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Fitting a Supercharger, Part 4

Inlets and outlets

by Julian Edgar

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At a glance...

  • Part 4 of a 4-part series
  • Inlet and outlet plumbing
  • Bypass valves
  • Noise suppression
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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 is stronger.

Click for larger image

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 that shape.

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 position.

Bypass Valves

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 fitted.

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 functions:

  1. 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.)
  2. 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.
  3. It progressively shuts as the throttle is opened, so bringing on boost.
  4. Depending on the way in which the valve is plumbed-in, it can be used to set maximum boost.
Click for larger image

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.

Click for larger image

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.

Click for larger image

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 pre-load control.

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 this way.

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 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.

Click for larger image

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 components.

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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.

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