What does a Dump Valve do and what aftermarket ones are available
The purpose of a dump valve is to release excess pressure in the boost system when quickly coming off or closing the throttle usually during gear changes in order to take the load off of the turbo. When the car is on boost the intake system fills with air pressurised by the turbo and upon releasing of the throttle the dump valve feeds excess pressurised air back into the intake system, pre-turbo for it to be sucked back up and pressurised again by the turbo. Recirculated air if picked up by the Mass Air Flow sensor and the ECU adjusts the air fuel ration accordingly.
There’s two main types of dump valves – Recirculating and Vent To Atmosphere (VTA) or ‘Blow-off’ Valves. All turbocharged Subaru models such as the WRX and STI are fitted with a recirculating version during build. These are almost silent in operation but work really well. Vent to Atmosphere Dump Valves send the excess pressure between gear changes straight into the engine bay, normally accompanied with a loud whoosh or chirp noise.
Generally VTA dump valves cause no running issues with the car, although can cause the car to run slightly richer than the factory fitted dump valves. The VTA dump valves are extremely popular in the aftermarket scene, usually fitted to aid correct function of a larger turbo, where the factory dump valve would not cope with the increase in pressure or simply fitted to gain the whoosh or chirp sound upon gear changes.
Usually owners choose to upgrade to a Vent To Atmosphere Dump Valve to either cope with higher boost pressures that the factory dump valve would not or simply to attain the ‘woosh’ sound between gear changes.
Dump Valves Available
Replacement Dump Valves including both recirculating and vent to atmosphere models are readily available from well respected brands such as Forge Motorsport, HKS, Turbosmart and Go Fast Bits (GFB), a few of which are outlined below in more detail.
Forge Motorsport have been manufacturing dump valves for the Impreza models since the early days of the classic Impreza models of 1993, and are still as popular today.
Forge produce a recirculating dump valve, part number SUB03 and is suitable for the 2001-2007 WRX and STI models including the Bugeye, Blobeye and Hawkeye models. The SUB03 is an alloy valve with a mirror polished body and highly rated spring meaning its able to deal with increased boost pressures, so ideal for tuned cars. Its fitted to the front of the standard intercooler as a direct replacement for the original with fittings to connect to the standard recirculating pipe and vacuum pipe. The SUB03 can be purchased here https://scoobyparts.com/subaru-impreza-wrx-sti-performance-parts/dump-valves-blow-off-valves/dump-valves-for-subaru-2001-2007-wrx-sti/forge-re-circulating-dump-valve-impreza-wrx-sti-01-07
Forge Motorsport SUB03 Recirculating Dump Valve
Forge also produce a version of this valve for the early models 1999-2000 part number being FMDVV5013 , this can be found here https://scoobyparts.com/subaru-impreza-wrx-sti-performance-parts/dump-valves-blow-off-valves/dump-valves-for-subaru-impreza-classic-gc8/forge-dump-valve-re-circulating-impreza-turbo-wrx-sti-p1-99-00
Forge also produce their more popular Vent to Atmosphere dump valves, again manufactured in alloy and mirror polished. These are available right from the early models of classic Impreza to the current models including the STI.
Forge Motorsport SUB01 Dump Valve
Forge’s SUB01 dump valve is a Vent to Atmosphere design suitable for the 2001-2007 Bugeye, Blobeye and Hawkeye WRX and STI models as well as the later STI Hatchback and Saloon models 2008-2019. These again are a direct replacement for the original factory unit and come with an alloy bung for sealing up the original factory recirculating hose. These valves come with an internal spring rated for most applications, but additional higher rated springs can be purchased separately. See this link for the SUB01 product https://scoobyparts.com/subaru-impreza-wrx-sti-performance-parts/dump-valves-blow-off-valves/dump-valves-for-subaru-2001-2007-wrx-sti/forge-atmospheric-dump-valve-impreza-wrx-sti-01-07-2008-gr-sti
Forge also produce a similar valve for the last of the classic Impreza version 5 and 6 models 1999-2000 being the FMDVV5012, as well as products for the 1993-1996 and 1996-1998 version 1-4 models. All can be found at this link https://scoobyparts.com/subaru-impreza-wrx-sti-performance-parts/forge-tuning-products
Forge Motorsport Spring Tuning Kits
Forge Motorsport also make a range of uprated Dump Valve springs for those running higher boost pressures.
So, if after fitting a Forge Dump Valve you are finding that the valve is dumping more than required, or if you are losing boost pressure it is normally an indication that a stronger spring is required. Alternatively, if the valve is failing to dump it may be necessary to install a weaker spring.
Please note that fitting a stronger spring into a valve will not increase boost pressure, unless existing pressure is being lost. Failure to allow the valve to operate by installing a spring which is too strong, may cause damage to the turbocharger.
Forge Springs come in 3 colours depending on required boost pressures : Green – 5-15 PSI, Yellow – 15-23 PSI, Blue – 23-30 PSI, Red – 30 + PSI.
The HKS Super Sequential Dump Valve is manufactured by the HKS Corporation of Japan and is wildly regarded as one of the loudest and most distinct Dump Valves available. The SSV is a Vent to Atmosphere design and comes in various fitments for both Classic and New Age 2001-2019 Impreza WRX and STI Models. Like the Forge SUB01 its designed as a direct replacement for the factory fitted recirculating Dump Valve and is therefore easily fitted in around 15 minutes.
The SSQV is available for almost all models of Impreza, and some models of Forester such as the XT and STI, from 1999 to 2019. Each is a model specific fitment, so easily fitted in around 20 minutes.
The HKS SSQV comes complete with a full fitting kit, including vacuum hose, ready fitted flange etc and is finished in a bright silver paint job.
Be aware that there are many fake HKS SSQV dump valves on the market, which can be easily mistaken for the genuine item.
You can check out the details of the HKS SSQV at this link https://scoobyparts.com/subaru-impreza-wrx-sti-performance-parts/dump-valves-blow-off-valves/dump-valves-for-subaru-2001-2007-wrx-sti/forge-atmospheric-dump-valve-impreza-wrx-sti-01-07-2008-gr-sti
GFB – Go Fast Bits
GFB are an Australian-made product range designed and produced by GFB’s in house team affording them total quality control over products from start to finish. Their philosophy is to produce exceptional automotive products that are built with the same precision, machining quality, and reliability that is the foundation of their reputation and their dump valve range is no exception.
GFB offer a wide range of dump valves from the best selling Respons model which works as both a recirculating and vent to atmosphere dump valve which switched from a recirculating to a VTA under hard throttle to an innovative Deceptor products, which allows for adjustment of the valves volume from an in-car control knob.
GFB Response Dump Valve
GFB state that their Respons valve offers better throttle response, faster boost recovery between gear changes and ultimate boost holding when in gear.
The Respons features a adjuster on the valve to correctly set up so that it works correctly with your Subaru, with no stalling issues once fitted, and like the Forge and HKS products its designed as a direct replacement for the original factory fitted valve.
The GFB Respons is available for 1999-2000 Classic Impreza and 2001-2007 Bugeye, Blobeye and Hawkeye WRX and STI and the 2008 onwards Hatchback and Saloon WRX and STI models. You can find further details of the full range of GFB products, including their Dump Valves at this link https://scoobyparts.com/gfb
- Patented Venting Bias Adjustment system – can be infinitely adjusted between full recirc (silent) and full atmosphere (loud)
- Change the venting bias simply by twisting the adjustment ring
- TMS Technology for up to 30% faster boost recovery on gearshift
- Huge range of direct bolt-on kits
- 100psi+ boost holding ability
- Manufactured in Australia in accordance with ISO 9001
GFB Deceptor Dump Valve
As mentioned earlier the GFB Deceptor is their dump valve with in-cabin volume adjustment. From GFB’s website the say “Go on, admit it. There are times you wish you could have that signature turbo blow off sound, but you’re not prepared to listen to it all the time. GFB’s Deceptor Pro II delivers just what you need – the ability to control how loud your BOV sound is, from the driver’s seat. To top it off, if your car typically suffers issues such as backfiring and stalling when venting more than a certain percentage of air to atmosphere, the controller can easily be configured to limit the maximum venting ratio. This way, you can achieve a blow-off sound without having to put up with the issues”
- DUAL OUTLETS WITH PATENTED VENTING BIAS ADJUSTMENT – CAN BE INFINITELY ADJUSTED BETWEEN FULL RECIRC (SILENT) AND FULL ATMOSPHERE (LOUD)
- CHANGE THE VENTING BIAS ELECTRONICALLY FROM THE DRIVER’S SEAT USING THE IN-CABIN CONTROLLER
- TMS TECHNOLOGY FOR UP TO 30% FASTER BOOST RECOVERY ON GEARSHIFT
- WIRING LOOM INCLUDED, ONLY REQUIRES 12V POWER SUPPLY
- HUGE RANGE OF DIRECT BOLT-ON KITS
- 100PSI+ BOOST HOLDING ABILITY
- MANUFACTURED IN AUSTRALIA IN ACCORDANCE WITH ISO 9001
Check out the GFB Deceptor at this link https://scoobyparts.com/gfb