This article was first published in 2005.
So far in this series we’ve talked about selecting a blower, making the
brackets and organising the belt drive. Now it’s time to cover the plumbing that
leads to and from the supercharger. The blowers commonly available from wreckers
are of the ‘plumbed-in’ type, where both connections are via pipes. But what if
the plumbing that comes with the blowers doesn’t fit? What are bypass valves?
And what about noise?
Inlet and Outlet
If it’s at all possible to use the inlet and outlet fittings that came with
the blower, do so! This is the case for a number of reasons:
- You won’t have to make new fittings
- The original fittings are likely to have breather hose connections, etc,
already on them
- They’ll be designed to minimise noise
- They’ll usually be in the form of cast alloy pipes that stay close to the
blower, giving you a compact overall package
On the other hand, if the original inlet and outlet pipes foul other
components, you’ll need to do some more work. Generally, new connections can be
made by having flanges cut from steel and then welding (or brazing) copper or
steel tube to them. If clearances are so tight the pipe will need dimples or
other shaping to clear the obstacles, you’ll find copper pipe easier to ‘work’.
(And you can use the tight, pre-formed copper bends available from plumbing
suppliers.) However, mild steel is more commonly used to make the plumbing, and
When the flanges are being cut, make sure the internal openings are a match
for the shapes of the inlet and outlet ports of the blower. In some cases, the
blower ports will be oddly shaped; this is done to reduce noise. If the ports
are other than round, the inlet and outlet pipes will need to be formed to match
All of the engine plumbing connections that originally connected to the
intake prior to the throttle body will need alteration. If it’s a system where
the supercharger blows into the throttle, these hoses will need to be re-routed
so that they connect ahead of the supercharger. If the throttle is placed in
front of the supercharger, these connections will need to be made ahead of the
throttle. Examples include breathers for the crankcase, fuel tank and charcoal
canister. If these changes aren’t made, the lines that formerly had a low
pressure on them (near ambient in fact) will suddenly be boosted by the blower!
So when the new intake plumbing is being made, don’t forget to include provision
for these hose connections.
Note that if the throttle has been moved, the idle air bypass hose
connections (and possibly idle air bypass valve) will also need to be altered in
If you are using a positive displacement blower (like the sort of ex-wrecker
superchargers we’ve been covering in this series), the throttle can be placed
either before or after the blower. (In most cases, it’s easiest to leave the
throttle where it originally was on the engine.) However, and especially if the
throttle is located after the blower, a bypass (or recirculation) valve must be
A bypass valve connects the outlet of the supercharger back to its inlet.
Normally, it’s opened by the action of manifold vacuum, ie like a turbo blow-off
valve. In a supercharger application, the bypass valve performs four
- It opens on throttle lifts, preventing a big pressure build-up between the
supercharger and the throttle. (In this respect it’s like a turbo blow-off
valve, but in the case of a positive displacement supercharger, the pressure
build-up when the throttle is closed can be very high – in fact, high enough to
stop the supercharger and slip it against its belt drive.)
- It is open at idle. This allows the supercharger to ‘freewheel’ – little
work is being done by the blower as its outlet isn’t pushing into boost
pressure. As a result, the supercharger is quieter and the idling fuel economy
is better. Significantly, the outlet air temp is then also lower when the
supercharger boost is actually needed.
- It progressively shuts as the throttle is opened, so bringing on boost.
- Depending on the way in which the valve is plumbed-in, it can be used to set
In this application, where a small ex-Subaru Vivio supercharger was being
fitted to a Toyota four cylinder, all four functions were achieved by carefully
setting up the action of a GFB recirculating ("plumback") blow-off valve.
The GFB valve contains a piston which is pushed into the ‘valve closed’
position by an internal spring. A small amount of spring preload adjustment can
be made by turning the knurled fitting on the end of the valve, which also
doubles as the vacuum hose connection. The internal piston, which has a mass of
only 60 grams, uses a tapered acetal piston seal and low-friction polyester
piston ring. The valve body is anodised for appearance and wear resistance.
The valve is plumbed-in so that boost pressure acts on the base of the
piston. The other side of the piston is connected by small diameter hose to a
source of manifold vacuum. Therefore, when there is pressure on the piston and
manifold vacuum is present, the valve opens – this is what occurs when it is
acting as a throttle-closed blow-off valve.
If the internal spring is shortened (or a softer spring substituted), the
valve can also be made to fully open at idle (idle vacuum is usually much less
that the vacuum which occurs on the throttle over-run). This allows the valve to
recirculate air from the outlet of the supercharger back to its inlet.
On systems where the supercharger is blowing into the throttle, it’s very
important when setting up the action of the bypass valve to measure the idling
pressure in the system between the supercharger and the throttle. In many cases,
despite the bypass being apparently open, there will be a pressure build-up. For
example, with the Toyota, until the recirc valve's internal spring tension was optimised, there
was 2 psi pressure between the closed throttle and the supercharger – even with
the bypass valve apparently open! In systems with greater idle air output from
the supercharger, two bypass valves may need to be used to stop this pressure
build-up. (An alternative approach is to use an external turbo wastegate.)
If too light a spring is used in the bypass valve, it will not shut as the
throttle is opened. (Remember, if the valve doesn’t shut, you won’t get any
boost.) Once the spring has pushed the piston shut, the valve will then stay
closed because the boost pressure acting on the base of the valve is
counterbalanced by the pressure from the manifold acting on the other side of
the piston. Well, that’s the theory. In fact, the GFB valve with a shortened
spring will open if subjected to boost pressure. This is not a disadvantage,
because it allows the adjustment of peak boost pressure by turning the spring
In the GFB catalog it repeatedly states that boost pressure cannot be
adjusted by turning the knob on the end of the valve. That’s the case when the
valve is being used conventionally, but in this specific application (where the
internal spring has been changed), peak boost pressure can in fact be altered in
Tuning the action of the valve so that:
- It opens on throttle lifts
- It opens at idle
- It progressively closes as the throttle is opened
- It opens a little at peak boost to control the level reached
...is a question of trying different springs and preload adjustments. In some
cars it may not be possible to achieve all four characteristics, but the first
three shouldn’t pose any problems.
Superchargers – especially Roots blowers – can generate a lot of noise. This
noise can be present on both the intake and outlet sides of the blower. It’s
caused by the pressure pulses of air being emitted by the supercharger, and it
is potentially much worse when the blower is discharging into a pipe rather than
directly into a larger volume like an intake manifold or intercooler.
Intake noise is best addressed by the design of the airbox and the intake
snorkel leading to it. In many cases it will be a ‘suck it and see’ procedure:
it’s not a good idea to finalise the airbox and other intake plumbing until
you’ve had a chance to listen to the system in operation. Airboxes with tortuous
intake paths (eg a "non-crossflow" design where the air enters and leaves from
either the top or bottom of the airbox (ie on the other side of the filter
there’s a common chamber) or with acoustic padding inside, are likely to better
damp intake noise. The length, shape and diameter of the intake to the airbox
will also affect noise – this is best optimised experimentally.
Outlet noise can be an even bigger problem. Running an expansion chamber
(like the aforementioned intercooler or plenum chamber) up close to the
supercharger outlet is very effective. However, if the supercharger has to
discharge into a pipe, soundproofing the pipe is the best way. This can be
achieved with acoustic lagging (eg the combined foam/bitumen material sold for
soundproofing firewalls) wrapped around the pipe. Inevitably, this expands the
diameter of the pipe, which in turn can cause clearance problems to other
If the sound is mostly of the same pitch, a tuned length chamber T’eed into
the outlet pipe may suppress the noise. This approach – called a Helmholtz
resonator – uses a sealed volume and connected to the pipe by a small neck. The
air within the neck vibrates out of phase with the vibration in the main pipe,
cancelling the noise. Some factory supercharged cars use small Helmholtz
resonators on the outlet to the supercharger for this purpose. While there are
equations available to design Helmholtz resonators, welding a small connecting
pipe to the outlet pipe (eg a 1-inch pipe on 2-inch outlet plumbing) and then
experimenting with various volumes on the other end is tedious but effective.
Unless you have evidence to the contrary, when planning the installation,
assume that the supercharger will be loud. If you don’t like that, expect to
have to undertake some noise suppression techniques.
While it is a heap more work than it first appears, fitting an ex-wrecker
supercharger is a viable path to DIY performance. If the welding and cutting are
farmed out, nearly everything can be done at home with just normal tools...
The GFB blow-off valve was supplied courtesy of the manufacturer.