Begin the installation by locating and
removing the factory blow-off (diverter)
valve, taking note of the hoses that connect
to it. If the fittings on your GFB Plumb Back
do not match the factory hoses, check the
adaptor listing at the end of the instructions.
For cars without factory blow-off valves, a
location must be found on the inlet piping
between the turbo and the throttle body (see
diagram below), and a suitable hose or pipe
for mounting the valve must be attached.
The standard base fitting can be mounted in two ways. It can either be clamped into
rubber/silicone hose or it can be pipe mounted directly onto the turbo piping. These two
methods are described below.
1) Hose mounting: The standard base fitting of the GFB Plumb Back will fit inside 32-
36mm I.D. hose, and should be secured using a hose clamp (not supplied). Make sure that
the supplied O-ring and grub screws are not used for this application, and that the hose
clamp band is seated on the straight section of the base, rather than the retaining bead.
2) Pipe mounting: Weld the supplied mild steel adaptor to a suitable location on the turbo
piping as shown in the diagram opposite. Also available from GFB are alloy and stainless
steel weld-on adaptors. Alternatively, check the guide below for the list of available bolt-
on pipe mount adaptors. Insert the supplied o-ring into the internal groove in the base of
your GFB valve, and partially thread the two grub screws into the tapped holes. Slide the
valve onto a 1” (25.4mm) O.D. pipe or GFB adaptor flange, making sure that the o-ring
slides completely over the end of the pipe, and then lock it in place with the grub screws.
Thread the supplied nuts onto the ends of the grub screws and tighten.
3) Most factory valves are fitted with a hose that returns the vented air to the intake prior
to the turbo inlet (plumb back or recirc system). This hose must be connected securely to
the plumb back outlet (on the side of the GFB valve) with a hose clamp (not supplied).
Various sized plumb back outlets are also available if your car has a different sized hose.
4) Connect the nipple on the top of the GFB valve to a vacuum hose from the inlet
manifold AFTER the throttle body. Make sure not to attach any other hoses to the
vacuum hose and keep the hose as short as possible, as this will ensure rapid response
from your GFB valve.
NOTE: The orientation of the GFB Plumb Back valve is critical for proper operation. If
installing this valve in an Audi/VW engine, note that it MUST be installed in the reverse
direction to the factory Bosch diverter valve. Boost MUST enter the bottom of the GFB
valve and dump through the side. Check the orientation of the valve in any installations
where the factory hoses for inlet and outlet are the same diameter.
Boost increases should always be made carefully, with the aid of an accurate boost gauge.
GFB recommends having large boost increases performed on a dyno by an expert.
Adjusting the spring pre-load
Contrary to popular belief, the spring DOES NOT need to be adjusted to hold different
boost levels. The valve will stay shut under full throttle conditions regardless of boost
pressure or spring pressure as long as the vacuum hose is properly connected. Rather, the
spring adjustment changes how easily the valve opens when you lift the throttle, and how
long it stays open when it vents. Note that the Plumb Back valve uses a softer spring than
the GFB atmosphere-venting valve range, which means the piston will be slightly open at
idle, and hence the Plumb Back cannot be used for atmosphere-venting applications
unless the spring is changed.
Unlike atmosphere-venting valves, the spring setting on Plumb Back systems is not
critical, there is very little noticeable difference between firm and soft settings, and there
is no chance of engine damage with any setting. Begin at the softest setting by winding
the vacuum nipple out until you feel the turning resistance “release”, drive the car and
then try the hardest setting by winding the nipple all the way in. If you notice no change
in the way the car drives, then simply leave it where it is.
If when driving a significant fluttering noise is heard (at full throttle or high RPM), turn
the nipple anti-clockwise a few turns. A little flutter can sometimes occur at lower RPM
under certain conditions, this is quite normal. Troubleshooting
Problem: The valve does not vent or blow off at all
• Check the vacuum source. This is the most common cause of the valve not opening.
The vacuum hose should be AT LEAST 3mm internal diameter, should be kept as short
as possible, and should not have any other hoses teed into it. Check that the hose you are
using is indeed a vacuum source, putting your finger over the end of it whilst the engine
is running will tell you if it is or not.
• Check the orientation of the valve. Boost MUST enter the bottom of the valve, and
dump through the sides. This is particularly important to check on Audi/VW engines, and
any others that use plastic Bosch style factory valves.
• Check the spring pre-load. Try turning the vacuum nipple anti-clockwise to lighten the
pre-load. Some cars (particularly those that run multiple throttle bodies such as the Pulsar
GTi-R) do not pull as much idle vacuum as others and require a softer spring. If this is
the case, contact GFB or your local dealer for a replacement spring.