While on my search for detailed information about managing compressor surge in our favorite little *MAFed* engine and a good BOV I came up on this information at the Go Fast Bits website and thought I might post it here for everyone to read. I know some of you don't see this as an extremely important or complicated issue, but there is more to managing the turbo charge than some people think, and this is great for noobs to know before they go out and make a BOV purchase just based on sound. I edited it a little because of course it is biased towards GFB valves, but I nevertheless found it to be very useful info. It might be a little sloppy and I apologise for any incinsistencies, but that's what happens when you borrow information. Please let me know about any errors etc. so I can fix them. I DID NOT WRITE THIS!!!
Blow Off Valves -
I want my BOV to be noisy, but I've been told that I can't vent a blow-off valve to the atmosphere. What's the deal?
There are many people who will say that if your car has a MAF (Mass Air Flow) sensor (which is most modern turbo cars), you can't vent a BOV to atmosphere. This is not entirely true. In most cases you can, but it pays to be aware of the possible side effects. Quite often you may find the side effects are so minimal that they are not really a concern. This section describes in detail what happens when you vent to atmosphere.
Most factory turbo cars run some form of MAF sensor (usually found directly after the air filter box), some use a MAP (manifold absolute pressure) sensor or even a combination of both. These sensors are used to determine the amount of air the engine is using so it can deliver the appropriate amount of fuel. In a car with a MAF sensor, when a BOV vents, air is escaping from a closed system. This air has already passed through the MAF sensor and has been measured, and the computer doesn't know that this air never made it to the engine. This will cause a brief rich mixture as the computer will still deliver the correct amount fuel dosage based on the amount of vented air.
There are two stages to a BOV venting, as initially it is evacuating the pressure from the inlet pipes and intercooler, which usually takes less than a second (depending on your inlet system). Then once the pressure is released, the valve stays open to allow the turbo to freewheel, thus reducing compressor surge and the associated thrust and torsional loads. It is mainly during this free-wheeling stage that causes the over-fuelling problems, since the turbo is basically pumping air through the MAF sensor and out to the atmosphere through the BOV, which accounts for the majority of air that escapes the system. The resulting rich mixture is what can sometimes cause backfiring and a puff of smoke in some cars. The severity of these effects usually depend on the state of tune of the engine. In cars that are modified (say with full exhaust, pod filter, a little extra boost etc) but still using the factory tuning, it is not uncommon for the ECU to compensate for the extra airflow it sees by running rich for engine protection. On a WRX for example, mildly modded engines can be running as rich as 10:1 with the factory ECU. It is this poor state of tune that can cause backfiring when an atmosphere-venting BOV is added.
Stalling is another common problem, many people have had bad experiences with atmosphere-venting valves causing stalling problems. However, with the correct spring adjustment this is never a problem. As long as the valve closes properly before the engine reaches idle, the ECU will have no problem maintaining a smooth idle. Most complaints of stalling actually come from people using certain Japanese brand valves, which often do not have the range of spring adjustment to compensate.
Some cars are affected by backfiring when venting to atmosphere, and some are not. Even two identical cars with slightly different mods can react differently. The bottom line is if you vent to atmosphere with a MAF sensor you MAY use fractionally more fuel (depending on the kind of driving you do) and there is a chance you may hear some popping in the exhaust. For people who just want the maximum noise from the valve this is usually not a worry.
How do I adjust the spring pressure to suit different boost levels?
You don't need to! At full throttle there is equal boost pressure on both sides of the piston, so it doesn't matter what boost level you are running, the pressure balances itself out. It then requires very little spring pressure to stay shut. The spring adjustment is used to match the VACUUM signal of the car. The idea is to adjust the valve so that the piston remains just closed at idle. This way, when you lift off the pedal, the spring will be neutralized so that the boost is free to push the piston open.
I have a blow-off valve on my 2002 WRX and there seem to be times when it doesn't blow off, and when it does it isn't very loud. Is there a problem with the valve?
The MY01-2 model WRX features vastly improved noise insulation from the engine bay when compared to earlier models, so the noise of the valve tends to be quite muffled. Rest assured that if you drive past a concrete wall with the window down you will hear it! The recirculation part of the WRX Hybrid valve is substantially larger than the trumpet opening, and will open with much smaller piston movement. It is designed this way for maximum flow and also because these WRX's can be more sensitive to atmosphere venting than earlier models. So the times when you can't hear the valve are a result of most of the air going back to the inlet and the extra sound insulation. How do you get a BOV to make the fluttering, chirping kind of noise?
This noise is actually compressor surge, and it does not come from the BOV, but from the intake of the turbo. Compressor surge is basically a situation where boosted air forces its way back through the compressor housing when the throttle is closed, causing that noise as the blades of the compressor "slip" in the air. Although many people like the sound of compressor surge, it can shorten the life of the turbo depending on its severity. The pulsing air in the inlet can cause excess thrust loads on the bearings, and also shock loads on the compressor blades.
It is best to set the valve spring pressure to avoid this noise, but for those that like it, it is possible to set the spring pressure a bit tighter to get the noise at lower RPM, whilst still allowing the valve to blow off properly at high RPM. The speed of the turbo and the forces it endures rise a lot with RPM, so low in the rev range you can get away with it without placing too much extra load on the turbo.
The adjustment of the valve should be made so that the piston will shut just before idle every time. There is a common misconception that adjusting the spring pressure will change how loud the valve is. This is not really true, but the spring pressure does determine the duration of the venting.
I'm having new intercooler piping made and I want to know where the best place to install the blow-off valve is?
I've not seen any conclusive results that prove that one position is better than another, but many people have different ideas about this. Some say that it's best to have it close to the throttle, since that's where the back-pressure builds from when the butterfly is closed and it will respond quicker. Others say that it's better to put it as close to turbo as possible so that the valve is venting hot air rather than post-intercooler cold air, so that the inlet pipes after the intercooler are still filled with cool air.
Both positions have their merits, but are really of minute benefit. There is a trap to watch out for when mounting close to the turbo however. There is often a measurable pressure drop between the turbo and throttle body, especially as the RPM and hence the airflow increases. A BOV uses two pressure signals to stay shut, one from the manifold and one from the inlet, which act on opposite sides of the piston and since they oppose each other they should balance out, and the spring then holds it shut.
The problem is when the revs rise and there becomes a pressure difference between the two signals. The pressure leaving the turbo may be say 16psi, while only 13 or 14 psi makes it to the manifold. So this means that there is 2 or 3 psi of pressure acting against the spring, which is enough to move the piston a fraction. In the Hybrid and Bovus Maximus valves, since the outlets are staged and one is placed very close to the seat for rapid response this slight spring compression can be enough to open the valve a small amount. It is not a problem with the other valves in the range, and it depends on the flow efficiency of your intercooler and pipes, but for the Hybrid and Bovus Maximus valves it is best to locate them after the intercooler.
If this problem does show up then it may also be a good idea to test the efficiency of your piping by measuring boost at redline just after the turbo, and after the throttle body. If there's any more than about 3 psi difference then your turbo is working harder than it needs to, and reducing this drop would help performance a lot.
Boost Controllers -
How much extra boost can I run when using a boost controller?
It is very important to be sensible when raising the boost limit on turbocharged cars. Power is directly related to the amount of air and fuel that makes it to the engine cylinders, but lifting the boost level is only one way of doing this. While in most cases it is quite safe to increase the boost by about 10-20%, keep in mind that every component in an engine comes from the factory designed to operate most efficiently at the level it leaves the factory at. If you intend to raise the boost to any more than 20% above factory, you will need to check things like fuel delivery, the intercooler, ignition timing, etc to ensure that they are capable of supporting the increase in power. It is also important to realise the mechanical limits of components, a good example being the ceramic turbine wheels in the turbos found in Skylines, which will take no more than about 14psi without losing blades. A GFB boost controller will give you the added benefit of not only being able to adjust the peak boost level, but in most cases will bring the boost on earlier, resulting in more power throughout the rev range. To get the most out of the extra boost it is best to ensure that the engine can breathe properly. A good exhaust is the first place to start. Then follow the inlet tract all the way to the engine, looking for restrictions in the form of resonators, sharp bends or reductions in diameter. Manufacturers will often purposely build in such restrictions to muffle intake noise or prevent overboosting. Removing airflow restrictions means the turbo doesn't have to work as hard to get the boost to the engine.
Why does the boost in my car tend to taper off with increased boost at high RPM?
This is a sign that your turbo system is nearing the limit of its efficiency. Most factory turbos have small turbine housings to reduce lag, but the problem is at high RPM it can cause a large enough restriction to push the wastegate open as the revs increase, often referred to as wastegate creep.
This can also be caused by restrictions in the factory turbo compressor inlet and outlet pipes that also contribute to tapering boost levels, such as convoluted hose sections. The boost controller usually gets its boost signal from directly after the turbo, so the boost at this point is often likely to hold steady. However, restrictions further downstream (corrugated pipes, small intercooler, sharp bends or reductions in diameter) will cause a pressure drop at elevated RPM, particularly if the car is modified and making more power than factory.
It is also important to realise that in stock or mildly modified engines it is probably much safer to drop the boost level a little towards redline. The inertial loads on the reciprocating components increase exponentially with RPM, which is why you will often see the factory ECU drop a few psi near redline. Generally this will occur on cars with small factory turbos, and it is probably in the best interests of the health of the turbo for this to happen unless your engine is built to handle it.
Please stickey:BOV and boost controller FAQ
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captain nismo
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hokiruu
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'95 240sx/s14 SR20DET (in 2000) SOLD - Location: Northern CA
Yeah, I thought I wanted the Blitz too. But anything that doesn't even offer the option to recirculate is less worthwhile. So now I am just looking at Greddy and HKS for BOVs, or I may just get a bypass/recirculating valve like TurboXS or the HKS racing bypass. Seems like you can't really go wrong with recirculating.
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badass240sx
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Cyberkreig
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badass240sx
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It's been dyno tuned, not running rich at all...those who ran rich using the Blitz BOV obviously didn't adjust it right...you've got to tighten the piston just right. I have had hardly any backfires, only when I give too much gas and shift way soon because of the excess gas it'll backfire. We're talking once in a while, not much at all.hokiruu wrote:Yeah, I know, the sound, the sound. What about things you can't feel or see on the *** dyno like running rich? Recirculating also helps turbo response. But we'll see what I actually end up using.
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chillmieste
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Im thinking of going with the hks ssqv. If anyone wants to hear the bov sounds on different cars go here:
http://www.blowoffvalves.com
Hope you like the sound clips.
Waaaaaa pshhhhhh!!
http://www.blowoffvalves.com
Hope you like the sound clips.
Waaaaaa pshhhhhh!!
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badass240sx
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chillmieste
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SquirrelOnFire
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hey guys, im a noob and this question might be preety dumb to you, but to be on the absolutely SURE side the recirculate option that some of the BOVs have, as it sujests basically sends the atmoshpere bound air back to the engine (or em I wrong?) and I was wondering how. I mean after all the air has to go somewhere for that split second, so how exacty does it work?
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[GoaTSLonG]
- Posts: 185
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ever since i put on my Blitz SS my car backfires at 1400rpm REALLY LOUD, my car runs really rich, and it stumbles when being brought back to idle really fast. The backfire is kidna fun sometimes cuz its loud enough to scare the **** outta people. Is there anyway to adjust it so that it doesnt run so rich and stuff? another question i have is, if i install a manual boost controller and i have the stock intercooler and injectors but i have full 3" exaust, intake and a walbro 255 what is the highest boost i can run safely? and with the MBC all i have to do to raise the boost is unscrew the knob and it will raise? i never really understood a MBC
