GFB Blow Off Valve FAQs

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More details on GFB products can be found on our on-line shop e-CAT here.

  • How a GFB BOV works: A GFB valve operates on a pressure differential between the intercooler and the intake manifold. If they are both at the same pressure, i.e. WOT, the pressure top and bottom is equal and there is no resultant force. The spring then keeps the valve in the closed position. When the manifold pressure drops below the intercooler, there becomes an imbalance in the forces and the valve opens. Once the intercooler pressure is relieved, the spring closes the valve again
  • Spring pre-load – as above, the spring pre-load had nothing to do with the amount of boost pressure – a GFB valve stays shut regardless of whether boost is 5psi or 500psi. The spring pre-load is adjusted to balance the IDLE VACUUM. At idle, the spring should push the piston down just a little bit harder than the vacuum trying to open it, so the piston stays shut, but will open very easily if there is boost in the pipes that needs venting.
  • GFB vs factory: The operation method described above means that a GFB valve is shut until it is required to open, whereas a factory valve is usually open until required to shut. GFB’s method improves throttle response over the factory valve.
  • Factory valve operation:  typically factory valves have one extra force acting on the valve that causes it to open at higher boost to prevent boost increases – manifold pressure on top of the diaphragm, IC pressure on the bottom of the diaphragm (thanks to the small transfer port), intercooler pressure pushing valve up, spring pushing down. At WOT, the forces on the diaphragm are equalised, leaving the spring to hold the valve shut against the boost trying to open it. So in this case the spring pre-load is DIRECTLY responsible for holding the boost, and will leak at some point. For example, a WRX valve starts leaking at about 19psi, STi at 22psi, EVO at 22psi, and a 200SX leaks from 0. This is also why the spring on a factory valve FEELS harder than a GFB one, but in actual fact the GFB spring is EFFECTIVELY harder.
  • Venting bias adjustment: GFB is the only manufacturer to offer venting bias adjustment – a feature patented by GFB. This allows you to choose how much air vents to atmosphere, and how much is recirculated, simply by twisting the cap. Other brands require plug kits and tools to achieve a maximum of 3 configurations. When the GFB Respons is the same price or less than these types, it’s not a difficult choice.
  • Vehicle applications: The Respons TMS comes in a bolt-on kit to suit just about any turbo car on the market, without modification|
  • Boost leaks: a GFB valve will show a leak when subjected to most common methods of pressure testing. This is important, because it does not actually indicate a malfunctioning valve, and the reasons for this are not well understood, even by people that would call themselves “experts”. A normal intake system pressure test will pressurise not just the intercooler downstream of the turbo, but the intake between the filter and the turbo too. If a GFB valve is fitted to the car and is connected to the recirc hose, that means the recirc outlet of the valve is pressurised during this test. GFB valves are not designed to hold pressure on the recirc outlet, simply because in practice there is no need to – the intake pipe only sees tiny amounts of vacuum, never boost pressure.

    Secondly, the piston in a GFB valve is precision machined with a piston-to-bore clearance of 0.03 to 0.05mm, and features an Ertalyte (similar to Teflon) piston ring, similar in design to an engine piston ring. However, this does not form a perfect seal like an o-ring, and a small amount of leakage will occur past the sides of the piston. This is normal, and will show up during a leak test if the vacuum nipple is pressurised. The size of the leak however is approximately 2/5ths of f**k all, and about 10 times smaller than the amount of air leaked by a common boost control solenoid, and many, many times less than a factory valve at its threshold, and is therefore not capable of causing a boost pressure loss, performance loss, or any adverse effects to the engine whatsoever. The reason the piston does not use an o-ring is because o-rings cause friction, which prevents the valve from being able to vent to atmosphere properly without stalling the car, and they need frequent replacement. O-rings are in fact cheaper than the ertalyte piston ring we use, so this is an example of how we put performance over cost.
  • Venting To Atmosphere – is it really as bad as everyone says? Not with a GFB valve. A correctly installed and set up GFB valve won’t (and can’t) blow your engine up, damage or wear the engine or turbo, stall the car, or use more fuel. Fact is, GFB have been making valves that successfully VTA on cars with airflow meters for over 10 years. If it couldn’t be done, we wouldn’t still be in business.

    There is of course a valid technical reason why people say you shouldn’t. If the ECU measures air which is then vented out of the system by the BOV, it will continue to inject fuel for the measured amount of air which is now missing, and will result in a rich mixture. However, the conditions surrounding this need to be looked at. The only time this occurs is when the valve is venting, which if set up correctly, is for a brief period after the throttle is closed when the car is on boost. In addition, how rich the mixture goes depends directly on how much air you vent. Put too big a valve on and it will cause problems.

    So, the mixture goes rich ONLY for a second or two when the throttle is closed – not really detrimental conditions to the engine. And with the venting bias adjustment system of GFB’s valves, you can fine tune exactly how much air is vented, which will prevent problems such as backfiring as a result of a rich mixture.

    Stalling is caused by a VTA valve that does not close at idle, or does not close quick enough. Correct spring pre-load is the simple way to ensure this does not happen.
  • Throttle response: contrary to popular theory, venting as much air as possible through the BOV does not allow the turbo to “freewheel” and maintain RPM to reduce lag. What is does do is empty the intercooler completely of pressure, which ultimately means it takes longer to return to peak boost after shifting gears. The key to optimum throttle response is having the spring pre-load set firm enough to hold a little backpressure in the intercooler, but not so firm that it causes compressor surge. The end result is up to 30% less lag during a gearshift.

Further Reading

  • The truth about Compressor surge
  • Turbo Lag and the TMS solution