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The turbo pressure acts on the diaphragm in the Bosch valve. On the TurboXS.con valve, it acts on the piston face. I believe that Forge states that it can be applied to other port to put the valve into the "stock orientation". The action of the valve is quite different in a piston valve when you change the orientation. With flow acting on the piston face, once it starts opeing, the dynamic force of the flow against it, will cause it to open faster. This is not true of the Forge valve in the "stock orientation". So claims of valve speed must take this into acount. Bench measured opening speed is not useful, unless the test bench includes a reservior of pressurized air to be discharged. You cannot simply do timing tests on valve stroking by the vacumm line only.
The static pressure opeing forces do not take into account the pressure on the piston face when used or designed for this orientation. Valves that have low opening pressures may leak if the pressure drop between the valve and the throttle body are too high. This could occur with a 94-99 NG900 with the unmodified throttle body transistion casting at wide open throttle and high RPMs (high flow rates).
I have not seen a Forge valve in the flesh too know, but IF it has the turbo pressure applied to the underside of the piston, then the valve is really a diaphragm-less analog of the original valve. It will then trade pressure on the TurboXS piston face, for pressure on the piston non face piston area, just as with the diaphragm design. This will work well, but does forego the dynamic flow pressure of the discharge against the piston face which acts as a power booster. So IF that is how the forge valve works, then the forge valve and the TurboXS valve are very different devices.
TurboXS has a 1" by 1" variant of the one shown on their web site, for a bit less money. The Vacumm line hose points to the front of the car when installed properly, not at the battery. When installing, the aprox 2.5" coupling hose needs about 3/4" removed to get a proper fit. The Forge valve installs in teh stock orientation without any modification to the hoses etc. The Forge valve is less ecpensive. The TurboXS valve is red anodized aluminum. Either unit should preform well, and get rid of the rude noises that the Bosch unit makes, especially with increased boost pressures. The TurboXS valve has been totally silent on my Saab.
The discharge ports should never be left open to atmosphere, as the valve is open with manifold vacumms, and dust and dirt will get into the engine. The discharge could be to a small filter.
95SET, 115000 miles, 15 PSI
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