Paul Cowland

Paul Cowland


Project Car Builds

Project STi – Part 2 Forge Dump Valve

30 Jul , 2015  

That essential question; just what is the first modification to undertake on your latest project car? This time I was pretty sure what it had to be – and ended up making a quick visit to tuning scene legends Forge to get it fitted.


Having spent a wonderful first month with the STI, I have been relishing every moment that I can get to spend behind the wheel. As someone who has been driving Impreza, WRX and STI models for over 20 years now, there’s someone very comforting in piloting that familiar combination of grip and grunt, combined with that wonderful flat four burble that makes me feel very much at home in the seat.


Although the MY12 STI is very much a case of ‘business as usual’ from this enthusiast’s point of view, I did find myself missing an essential part of my Subaru soundtrack as I was blatting through the country lanes; an atmo dump valve! To my mind, there’s nothing like that ‘perrrtisssh’ sneeze between gear changes to give you the full Colin Mcrae experience.


I’ve always fitted Forge valves to all of my other Subarus, so that choice was easy. They look superb, they fit like OEM – and they always work perfectly. Since I fitted my last valve however, the company had added an innovative new twist to the kit – and invited me down to its Gloucestershire HQ to have a look while it was fitted. It sounded a lot more fun than me doing it, that’s for sure!


Forge Motorsport has long been associated with clever, ground breaking innovations, but the company’s latest design is one of the smartest ideas to appear onto the automotive aftermarket for quite some time; the Intake Pressure Compensation Valve (IPCV) – an idea so ingenious that the company has patented it!


Put simply, this engineering marvel prevents boost loss across the components of any turbo-charged, forced induction car, it allows any specification of dump or diverter valve to work perfectly, regardless of engine boost levels, and it ensures perfect operation for any diverter valve, irrespective of the spring or diaphragm rate within it. I have to be honest, some of the science behind it is quite intense, but luckily, Dom explained it to me in layman’s terms as he did a superb job of fitting the kit up while I snapped away.


The IPCV has been designed to address a potential issue in many turbo-charged cars. By increasing the load on the top part of the internal piston or diaphragm of a vehicle’s turbocharger pressure relief or diverter/dump valve (DV) the IPCV allows it to form a superior seal, irrespective of the boost pressure being generated.


In doing so, the IPCV allows an equal or greater pressure on the top side of the piston or diaphragm of a DV, allowing it to make a better seal onto its seat. This prevents the DV opening on high boost, load or rpm, and prevents the need for a stronger spring in the DV as the boost pressure is increased, which has always been the case up until now. By allowing the fitment of a weaker spring in the DV, significantly faster valve response is assured, thanks to less resistance to the vacuum operation when the unit opens to vent boost. This can help in-gear acceleration times, engine responsiveness and the integrity of the turbocharger itself.


Normally the boost feed to the top of the DV would have been through the restriction of the throttle body and inlet manifold, so typically is of a lower value than the boost level on the underside of the DV, due to pressure drops inherent in any system. The IPCV is able to swap between a positive pressure feed during boost and a vacuum on idling and low engine speeds. This is state of change is clearly evident to the ECU of the vehicle, meaning that the DV will open and close as would be expected by the ECU, but the valve will behave more consistently than would normally be the case.


So what does this actually mean? Well, put simply, no matter what boost setting I run in the future, I shouldn’t now need to keep upping the spring as I would have had to before. All great news in future-proofing the motor as the mods roll in.

Best of all, the under-bonnet area looks so much nicer now the coarse casting of the old unit has been replaced by the new precision CNC-milled beauty of the new parts – and as for the soundtrack. Well, it’s like being back in my old 22B again! Or it will be when I get some of that restriction out of the exhaust system and really unlock that Subaru sound!


The IPCV isn’t just a Subaru mod of course – it fits any turbocharged car, so it’s well worth having a look at if your Jap ride is of the FI persuasion. Having spent a couple of hundred miles with my set-up now, I can’t recommend it highly enough…

Until next time…




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