What is a blow-off valve?

A blow-off valve is an air pressure bypass valve that is placed between the turbo compressor and the throttle.

When your turbocharged car is on boost, the entire intake system is filled with pressurised air; from the turbo compressor, through the throttle body and inlet manifold and into the combustion chambers. When the throttle is closed, this pressured air can no longer enter the engine. The only path available for the air is to try to flow back the way it came, through the turbo compressor the wrong way. This creates a fluttering noise on the blades of the still-spinning turbo compressor.

In addition to making this fluttering noise, a noise that is probably unwanted in a nice new turbo car (though actually extremely popular amongst modified-car enthusiasts!) it is often claimed that the load placed on the turbocharger from this pressurised air flowing through it the wrong way can cause premature wear or damage. The jury is still out on this, as it’s quite difficult to directly attribute a turbo failure to not having a blow-off valve fitted. For that matter, we are yet to see a spectacularly damaged turbo from a street-driven car; they usually just plain wear out.

There are many other reasons car manufacturers fit blow-off valves to their cars, mainly to do with emissions, fuel economy and drivability. In aftermarket applications though, the main reasons for fitting a BOV are to hold higher-than-standard boost levels, to give better throttle response (than a factory BOV) by staying closed whenever it’s not venting, and of course to make noise!

A blow-off valve (also called a compressor bypass valve or diverter valve) is a valve, generally a piston type, which is placed between the turbo compressor and the throttle to bypass the pressurised air on a closed throttle, either plumbing it back into the turbo inlet for silent operation, or to the atmosphere to make the signature blow-off valve whoosh.

How a blow-off valve works?

A blow-off valve is vacuum/pressure actuated piston-type valve. It uses vacuum/pressure signals to tell the piston when to open and close.

At idle there is engine vacuum on the top of the BOV piston trying to suck it open, and no vacuum or pressure on the bottom of the piston. Since a vent-to-atmosphere BOV needs to be shut at idle to avoid air being drawn in through it, there is a spring inside a BOV with the job of holding the piston closed. The spring preload adjustment is to allow for differences in engine vacuum from car to car, and variations in atmospheric pressure at different elevations.

On airflow metered cars the air drawn in through an open vent-to-atmosphere BOV at idle would confuse the ECU and cause over-fuelling and stalling and in any case, the air drawn in is unfiltered.

Under cruise conditions (off boost) the BOV is experiencing similar conditions to when the car is at idle, but there is less vacuum present on top of the piston because the throttle is partly open. If the BOV spring has been adjusted to keep the piston closed at idle, it will also be closed at cruise.

On boost there is boost pressure on both top and bottom of the BOV, the forces from which counteract each other, so the BOV remains closed.

Immediately after the throttle is closed under boost there is vacuum on the top of the piston and boost pressure on the bottom of the piston, which together, quickly open the BOV to release the pressure. When the pressure has been released, the BOV closes.

Which type of blow-off valve makes the fluttering or ‘pigeon' noise?

The short answer is that there is no blow-off valve that makes this noise. Read on to find out why.

Without a BOV, the pressurised air being pumped into the engine by the turbo will have only one path when the throttle is closed: back through the turbo compressor. The fluttering sound is the sound of this air against the blades of the spinning turbo compressor as it tries to flow through it the wrong way.

Car manufacturers fit recirculating (plumb back) BOVs to give the pressurised air an alternate path when the throttle is closed: back into the turbo compressor inlet. This eliminates the ‘undesirable in a brand-new car’ fluttering noise.

Aftermarket BOVs typically vent the pressurised air into the atmosphere for the purpose of making noise, and are characterised by the ‘standard trumpet’ sounds that can be heard here. Some other brands do different things with the air to make different noises, but this is not to be confused with the fluttering noise. Our own ‘whistling trumpet’ is one example of this. It can also be heard here.

In some cases, aftermarket BOVs do not flow enough air either as a result of their design, or the way that they are adjusted. In this case, fitting an aftermarket blow-off valve will result in the fluttering noise being emitted from the turbo. While this is extremely popular, it is worth noting that if this is your objective, then simply removing the factory BOV and replacing it with a pair of hose plugs would have been more cost-effective!

Incidentally, fitting a pod air filter can make any fluttering noise that was already present more audible. Also, large front-mounted intercoolers can increase the likelihood of ‘flutter’ for any given BOV, due to the larger volume of air present in the intake system. If the BOV is any good, some adjustment of the spring preload would be all that is necessary to once again eliminate the flutter.

Finally, it is possible to set up your GFB blow-off valve to cause some ‘pigeon’ noise by increasing the spring preload slightly (turning the spring preload adjustment clockwise). The aim is to have the flutter occur at low rpm and boost, while allowing the BOV to vent freely at higher rpm and boost levels. Experiment with it; you can’t do any harm!

How to fit a blow-off valve?

The simplest way to fit a blow off valve is to buy a GFB ‘bolt-on’ blow off valve kit. Bolt-on kits are available for many vehicles including Audi 1.8T, Ford Falcon XR6 Turbo, Mitsubishi Lancer GSR and EVO, Mitsubishi Galant VR4, Nissan Skyline, Nissan 200SX, Subaru WRX & STi, Subaru Liberty/Legacy GT and Volkswagen 1.8T.

If there isn’t a bolt-on kit available, don’t worry, you can still fit a GFB BOV to almost any turbo car. Using the standard adaptors supplied with a Go Fast Bits blow-off valve you can…

1. Hose mount

Many OEM bypass/diverter valves use rubber hoses, which makes it very easy to replace them with a GFB unit as shown. A range of hose adaptors are available from GFB to suit all of the common hose sizes used.

Be careful about the orientation of the valve when the factory inlet and outlet hose are the same diameter. The majority of European manufacturers install their diverter valves in the opposite orientation to the way a GFB valve should be installed. Boost should always enter the bottom of a GFB valve, and dump through the side outlet(s).

2. Pipe mount

Two sizes of pipe mount bases are available – 1” or 1.5” (25.4mm or 38mm), and short lengths of pipe in these diameters are available in stainless steel or alloy.

Select a suitable location on the factory inlet plumbing (somewhere between the turbo and the throttle), and weld the suitable pipe into position. The GFB BOV then pushes onto this pipe and is sealed by the supplied o-ring that sits in a groove inside the base. The BOV secures on the pipe with grub screws and locking nuts (also supplied).

3. Vehicle specific adaptor

Some OEM valves bolt up to a flange, and GFB has a range of vehicle specific flange adaptors to suit many cars. The GFB blow-off valve then mounts onto the adaptor in the same way as the pipe mount described above.

Note that some GFB flange adaptors screw directly into the bottom of the GFB valve, thereby replacing the original base entirely.

You can find installation instructions for all GFB BOVs here.

What makes the GFB Stealth FX and Deceptor Pro blow-off valves different to other blow-off valves on the market?

Other manufacturers will claim that their valves allow the noise to be adjusted, but none can do it to the extent, or with the ease, that the GFB Stealth FX and Deceptor Pro can. Furthermore, no manufacturer will be able to offer a product with this degree of adjustability, since this technology is patented.

Neither of these blow-off valves change the noise by muffling the air vented to the atmosphere. They change the ratio of air that is vented to either the inlet or the atmosphere. This way the noise can be completely silent like a factory valve, or as loud as you want.

In cars that suffer from backfiring or throw the check engine light with a vent-to-atmosphere valve, the GFB Stealth FX and Deceptor Pro blow-off valves can be tuned to vent as much air as possible to the atmosphere before the problem occurs, so that having the noise is still possible.

So whether it is the Stealth FX hand-adjustable blow-off valve, or the Deceptor Pro in-car electronically adjustable blow-off valve, you’re getting the best and most adjustable BOV on the market!

What boost level can I use with a GFB blow-off valve?

Any level you like! The design of all GFB blow-off valves means that you can run boost levels that will more likely blow up hoses before the valve will leak. Because of the acetal seat the piston makes a perfect seal, and our valves have been tested in the factory to pressures of 110psi (if you can blow up one of our valves on a car we’d like to hear about it!).

I want my BOV to be noisy, but I've been told that I can't vent a blow-off valve to the atmosphere. What's the deal?

There are many people who will say that if your car has a MAF (Mass Air Flow) sensor (which is most modern turbo cars), you can’t vent a BOV to atmosphere. This is not entirely true. In most cases you can, but it pays to be aware of the possible side effects. Quite often you may find the side effects are so minimal that they are not really a concern. This section describes in detail what happens when you vent to atmosphere.

Most factory turbo cars run some form of MAF sensor (usually found directly after the air filter box), some use a MAP (manifold absolute pressure) sensor or even a combination of both. These sensors are used to determine the amount of air the engine is using so it can deliver the appropriate amount of fuel. In a car with a MAF sensor, when a BOV vents, air is escaping from a closed system. This air has already passed through the MAF sensor and has been measured, and the computer doesn’t know that this air never made it to the engine. This will cause a brief rich mixture as the computer will still deliver the correct amount fuel dosage based on the amount of vented air.

There are two stages to a BOV venting, as initially it is evacuating the pressure from the inlet pipes and intercooler, which usually takes less than a second (depending on your inlet system). Then once the pressure is released, the valve stays open to allow the turbo to freewheel, thus reducing compressor surge and the associated thrust and torsional loads. It is mainly during this free-wheeling stage that causes the over-fuelling problems, since the turbo is basically pumping air through the MAF sensor and out to the atmosphere through the BOV, which accounts for the majority of air that escapes the system. The resulting rich mixture is what can sometimes cause backfiring and a puff of smoke in some cars. The severity of these effects usually depend on the state of tune of the engine. In cars that are modified (say with full exhaust, pod filter, a little extra boost etc) but still using the factory tuning, it is not uncommon for the ECU to compensate for the extra airflow it sees by running rich for engine protection. On a WRX for example, mildly modded engines can be running as rich as 10:1 with the factory ECU. It is this poor state of tune that can cause backfiring when an atmosphere-venting BOV is added.

This is where the idea for the GFB Hybrid valve came from. It is made to eliminate such backfiring by its unique design that evacuates the boost pressure to the atmosphere, while sending most of the turbo overrun air to the inlet, still giving the distinct BOV sound but reducing the effects of over-fuelling.

Stalling is another common problem, many people have had bad experiences with atmosphere-venting valves causing stalling problems. However, with the correct spring adjustment this is never a problem. As long as the valve closes properly before the engine reaches idle, the ECU will have no problem maintaining a smooth idle. Most complaints of stalling actually come from people using certain Japanese brand valves, which often do not have the range of spring adjustment to compensate.

Some cars are affected by backfiring when venting to atmosphere, and some are not. Even two identical cars with slightly different mods can react differently. The bottom line is if you vent to atmosphere with a MAF sensor you MAY use fractionally more fuel (depending on the kind of driving you do) and there is a chance you may hear some popping in the exhaust. For people who just want the maximum noise from the valve this is usually not a worry.

Are GFB blow off valves adjustable?

Yes, all of the GFB blow off valves feature spring pre-load adjustment.

This should not be confused with the noise adjustability of the Stealth FX and Deceptor Pro, which have a second system to for this purpose. Spring pre-load is used to keep the piston shut at idle, and therefore should be adjusted to suit the idle VACUUM, not full throttle boost (it has no effect on the boost holding ability of the valve). The adjustment of the valve should be made so that the piston will shut just before idle every time.

Have a WRX Hybrid blow-off valve on my 2002 WRX and there seem to be times when it doesn't blow off, and when it does it isn't very loud. Is there a problem with the valve?

The MY01-2 model WRX features vastly improved noise insulation from the engine bay when compared to earlier models, so the noise of the valve tends to be quite muffled. Rest assured that if you drive past a concrete wall with the window down you will hear it! The recirculation part of the WRX Hybrid valve is substantially larger than the trumpet opening, and will open with much smaller piston movement. It is designed this way for maximum flow and also because these WRX’s can be more sensitive to atmosphere venting than earlier models. So the times when you can’t hear the valve are a result of most of the air going back to the inlet and the extra sound insulation.

Is there a possibility that dirt could enter the engine through the trumpets?

No. A blow off valve is a one-way device, air will only ever flow OUT of it.

The turbo piping before the throttle body will only ever be at atmospheric or positive pressure, except for a very brief period when you rapidly open the throttle from off boost conditions.

In such a situation, if the valve is open, it will immediately close as soon as the throttle is opened, because the manifold vacuum no longer holds the piston open, and any vacuum in the turbo piping will also tend to pull the valve shut. Therefore there is no way unfiltered air can ever enter the engine through the valve.

Do I need to buy extra parts to configure my WRX Hybrid as a plumb back or vent-to-atmopshere only BOV?

The blanking plate, plug and plumb back block off plug come standard with the WRX Hybrid, so that it can be configured in three ways; either as a hybrid, a plumb-back or full vent-to-atmosphere, for maximum noise.

Which blow off valve kit do you recommend for an MY98 WRX with a cat back exhaust and pod filter? Everything else is standard.

It all depends on how loud you want it! Our range includes a full plumb-back valve for silent operation, the twin trumpet Bovus Maximus for maximum noise, or the Hybrid, which offers the best of both worlds. For the best versatility, we recommend the Hybrid. Due to the modular design of GFB blow-off valves, it can easily be configured (using a supplied plug) for vent-to-atmosphere operation, or as a plumb back to solve any airflow meter related problems or excessive BOV noise issues. All of these valves will fit directly onto the factory blow off valve hose on the MY98.

How does a boost controller work?

There are 2 main components of a boost control system on a turbocharged engine: the wastegate with actuator and the boost controller. Note that some turbo cars do not use a boost controller as such, instead they rely on the spring rate in the wastegate actuator to achieve the desired boost level.

There are two types of wastegates; internal and external. Internal wastegates are found on the vast majority of turbo cars, where a flap-type valve is built into the turbocharger exhaust housing, and is opened by a remote diaphragm actuator. The external type usually use a poppet-style valve attached directly to the diaphragm actuator as one unit, which is plumbed into the exhaust header before the turbo, and dumps back into the exhaust after the turbo.

Both types of wastegate open a path for exhaust gas to bypass the turbo, thereby limiting the amount of gas available for making boost. The diaphragm actuators in both cases are controlled by the boost pressure itself, which creates a feedback loop boost control system; as the boost pressure increases, it pushes the diaphragm actuator (and therefore the wastegate valve) further open, which results in the boost stabilizing at a set level determined by the spring rate in the actuator.

A boost control device is commonly used in modern turbo engines which alters the boost signal that reaches the wastegate actuator. If the actuator sees a lower boost signal than is actually present, the wastegate will open less. This results in greater exhaust gas flow through the turbo, which increases boost. For example, if the actuator spring is set to 7 psi (i.e. when the valve is fully open, boost level is 7 psi) but the boost control device only allows 5 psi to reach the actuator, it will open the wastegate valve less, so more exhaust flows through the turbo, spinning it more quickly, so that the actual boost produced will be greater than 7 psi; say, 10 psi.

The boost control device is designed to reduce the pressure reaching the actuator, which it does by ‘bleeding off’ a specific amount of air from the supply hose. This can be done electronically through the ECU by pulsing a solenoid-type valve in the supply hose, or mechanically by placing a ‘controlled leak’ in the supply hose.

GFB boost controllers are the latter type, which may not sound incredibly scientific, but there are a few design criteria that are critical to performance. Essentially, a GFB boost controller receives boost pressure from the turbo compressor, passes it through a restrictor (which is necessary, or the turbo would simply fill the supply hose as fast as the controller could bleed it off), where a measured amount of air is bled out of the hose that leads to the wastegate actuator. Note that it is not possible to reduce the boost level below the setting of the actuator spring, only increase it. It is however possible to reduce boost below standard on cars that originally had ECU/solenoid controlled boost.

I'm having new intercooler piping made and I want to know where the best place to install the blow-off valve is?

I’ve not seen any conclusive results that prove that one position is better than another, but many people have different ideas about this. Some say that it’s best to have it close to the throttle, since that’s where the back-pressure builds from when the butterfly is closed and it will respond quicker. Others say that it’s better to put it as close to turbo as possible so that the valve is venting hot air rather than post-intercooler cold air, so that the inlet pipes after the intercooler are still filled with cool air.

Both positions have their merits, but are really of minute benefit. There is a trap to watch out for when mounting close to the turbo however. There is often a measurable pressure drop between the turbo and throttle body, especially as the RPM and hence the airflow increases. A BOV uses two pressure signals to stay shut, one from the manifold and one from the inlet, which act on opposite sides of the piston and since they oppose each other they should balance out, and the spring then holds it shut.

The problem is when the revs rise and there becomes a pressure difference between the two signals. The pressure leaving the turbo may be say 16psi, while only 13 or 14 psi makes it to the manifold. So this means that there is 2 or 3 psi of pressure acting against the spring, which is enough to move the piston a fraction. In the Hybrid and Bovus Maximus valves, since the outlets are staged and one is placed very close to the seat for rapid response this slight spring compression can be enough to open the valve a small amount. It is not a problem with the other valves in the range, and it depends on the flow efficiency of your intercooler and pipes, but for the Hybrid and Bovus Maximus valves it is best to locate them after the intercooler.

If this problem does show up then it may also be a good idea to test the efficiency of your piping by measuring boost at redline just after the turbo, and after the throttle body. If there’s any more than about 3 psi difference then your turbo is working harder than it needs to, and reducing this drop would help performance a lot.

Do GFB boost controllers use a ball and spring?

No. Some manufacturers use a ‘ball and spring’ arrangement in their manual boost controllers, with the claim that it brings the boost on faster by keeping the wastegate closed until the boost has nearly reached it’s peak level. This is a sound idea in theory, however it is worth noting that the “gated” ball-and-spring system offered by other manufacturers is incapable of holding back more than a paltry 2psi – certainly not enough to live up to the advertising claims.

Another nail in the coffin for the ball-and-srping design is poor boost stability. Since it is a dynamic system, and the ball can move slightly differently each time your engine comes on boost, the size and flow properties of the flow path through the restrictor and around the ball can change each time. This can lead to poor boost stability and random variations in peak boost level. The ball can even flap around like a pea in a referee’s whistle whilst air is flowing through, resulting in a fluctuating output signal from the controller. Grab a boost gauge, a compressed air supply and a pressure regulator and see for yourself.

GFB boost controllers use a more consistent needle-valve bleed system, which brings brings boost on just as fast as any ball-and-spring system, and generally more quickly than a factory boost control system.

How much extra boost can I run when using a GFB boost controller?

It is very important to be sensible when raising the boost limit on turbocharged cars. Power is directly related to the amount of air and fuel that makes it to the engine cylinders, but lifting the boost level is only one way of doing this. While in most cases it is quite safe to increase the boost by about 10-20%, keep in mind that every component in an engine comes from the factory designed to operate most efficiently at the level it leaves the factory at. If you intend to raise the boost to any more than 20% above factory, you will need to check things like fuel delivery, the intercooler, ignition timing, etc to ensure that they are capable of supporting the increase in power. It is also important to realise the mechanical limits of components, a good example being the ceramic turbine wheels in the turbos found in Skylines, which will take no more than about 14psi without losing blades.

A GFB boost controller will give you the added benefit of not only being able to adjust the peak boost level, but in most cases will bring the boost on earlier, resulting in more power throughout the rev range.

To get the most out of the extra boost it is best to ensure that the engine can breathe properly. A good exhaust is the first place to start. Then follow the inlet tract all the way to the engine, looking for restrictions in the form of resonators, sharp bends or reductions in diameter. Manufacturers will often purposely build in such restrictions to muffle intake noise or prevent over boosting. Removing airflow restrictions means the turbo doesn’t have to work as hard to get the boost to the engine.

Why does the boost in my car tend to taper off with increased boost at high RPM?

This is a sign that your turbo system is nearing the limit of its efficiency. Most factory turbos have small turbine housings to reduce lag, but the problem is at high RPM it can cause a large enough restriction to push the wastegate open as the revs increase, often referred to as wastegate creep.

This can also be caused by restrictions in the factory turbo compressor inlet and outlet pipes that also contribute to tapering boost levels, such as convoluted hose sections. The boost controller usually gets its boost signal from directly after the turbo, so the boost at this point is often likely to hold steady. However, restrictions further downstream (corrugated pipes, small intercooler, sharp bends or reductions in diameter) will cause a pressure drop at elevated RPM, particularly if the car is modified and making more power than factory.

It is also important to realise that in stock or mildly modified engines it is probably much safer to drop the boost level a little towards redline. The inertial loads on the reciprocating components increase exponentially with RPM, which is why you will often see the factory ECU drop a few psi near redline. Generally this will occur on cars with small factory turbos, and it is probably in the best interests of the health of the turbo for this to happen unless your engine is built to handle it.

Do I need different belts for the pulley kit?

Yes, however the required belts for the WRX kits are included. All other kits list the correct sizes required.

Is there a dyno graph of the Pulley Kit on the WRX?

Yes, we tested an MY97 WRX on the dyno with and without the kit, and you can see that the chart shows significant gains at the bottom and the top of the rev range. The reduction in rotating inertia (we’ve removed about 2kg by using 6061 T6 aluminium billet) means that the engine can accelerate much quicker during the times it is making very little power before the turbo spools up. The result is better driveability off-boost and better acceleration, which is especially useful for those with large turbos that only make power in the upper rev range. The smaller size of the crank pulley under-drives the power robbing accessories so that the upper end of the rev range is less burdened, while still allowing them to operate properly.

It is difficult to show the true benefit of the kit on the dyno, since the pulleys reduce INERTIA, which by definition is an object’s resistance to acceleration. This means that the benefit only comes when the engine is accelerating, much like removing weight from your car. On a dyno the revs are brought up relatively slowly, and the difference is still noticeable. So on the road, a bigger gain is noticed when you accelerate quickly.

When using a GFB Power Up Pulley Kit, is there any problem with the removal of the harmonic balancer?

Since the crank pulley in our Power-Up Pulley Kit replaces the factory harmonic balancer, people often express concern for the effect that this will have. It is important to understand the task of a harmonic balancer and why they are fitted to explain the effects of fitting a lightweight pulley.

What is a harmonic balancer?

A better name for a harmonic balancer would be “torsional dampener” since its main task is to absorb the rotational pulses inflicted on the crankshaft by the pistons. Most often it is incorporated into the crank pulley by attaching the outer belt drive ring to the inner by means of vulcanized rubber. At the right RPM, it is possible for a resonant frequency to be set up torsionally on the crankshaft. Resonant frequency occurs when the pulses of the engine correspond with the natural frequency of the crankshaft and it ancillary components. However, since factory pulleys are often comparatively heavy (reasons for this are described later) it is actually the large mass (and therefore inertia) of the factory harmonic balancer and flywheel that will help to excite this natural frequency. So by dramatically reducing the weight and inertia of the crank pulley, the natural frequency of the crankshaft is shifted and its ability to self-excite is greatly reduced. So in fact it is the harmonic balancer’s own weight that necessitates the dampening, and since the weight of a GFB crank pulley is typically about 20% of the factory component it cannot supply an exciting force significant enough to damage the crankshaft.

An opinion often expressed is “if the manufacturer put it there, it must be there for a reason”. However, if you look at it from the car manufacturer’s point of view, casting pulleys from steel is very cheap and easy, because they can be produced in large numbers and there is no waste (as opposed to machining them from billet). But because the resulting pulley weighs significantly more than one made from aluminium alloy, it requires dampening.

Manufacturers will always build cars (even high performance cars) to suit the widest possible selection of driving scenarios and drivers, which means there are always compromises. The weight of the flywheel and pulley also affect how fast the revs drop between gear shifts, and a production car is designed to only allow the revs to drop fast enough for average shifts. If you hurry the shift the revs will be too high for the next gear, resulting in a sharp jerk as the momentum of the engine transmits through the drivetrain. Reducing the engines’ inertia with a lightweight pulley kit allows faster and smoother shifting.

When looking at high performance engines such as those found in Honda VTEC equipped cars and the S2000, it is obvious that manufacturers do understand the benefits of reducing engine inertia, and have utilized lightweight pulleys to help the power output and responsiveness without the use of a harmonic balancer.

However, this is not the case for all engines, many of them do require the use of the harmonic balancer to prevent failure. Skylines with the RB20, 25 and 26 are a good example of this, which is why we don’t make a pulley kit for them. The pulley kits we do make are for engines that do not rely on the balancer to any significant degree.

Isn't there a loss of torque associated with fitting lightweight pulleys, since the flywheel effect is lost?

No. In early days it was common to fit large heavy flywheels to increase torque, which to a certain extent does work, but it is important to look at the context of the application. A heavy weight acting as a flywheel on the crankshaft has a greater resistance to a change in rotational velocity, which is termed inertia. The term “resistance to change in rotational velocity” applies to both acceleration and deceleration, so a heavy flywheel or pulley does oppose deceleration in situations such as towing caravans up a hill, but it will also oppose fast acceleration.

It is easiest to think of inertia as an energy reservoir, and the laws of physics state that you can only get out the same amount of energy as you put in (actually in the real world it is often less due to irreversible losses). So an engine needs to put in a significant amount of energy to accelerate a heavy pulley before any useful torque can be extracted from it. Don’t forget that when the engine is at low RPM before the turbo spools up, it is only making about 20% of its maximum power, and in this situation the amount of energy required to accelerate the pulley becomes a much larger percentage of the available power.
If you were to measure an engine’s torque whilst applying a load high enough to cause the revs to drop, then a heavy pulley will show a higher figure. But if you measure the torque whilst the revs are increasing, the lighter pulley will come out on top. If you can think of a case when your 200SX or WRX is at full throttle and the revs are DROPPING, then it means your engine is probably running on 1 cylinder. In a high performance engine the emphasis is on acceleration, not momentum.

How much lower is a GFB shifter than the factory one?

The GFB Short Shifter is the same height as the factory shifter, it is the actual distance that the gear stick moves between gears that is reduced.

Are there any problems associated with under driving the alternator and power steering pump when using a Power-up Pulley Kit?

No, the ratios are only reduced by about 15% on the alternator, and 20% on the others. This means that you get full charge at 1000 RPM instead of 850 RPM which will only cause problems if you plan to drive your car at idle for long periods of time with the fan, high beams, air conditioning, rear demister and stereo full blast.

The reason for under-driving is that from the factory the accessories (alternator, A/C, power steering) are designed to run at almost 100% at or near idle, so at high RPM they are causing excessive and unnecessary drag. Reducing the drive ratio by 15% doesn’t have a large impact at idle, but at say 6000 RPM, 15% becomes a much larger proportion and the reduction in drag is much more noticeable.

I've heard bad stories of gearbox damage from using a short shifter - is this true?

Absolutely not!

We’ve heard this concern many times, often from ‘internet experts’, and surprisingly even from so-called ‘performance mechanics’. Read on, and you can smile to yourself knowing that when someone claims to have damaged their gearbox because of a short shifter, it’s more likely they are in fact a sloppy driver.

Here’s the hard truth – a gearstick is simply a lever, connecting the driver’s hand to the gearbox. Any wear or damage is proportional only to the way in which it is used (or abused!), no if’s, but’s or maybe’s about it. If you shift hard and fast (or time the clutch poorly), the synchros will wear out faster regardless of what type of shifter you have.

The GFB Short Shifter reduces the travel of the gearstick – if you reduce the throw by 20%, there is a 20% increase in the effort required to shift gears – this is the very basic principle of levers. Whilst this increase in shift effort may give the feeling that you are stressing the ‘box more by having to push the stick harder, you are in fact exerting the same shifting force to the gearbox. Additionally, the reduced throw may falsely lead the unsympathetic driver to believe faster shifts can be performed – you can only shift as fast as the synchros will allow.

How hard are these units to install?

Very simple. All of the installation is done from inside the car, without having to jack it up and crawl around underneath. Only a few basic hand tools are required, and the necessary hex keys are supplied.

How much is the throw reduced?

The maximum throw reduction varies for different models. Early WRXs had a particularly long throw, so the maximum reduction with the GFB kit is around 50%, whilst on later models it is closer to 40%. In either case, it is possible to reduce the distance per gate to about 30mm. Whilst it can go shorter, the shifting force becomes too great for comfort, and some of the shift feel is lost.

What does the ‘Whistling Trumpet’ sound like?

In normal driving conditions it gives a typical “whoosh” sound, but once the revs and boost level rise, a high pitch whistle noise takes over. Visit our sounds page to hear it in action.

Which blow-off valves will the ‘Whistling Trumpet’ work on?

The optional whistling trumpet works on the Stealth FX, Deceptor Pro and the updated model WRX Hybrid (not the old model WRX Hybrid or the 1003 Hybrid ‘universal’). For the whistling trumpet to be effective, the BOV must be set to full vent-to-atmosphere, and boost must be at least 12 psi (0.81 bar).

The whistling trumpet does not work on any other blow-off valves due to their different airflow characteristics.