What is a blow off valve and how does it work?

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A blow off valve goes under many different names such as dump, bypass, recirc (short for recirculation), diverter, and pop off valves. No one term is correct, the lingo just tends to change in different regions and countries. Basically these valves are all designed to the same job, and we will refer to all types as blow-off valves to save confusion.

What it does: The only job a blow-off valve is supposed to perform is to relieve excess turbo pressure that results from shutting the throttle rapidly under boost conditions. This then answers one of the questions we get asked a lot – yes you need a turbo to fit a blow-off valve! Otherwise you will have a rather expensive paper-weight. It is necessary to release this pressure because it will try to find a way out of the system, and the only way is back out through the turbo. This produces a fluttering noise as the air passes backwards through the turbo, which is often unwanted. There is also a school of thought that a blow-off valve can improve turbocharger longevity or, in extreme cases, prevent damage to the turbo.

There are two ways in which the majority of blow-off valves work. Most aftermarket and many factory types use two pressure signals to determine when the valve should open. The other type uses only one pressure signal to open. We will discuss the former type first.

How it works: The usual arrangement is to have the pressure side of the valve attached to the pipe between the turbo and the throttle, and a vacuum hose on the top of the valve hooked up to the inlet manifold after the throttle body. A spring holds the valve shut. When you are on the throttle, the pressure in the turbo piping and the inlet manifold is equal, meaning that the pressure on each side of the valve is the same and therefore cancels itself out, leaving the spring holding the valve shut. When you lift off the throttle, you have high pressure in the turbo piping, and a vacuum in the inlet manifold. The pressure on the bottom of the valve and the vacuum on the top combine to lift the valve open and release the pressure in the turbo piping, since it can no longer go into the engine.

Blow-off valve myth #1: One of the biggest misconceptions about blow-off valves is that you need to tighten the spring to run higher boost. This is totally incorrect (at least, for a GFB valve anyway), as you can see from the last paragraph, at full throttle, the pressure on the top and bottom of the valve is equal, therefore cancelling itself out. So no matter what boost pressure you run, the valve will stay shut.

Leaking factory valves: Having said that, some factory valves have systems designed into them to crack open at high boost, in order to protect the engine from boost spikes. They do this by designing the pressure to be unequal on the top an bottom of the valve, thereby overcoming the spring at a certain pressure.

If you are intentionally raising the boost level, this is bad news as the valve will begin to leak pressure. Examples of factory valves which exhibit this behaviour are the Subaru WRX MY95-98, and MY01-04, Mitsubishi Lancer GSR, and Nissan 200SX to name a few. Replacing these valves when increasing the boost is a good idea.

Blow-off valve myth #2: The fluttering sound is usually believed to be the blow-off valve. In reality, it is caused by a blow-off valve, but does not come from the blow-off valve. If the spring pre-load is adjusted too tight, this will cause compressor surge, which as described above is the sound of air exiting the turbo.

Compressor surge: You can think or surge as the point at which the compressor blades begin to “slip” in the air, losing their pumping ability, much like an aircraft wing loses its lift when it stalls. In a turbo, this happens in a series of bursts, as the blades slip, then bite, slip then bite. This sets up a pulsing wave in the turbo piping and explains why the sound has that characteristic “flutter”.

The interesting thing about compressor surge is that it occurs much more readily at low turbo shaft speeds. At these low shaft speeds, on road cars this is generally between 2000 and 3000 RPM, compressor surge is not much of a problem, as the loads generated by the surge are miniscule compared to what the turbo encounters at high boost. However, if surge occurs at high RPM and boost, it is possible to reduce the turbo life and/or damage the compressor.

Boost controllers:

Raising the boost: The power produced by a turbocharged engine is (in perfect conditions) directly related to the amount of air that fills the cylinders. In practice, other variables such as temperature, humidity, ignition timing etc etc make it a less-than-direct relationship. That aside, raising the boost pressure is a very simple and effective way of increasing the amount of airflow into the engine, thereby increasing the power output.
Cautions: While increasing the boost is a very easy way of extracting power, it should be done sensibly and with a little appreciation of the mechanical limits of the engine. Therefore it is important to use a boost gauge and make small increments. Raising boost levels will increase the amount of mechanical and thermal stress on all engine components, however, in most applications boost increases of 10-20% are quite safe. Also be aware that any engine components that may fail as a result of careless adjustment are not likely to be warranted by the manufacturer.

How boost control works: All turbocharged engines have some form of factory boost control, all of which work on a pneumatic system. To understand how a boost controller works, we must first look at this system. Ultimately, the boost pressure is determined by the wastegate, which on most factory turbos, is integrated into the exhaust (turbine) housing. The purpose of the wastegate is to dump a controlled amount of exhaust gas from the exhaust before the turbine to keep the turbo shaft speed, and therefore the boost, under control. If not for the wastegate, the boost pressure would continue to rapidly rise to disastrous levels.
The wastegate actuator, which is the can shaped object mounted on the turbo (except on external wastegate systems), forms part of the pneumatic system which controls the wastegate. Boost pressure is delivered to the actuator via a small hose from the compressor outlet, thus forming a control loop. As the boost pressure rises, this pressure begins to open the wastegate via the actuator to slow the build up of boost until the set level is reached.
Increasing the boost: Here's where the GFB boost controller comes in. When plumbed correctly into the hose that feeds the wastegate actuator, the controller "bleeds off" a measured amount of air (set by the adjusting screw on the top) to reduce the pressure in the hose. This sends a lower boost signal to the actuator, so that the wastegate will remain closed for longer, thus increasing the boost level. The end result is that the turbo is producing more boost, but the wastegate doesn't know!
Ball-and-spring, or bleed type? Many other manufacturers use what is described as ball-and-spring, or “gated” type boost controllers. The claim is that pressure is held back from the wastegate to prevent it from opening prematurely, and only allowing pressure in once the boost level is almost reached. Whilst this sounds good in theory, in practice it does not always work.
Bleed type: GFB boost controllers are all bleed-style, which uses a restrictor (a small precision hole in the pressure inlet nipple) and a taper needle adjustment. The restrictor plays a very important role, which must not be underestimated. Basically, without the restrictor, the adjustment needle would not be able to bleed off enough air to lower the pressure reaching the wastegate actuator. The turbo is pumping more than enough air to overcome such a small bleed. By placing a restrictor in the flow path, with the bleed on the other side, the air cannot pass fast enough to overcome the bleed, therefore the pressure will drop at the actuator and boost will rise. This explains why if you install the controller backwards, you will not be able to raise the boost.
The diameter of the restrictor hole is very critical, and the wrong size can affect spool-up, or cause boost spikes. GFB has spent a lot of time and development ensuring the restrictor is optimum, and the results show.
Ball-and-spring types: These controllers still use a bleed, but the most common technique is to place a steel ball behind the restrictor hole, held shut by a spring. The main problem with this technique is that since the restrictor hole is so critical to boost, placing a free-floating ball behind it adversely affects its performance. Because the ball is free-floating, it can act like a pea in a referee’s whistle, fluttering around in the restrictor hole. This then causes boost fluctuations, and (if the spring is set incorrectly) boost spikes. This is much easier to see on a dyno which accurately logs boost pressure.