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Tech <strong>Tune</strong> BLOW OFF VALVE Our Tech <strong>Tune</strong> segment for this issue concerns that wonderful noise-unleashing contraption turbo lovers can’t get enough off – blow off valve. Never mind that its namesake might take on a rather ‘naughty’ connotation, a blow off valve is an important part to any turbocharged or supercharged vehicle. These valves save our forced induction components in a way that most users may over look. Other terms you may hear or read about that are essentially the same thing to Blow Off Valve, are recirc (re-circulating) valves, diverter valve, bypass valves, or the chic ‘stylized’ abbreviated BOV. A blowoff valve (BOV) is similar to a wastegate, except it works on the intake side of the turbocharger. Once a certain level of pressure is reached, the inlet air opens a spring, allowing for the intake air downstream of the turbocharger to either vent to the atmosphere or be redirected back into the turbo. Blowoff valves which open to the atmosphere give the iconic “pssshh” sound, which is heard when you let off the throttle. Instead of the high pressure air trying to work its way back through the compressor (surge), a BOV allows it to escape. Why we have blow off valves? Blow off valves are important in helping with the longevity of turbochargers and centrifugal superchargers. These valves open whenever there is enough boost pressure in the intake pipes, and vacuum from the intake manifold. Essentially, these valves operate every time the throttle is opening enough to create boost pressure, and then backed off to create vacuum. When this valve opens it releases the pressurized air in the intake piping either back to the turbo intake piping or vents it to the atmosphere. The reason this is important is because if that pressurized air is not allowed to escape once the throttle buttery closes, then the air will apply pressure on the turbo or superchargers compressor wheel putting un-wanted stress on it. While this stress is very small, over time it can cause added wear to the bearings and cause the unit to fail prematurely. Another great benet of a BOV, other than adding longevity and reliability to your set-up, is that it can help maintain turbine speeds in-between shifts adding a performance enhancement as well to the vehicle. There are not many modications one can do to an engine that can give them reliability and POWER, so a BOV should be an essential component on your list if running a turbocharger or centrifugal blower. How a Blow Off Valve Works A Blow Off Valve works the same as a Wastegate does with a valve controlled by a diaphragm. The diaphragm has a spring and a vacuum port input to help open and close the valve. A great article has been written on this topic in the How a Wastegate Works section. If you are un-familiar with how a Wastegate works as well as a BOV follow this link over to the Wastegate article for a more in-depth look at how Wastegates and Blow Off Valves work. 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. Re-circulating Vs Vented Blow off Valve Recirculating BOV
A re-circulated BOV is one where the boost pressure is re-circulated back into the intake just before the turbo. This type of set-up is a must for those that are running a Mass Air Fuel (MAF) sensor. The reason for this is that the MAF has already accounted for the air that is in the intake manifold, and if it was allowed to escape to the atmosphere, the car would run overly rich due to the fact that the ECU still believes that the air is in the intake tract. A bonus to running a re-circulated BOV is that the air being re-circulated can be brought back into the turbochargers intake pipe at a location and angle so that the air pressure is forced directly onto the compressor wheel. This will aid even more in helping to keep the turbo spooled up and running inbetween shifts, or times when you are quickly on and off the gas pedal. A turbo type made by Borg Warner turbo systems called the EFR series has a build in, re-circulated BOV built into the turbo’s compressor housing. Vented BOV A vented BOV is one where the air is just released to the atmosphere instead of being re-circulated back into the system. Many argue that all BOV’s should be re-circulated back into the turbo’s intake tract to gain that slight performance increase in response, but some argue the sound of a vented BOV makes up for any performance loses. Another thing to note is that setting up a Vented Blow Off Valve is simpler than a Recirculated set-up just for the pure fact that there is really nothing to set-up, you just vent the air out. Ultimately its up to the end user which method they choose to employ on their particular set-up. What are the components of a turbocharger? The layout of the turbocharger in a given application is critical to a properly performing system. Intake and exhaust plumbing is often driven primarily by packaging constraints. We will explore exhaust manifolds in more detail in subsequent tutorials; however, it is important to understand the need for a BOV or (compressor bypass valve) on the intake tract and a Wastegates for the exhaust ow. The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo’s compressor from going into surge. The BOV should be installed between the compressor discharge and the throttle body, preferably downstream of the charge air cooler (if equipped). When the throttle is closed rapidly, the airow is quickly reduced, causing ow instability and pressure uctuations. These rapidly cycling pressure uctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads associated with it. Blow-Off valves use a combination of manifold pressure signal and spring force to detect when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the intake tract to atmosphere to relieve the pressure; helping to eliminate the phenomenon of surge BOV Myths Blow-off valve myth #1 One of the biggest misconceptions about blowoff valves is that you need to tighten the spring to run higher boost. This is totally incorrect (at least, for a GFB v a l v e 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 uttering sound is usually believed to be the blow-off valve. In reality, it is caused by a blowoff 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 “utter”. 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. In summation Their names are many, blow-off valve, dump tube, recirculation valve, bypass valve, diverter valve, and their function is similar but as with anything you need the right tool for the job. According to experts the most important consideration is to match the size of the valve with the ow of the system and when in doubt it’s better to go a little big than a little small. The same certied experts also said )and I quote following reminder: Ultimately, Picking the best valve comes down to the how much boost you’ll be running and the physical space available for installation,” says Staggs. “Many units have an adjustment screws or swappable springs to tailor valve activation to each particular application. Use the resources of the manufacturer or a trusted tuner to get the right size the rst time.
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