. So here it goes:
Q1. What is it for?
Suppose you are running 125mph by the end of the front straight
under 17psi of boost in the 5th with the wide open throttle.
You are approaching Turn No.1 (called "the 90" for a reason). If you
are brave enough, you'll start braking by the second pylon (braking by the
first would be suicidal). At this point you ubruptly drop the gas pedal
("lift-off") and throw your 90kg of weight on the brakes (the right
foot) and the clutch (the left foot), 'coz you need to scrub off a lot of
speed and drop down to the 3rd befor you enter that corner (it REALLY helps
to double de-clutch here, BTW).
Meantime the air charge is rushed to the throttle @17psi 'coz the
turbine inertia keeps the impeller spinning.
But, since you've just lifted off, you have closed the throttle shut.
The air charge, all upset by the obstacle, redirects it's pressure
partially towards the "accordeon" hose (hence it's shape=> it has to be
able to expand), and partially back, towards the turbo's impeller. It
will effectively slow it down or stop the turbine completely. In a worst
case scenario it will also damage the impeller's blades.
Buy that time you are in 3rd at the apex and throw your 90kg of
weight on the gas pedal, 'coz you want to power yourself through that
corner. Ooops, the turbine has stopped spinning! You nail it, but... can
you say *turbo lag*? Net result - that Jetta, that you've just blown away
on the strait, is somehow back on your heels in that corner.
There are three ways to deal with this phenomena, that are known to me:
1. A really expensive one (a Sport Quattro approach): an auxillary compressed
air tank in the car. You lift off, a valve opens, the air charge gets released
onto the turbine, preventing it from stalling.
2. Heel-and-toe. Since you really need a third foot to be on the gas
pedal (to keep the rpms above 3000 and, therefore, the turbine spinning)
while braking, clutching and shifting, that role can be delegated to your own
righ foot's heel. Make sure you install a good set of racing pedal covers (mine
are MOMOs, which are really well made for heel-and-toeing).
3. Install a dump valve, which would release positive mfld pressure
on the lift-off and prevent the turbine from stalling.
Q2.Why a closed loop blow-by valve? Isn't it easier to just dump the air charge
into the atmospere (a TAP dump valve)?
Two main reasons:
a) this is what our esteemed QSHIPQ has been warning about:
On a CIS car the fuel, which gets delivered to the injectors, is metered by the
fuel distributer, which does it by measuring the amount of air entering the
Unlike the later *hot-wire* sys ('91 200 and later), the CIS is a fairly stupid
all-mechanical *canteliever* device. It's a closed sys, that won't run if open,
that's why any vacuum leak hurts the performance, a large leak (hello,
accordeon hose!) would stall the engine. The sys runs on vacuum. When unable to
develop vacuum, which is strong enough to hold the air plate in the air (the
*air* side of the canteliver), the fuel distributor won't let any fuel out (the
*fuel* side of the canteliver)=> engine stalls.
If you let a certain amount of air in this closed sys (the throttle is open)
the fuel distributor calculates that amount of air and meteres out around
1/14th of fuel to prepare that perfect mixture with 14:1 air to fuel ratio.
Now, if you slam the throttle shut_AND_dump that extra air out of the closed
sys, the fuel, that has been metered out to match the dumped air, is still
remaining in the closed sys. Boom! You can get an enrichment spike with a
rather cool potential afterburn in the exhaust. Looks good on TV, when I watch
those rally tapes. Not sure if I want to have it on my daily driver, tho.
That's why the air, dumped by the valve, has to remain in the sys. In the right
set-up it gets returned back before the intake of the turbo and just circulates
by a freewheeling turbo impeller in the closed loop.
b) my own (admittedly paranoid) consideration:
Call me too cautious, but gasoline vapor evacuates from the gas tank through
the charcoal canister and subsequently enters the airbox en route to the intake
mfld. As crazy as it sounds, I couldn't completely rule out the possibility of
getting a long tongue of flame from under the hood on the downshift, when the
air, soaked in gasoline vapor, gets dumped into the proximity of the scorching
hot turbine (our James Marriott once sent me a jpg file with a photo of a
cherry-red exhaust mfld of a turbo Audi. *Scary* would be an understatement). I
figured this was one project where an overkill won't hurt.
Q3. How to do it?
Disclaimer: all the lengths and sizes are as being recalled to the
best of my memory. Be creative and very JUDICIOUS. Trial fit EVERYTHING!
As they say in the old country: measure seven times, cut only once.
1. The valve itself. Bosch p/n 280 142 102.
2. 2' of rubber hose, 7/8" ID.
3. 2' of rubber hose, 5mm ID.
4. A copper "T", 2.25"x2.25"x7/8" (it's a 2.25" copper pipe with a 7/8"
5. A copper elbow, 7/8"x7/8", bent at 90°.
6. A brass "T", 1/8"x1/8"x1/8" OD.
7. A 2.25"x2.25" rubber coupling, encased in a goffered tin collar
w/hose clamps riveted at both ends.
8. 4 hose clamps, 25-35mm clamping dia.
Items 2 (washer hose) and 7 (sewer coupling) can be bought at Home Depot. Items
3,6,8 at Pep Boys.
Items 4,5 at any professional plumbing supply.
You should install the blow-by valve as close to the throttle body, as
possible. The idea is to let the boost pressure escape IMMEDIATELY after
you lift off. The air charge gets returned into the sys before the turbo inlet.
a) Prepair the piece, that will go between the turbo and the airbox.
Insert Item 5 into Item 4 and align the elbow with the main body of the
"T". BRAZE the connection with silver solder. NO REGULAR Pb/Sn solder!!!
7/8" | \
|_____|braze this seam well!!
b) Remove the L-shaped black 55mm OD metall pipe, that connects the air
box rubber boot with the rubber turbo inlet hose. Insert Item 4 into one
end of Item 7, and the air box side of the L-pipe into another and make
sure they fit snug. Tighten up the worm hose clamps on both sides of
Item 7 and check that both connections are air-tight.
c) Disassemble Items 4 and 7 and the L-pipe. Carefully measure the
length of Item 4 ("A") and mark the same length on the L-pipe, starting
from the air box end. CAREFULLY cut off that piece using a plumber's
d) Reassemble Items 4, 7 and the L-pipe as per §b, but don't tighten up
the clamps yet. Now you should have basically the same L-pipe, only with
a 7/8" outlet on it's side. The 7/8" outlet should be pointing forward
in the intercooler direction.
e) Reinstall this modified L-pipe in the car. Align Item 4 in such a way
that the 7/8" side outlet is clear of the fuel injector lines and
unobstructed. Tighten up all 4 hose clamps now.
f) The blow-by valve (hereafter "the Valve") has three openings: Inlet
(22mm OD, on it's side), Outlet (22mm OD, on it's bottom) and Vacuum
Port (5mm OD, on it's top, opposite to the Outlet). ACHTUNG! DO NOT
confuse it's Inlet and Outlet or your car will be slower than the
snail's sh*t (unable to develop positive mfld pressure).
g) Cut a ~40mm long piece off Item 2. Fit it onto the Inlet of the
Valve. Secure it with a clamp.
h) Fit the remainder of Item 2 onto the Outlet of the Valve. Secure it
with a clamp.
i) Unscrew the clamp and pull the rubber cap off the third (central)
outlet of your intercooler.
k) Push the Inlet of the Valve (the open end of the 40mm hose) onto that
outlet. Orient the valve in such a manner that the 7/8" hose (Item 2) is
pointed upwards and can be run along and parallel the L-pipe. Secure
with a clamp at the intercooler outlet.
l) Push the free end of the 7/8" hose onto the 7/8" outlet of the
modified L-pipe (you will need to cut it to size on the spot). Secure it
with a clamp.
m) Push Item 3 onto the Vacuum Port of the Valve. Bend it, (while
avoiding pinching it) and run it to the rear end of the intake mfld.
n) Cut either one of the two vacuum hoses at the end of the intake mfld.
Reconnect them with Item 6. Push the remaining open end of Item 3 over
the remaining open end of Item 6.
o) DOUBLE, no TRIPLE check that the whole assy is air tight and does not
interfere with any OEM equipment.
p) Gentlemen, you may start your engins now.
Those who don't have a new style intercooler with a third outlet, should buy a
Samco hose with a third (side) outlet.
In this case skip §§g) and i).
§k) should read: push the Inlet of the Valve into the side outlet of Samco
hose. Secure it with a clamp.
Happy motoring with no turbine stumble under lift-off!
Copywright: Igor Kessel, 1997
'89 200TQ - 18psi (TAP)
'97 A4TQ - on the boat