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Technical info on BOV/BPV Set up and operation

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This info is posted with the best intentions.


I make no claims to its validity.


Some might be propaganda, or sales pitch. But, I get asked for it quiet often, so I thought it a good idea to post it all in one place.


There is a lot of info and reading. But there is some very good info in this. It should help people to better understand their set up, and how to get it dialed in properly.

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Learn stuff - DV (Diverter Valve or Blow off valve) is it just performance noise?


Is a BOV (blow off valve) bad for my engine?


There are two main reasons why most people want to put an aftermarket BOV on their car: noise, and performance. We’ll take a look at these two separately.


Noise: venting a BOV to atmosphere has no benefit, other than to make noise. The aural benefits of this and your increased ability to pick up chicks is beyond the scope of this posting, and remains purely a personal choice!


It is common knowledge that venting a BOV to atmosphere on a car with a MAF sensor does have the potential to cause a few issues, although the scale of the problem is often blown out of proportion and is not well understood. GFB takes great care with the design of our BOVs to ensure that those who want to noise are able to get it without risking the commonly associated problems.


I’ll refer to the linked thread http://forums.nasioc.com/forums/showthread.php?t=468038 , as this is a very thorough and appropriate summary, although there are a few points I’d like to clarify. Dan has tackled the old “running rich” statement very well. Basically, when you shift gears and the valve vents, a certain amount of air passes through the MAF and is measured. The ECU continues to inject fuel for the amount of air measured, but some of it has escaped to atmosphere, resulting in a brief rich mixture. How brief? A lazy shift lasts for about a second, and the injectors will typically cut out after a second anyway on a closed throttle above 1200RPM in a Subaru. So really the longest you can have a rich mixture as a result of an atmo-venting BOV is about 1 second.


How rich? As Dan says, it may dip below 10:1, which is nothing serious, but it should also be noted that how rich the mixture goes for a given ECU tune is ENTIRELY dependent on how much air you vent to atmosphere. Here’s where I would argue Dan’s point written about Hybrid BOVs:



“What about a 50/50 or BOVs that you can portion the VTA portion? This is a bad analogy, but if a BOV is a person in a wheelchair, a 50/50 BOV is a person in leg braces. It's not as bad, but not good enough to say bolt it up. If you find a deal on one or happen to like the sound of a particular model, go for it, but don't think you are doing your car better vs. a 100% VTA model”.

I’d have to disagree that it’s a bad analogy, or that it’s no better for the car than a 100% atmo model. As per my statement above, the amount of air vented to atmosphere is directly proportional to how rich the mixture goes during this brief period. If you put on a large atmo valve, say like a GFB SV45 designed for 500-1000hp, as soon as that valve opens it will let out a HUGE amount of air, and the mixture will go measurable richer. This can be felt as you go into the next gear and the car needs to “clear its lungs” of the extra fuel.


If on the other hand, you put on a valve such as the GFB Hybrid or Stealth FX in which you can alter the venting ratio, and 50% of the air is recirculated, then the mixture will not go as rich, and the chance that you will run into problems such as backfiring or loss of performance is proportionally less.


There is typically a certain amount of air that you can vent to atmosphere without causing any problems. In most Subarus, GFB valves can usually be vented fully to atmosphere without any problems provided the spring is set correctly (Dan’s explanation of this procedure is quite thorough). Possible exceptions for this are on cars that run the standard ECU, but have other mods such as more boost, full exhaust, intake etc. This is because the factory ECU runs quite rich to begin with, and increasing airflow via these mods will cause the ECU to run even richer – we’ve seen cars with stock ECUs run down below 10:1 because of mods that haven’t been compensated for. Throw an atmo-venting BOV on top of that and the likelihood of problems increases.


Now, let’s take a look at performance. It is fair to say that the factory Subaru valve is not completely useless. It will hold boost just fine at stock and slightly increased levels, but it’s not correct to say STi and WRX valves are the same. These valves are specifically designed to begin leaking above a certain boost pressure, which is set by the spring pre-load. Whilst unfortunately we don’t have a library of year models and factory bypass valve part #s, I can say with 100% certainty that STi and WRX factory valves have different spring pre-loads and will leak at different levels.


Now, a very important point. The fact that the spring pre-load on the factory valve dictates the point at which the valve begins to leak DOES NOT HOLD TRUE for GFB valves. A GFB valve will stay shut under WOT (wide open throttle) conditions REGARDLESS of the boost pressure or the spring pre-load setting. This is because manifold pressure and intercooler pressure are equal under WOT, and the piston area of the GFB valve top and bottom is equal, and since the manifold and intercooler pressure are fed to the top and bottom of the piston, the resultant force is zero – they cancel each other out. Therefore only a very small amount of spring pre-load is required to hold the valve shut.


Now, with the above in mind, let’s talk leaks. Here’s what Dan has to say:


“I have had XXXX brand valve for years, it has never leaked. How do you know? By looking at your boost gauge? Looking at the boost levels in your datalog? Neither of those prove that the valve isn’t leaking”.

I should also point out that a lower boost pressure reading doesn’t prove the valve IS leaking. “Losing boost pressure” is one of the most commonly assumed symptoms of a leaking BOV/BPV, but is in fact very close to useless for diagnosing leaking.


The term leak unfortunately does not in any way indicate a MAGNITUDE, and hence should be very carefully used. The pistons in your engine leak, but the engine still works. Your boost control solenoid leaks boost, but you don’t see a drop in boost pressure because of it. The SIZE of a leak is very, very important. For you to see a loss of boost on a gauge or data log, it needs to be quite significant – consider the amount of air entering your engine at full boost.


For argument’s sake, let’s assume your engine normally consumes 320CFM @ 14psi of boost. If the turbo does nothing to compensate for a leak (it will to a certain extent because of the nature of the wastegate system, but will be ignored for this example), and you see a boost loss of 2psi, for this to occur you would have to be losing 22CFM. The size of the leak would have to be the equivalent of an 8mm hole (derived from standard flow tables), which is pretty large.


So if you pull your aftermarket BOV off and find that putting a vacuum pump or compressor on it results in a few bubble coming out of it, you shouldn’t immediately panic. As I mentioned before, your boost control solenoid leaks in order to control boost (which is approximately equivalent to a 1mm hole), and if the leak you find on a BOV is similar, then you can be pretty sure it’s not costing you power. If on the other hand you could comfortably and constantly draw breath through it without suffocating, that’s a different story.


If you suspect a BOV leak IS causing a loss of boost/power, the only way to actually PROVE the BOV is the cause is to plug it up. You could perform a smoke test, you could put the car on a dyno and feel around for air leaks from the BOV under boost conditions, you could hook up a sophisticated air mass sensor in the BOV outlet to measure the air loss, but all of these tests either do not show magnitude, or are not feasible for most DIYers. Plugging off the valve and going for a lap around the block to measure boost is the simplest way to confirm the answer. One or two squirts without the BOV working won’t kill the turbo.


Finally, another point about performance. Fitting a GFB valve can give you a small but noticeable improvement in throttle response. The difference in the way the factory valve works compared to a GFB valve can be summed up as:


A factory valve is typically open until required to shut, whereas a GFB valve is shut until required to open.


This means that boost pressure can be made and held in the intercooler during conditions under which the factory valve would normally vent. A good example would be mid-corner, when modulating the throttle to balance the car – under light throttle, when the manifold is still slightly in vacuum, the turbo is capable of making boost, but not with the factory valve since it would be open under such conditions. A GFB valve on the other hand would be shut, the intercooler would be in boost ready for the throttle to open. The result is a quicker rise to peak boost when you do snap open the throttle. Connect your boost gauge prior to the throttle and you will see this effect.


In summary, atmo-venting BOVs aren’t “bad” for your engine. A correctly set-up GFB atmo-venting BOV should not cause any driveability issues, and does offer the benefit mentioned above. If anyone does have driveability issues with a GFB valve, I would invite them to speak to us about it and we can help.


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Here's an answer from the Go fast Bits engineers



The spring pre-load of all atmo or recirc venting bypass valves is determined by a number of factors, mostly based upon the design of the valve. For the design of the GFB valve, the spring pre-load is designed to hold the piston shut at idle – only just. The adjustment range of the spring pre-load should be such that the amount of force holding the piston shut at idle ranges from almost none to about 1.5lb. To accurately test this, don’t bother with vacuum pumps and the like, connect a vacuum hose from the manifold of your idling engine to the top of the GFB valve and then feel the force holding the piston shut – it will be almost zero. Therefore, when the throttle is closed after having been on boost, it only takes a very small amount of boost pressure to push the piston open and vent. This will be obvious during use, because even revving the car in neutral should cause the piston to partially open and vent audibly.


It follows from the above that the spring pre-load is not related to boost, but rather closed-throttle vacuum. The adjustment range simply changes how easily the boost pressure in the pipes can push the piston open when the throttle is snapped shut. Note that at the other end of the scale, i.e. when you’re at wide open throttle and full boost, the design of the GFB valve is such that the piston will stay shut regardless of the spring pre-load adjustment setting. This is because the area of the piston top and bottom is equal, and both the top and bottom are exposed to the same boost pressure at WOT, therefore the forces on the piston cancel each other out, leaving even the smallest amount of spring pre-load the simple task of keeping the piston shut – in the GFB valve the lowest amount of spring pre-load is about 10lbs, so it doesn’t matter if you are running 5psi or 50psi, there will always be at least 10lbs of force holding a GFB valve shut at WOT regardless of the boost or spring pre-load adjustment.


A note to anyone who sees boost dropping off as RPM increases, this does not necessarily mean it’s the valve leaking it off – most commonly this occurs because the turbo simply runs out of puff – a stock EVO turbo is really being pushed above 20psi, and as such the exhaust backpressure rises exponentially, the compressor efficiency drops right off and the air becomes excessively hot, to the point of choke – i.e. the region beyond which increasing the turbo’s RPM yields very small increases in airflow.


Going back to the first point about spring pre-load, why is the GFB valve designed to be shut at idle? This is because any BOV that vents any amount of air to atmosphere on a car with a MAF sensor needs to be shut at idle, or the car will stall, backfire, stumble and generally drive poorly – try venting a stock valve to atmosphere and see what happens…


So from this we can establish that if you have an atmo-venting valve, there is a minimum amount of spring pre-load that you can run – if the spring pre-load is too soft the valve will open at idle and cause the aforementioned problems. Now, in regards to flutter caused by the BOV, there is a maximum amount of spring pre-load above which flutter will occur, most noticeably beginning with low RPM and light-throttle lift offs, becoming more and more noticeable at higher spring pre-loads, until it begins occurring during higher boost and RPM lift-offs. Note that it is not uncommon for the minimum spring pre-load and the maximum spring pre-load as defined above to overlap, meaning and ideal solution is not possible – make it too soft to cure flutter and bucking at part throttle, and you may find the car idles poorly. Make it firm enough for smooth idling and you may find it flutters a little during light throttle lift-offs. This now leads into a whole new discussion about fluttering and its effect on the turbo.


In my opinion, if you are running a stock-cored turbo higher than factory boost, then you are placing quite a bit of load on the turbo. How much? Running some rough numbers off a TD05H compressor map, going from 18psi @ 5000RPM to 22psi @ 5000RPM increases the turbo RPM by about 20,000. By the time you get to redline, this number gets a lot worse. Then consider how much time you spend on boost compared to the next guy – there’s one of the biggest variables right there – two cars running the same boost and with the same miles on them may have seen the turbos do vastly different amounts of work simply be differing driving styles. Now throw flutter into the mix – the loads on the turbo created by a flutter when lifting off from 3000RPM at light load pale into insignificance. Compressor surge as defined as occurring when the turbo compressor is incorrectly matched to the engine and is being driven by the exhaust whilst it is surging, that’s an entirely different matter – that type of surge CAN do a lot of damage to a turbo – a small flutter at low boost and RPM in my opinion won’t.


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We've been seeing a number of threads and posts on forums about testing for boost leaks in the turbo system with many suggesting the "smoke test" as a good means by which to test for BOV or dump valve "leakage".


That concept of "leaks" and their practical significance is a whole subject on its own but we thought we'd start with our view of smoke tests and what you can expect to learn from using this method.


Below is an extract from our engineers paper on this and we hope you find it educational. As usual, we don't believe we know everything, this represents our view and we would love to hear from anyone that might disagree with these points.



Smoke testing for leaks:


When chasing boost leaks, the humble and effective smoke test is often used. Unfortunately, the results may not always be what they seem.


If you have a GFB valve fitted to your car, a smoke test will very likely return a “false positive” for a boost leak. There are a few things about the common smoke test that make it difficult to determine if a detected leak is really there.


To begin with, let’s look at a typical turbo system.


(Should be an image here but having trouble uploading, will have another attempt)


When you are driving, you floor the throttle and reach peak boost. In this situation, where you sure don’t want any leaks, the intake manifold and intercooler (and associated piping) are effectively at the same pressure (give or take a small amount for losses through the intercooler or throttle).


Therefore, in the diagram above you’ll see that the GFB valve is subjected to the same pressure top and bottom. This is essential for a GFB valve, as is the case for most valve types, because the pressure from the manifold at wide open throttle (WOT) counter balances the boost trying to push the piston open. Were the vacuum hose (poorly named in this example because it is currently in boost) removed from the top of the valve, it would simply blow wide open.




During a smoke test, the operator will typically remove the air filter and insert a large plug with a hole through the middle where the smoke is injected.


Here’s the first problem: this test does not typically pressurise the intake manifold, therefore the top of the valve is left un-pressurised, and will therefore blow open under a relatively small amount of pressure – this is the first case where a false positive can result.


The second problem is during this test, the turbo’s intake side is pressurised, which in real life never occurs. At worst, there may be a small vacuum in the intake pipe as a result of flow restriction at high boost/RPM.


A GFB Hybrid-type valve that features two outlets, one for recirc and one for atmosphere (and any other valve of this type on the market for that matter), will typically show a leak through the recirc port to the atmosphere port. Since during the smoke test the recirc port is pressurised (where it normally wouldn’t be), a second false positive can occur. Hybrid-type valves are simply not designed to hold boost pressure on the recirc ports, hence the leak.


As a separate issue, it should be noted that although a smoke test can point to a leak, it doesn’t actually give any kind of indication of the size of the leak. Sure, a leak is a leak, right? Well yes, but even leaks that are detectable by smoke test shouldn’t necessarily be assumed to be the cause of boost loss without further investigation.


If the symptom that warrants the smoke test in the first place is a loss of boost pressure noted on the car’s boost gauge, keep in mind the size of the leak you are looking for – a pinhole leak (or even a vacuum hose popping off the manifold) that can be detected by a smoke tester won’t even register as a boost drop on a gauge when driving.


If boost pressure drops when driving according to a boost gauge, in reality you’re going to be looking for a leak equivalent to a 6-8mm hole or larger.


Note that a smoke tester pumps a miniscule amount of air in comparison to a turbo, so therefore if it is able to build pressure in the system any existing leak is highly unlikely to be larger enough to be the cause of boost loss.


If on the other hand the smoke tester is simply unable to build pressure in the system at all, then that would indicate a reasonably large leak that could well be the cause of boost loss.




So, what can you do to eliminate the “false positives” during a smoke test? First thing to do is ensure the top of the valve gets the same pressure as the bottom – this will ensure the valve stays closed as it would when driving at WOT. The second thing to do is remove the recirc hose from the valve and plug it – this will prevent pressure leaking out through a path that is never pressurised in real life.


Finally, if you’re chasing a boost drop and the smoke test keeps nagging that the blow-off valve is the problem, there’s one sure-fire way to prove it either way – it’s very simple, requires no tools and you’ve probably already performed it without even knowing! This test is relevant to any GFB valve that you can hear when it vents (through the filter if it’s fully recirculated, or to atmosphere).


Here’s the test. If a GFB valve goes “whoosh” when you shift gears, it’s about 99.9% certain that it’s not the cause of a boost pressure drop.


Bold statement, I know - here’s the reasoning behind it. If the valve vents normally with a “whoosh”, it means the piston is not physically jammed, and the vacuum hose is connected properly, since without the vacuum hose the valve would not stay shut under boost, nor would it vent when the throttle is closed.


Since the piston is not jammed and the vacuum hose is connected, this also means that the piston cannot possibly open under WOT conditions, because the pressure on both sides of the piston is equal, which therefore cancels itself out (no matter how high the boost is) – in fact, a GFB valve is capable of staying shut under WOT conditions even if the spring were removed entirely!


Put simply, if a GFB valve vents normally when you shift gears, it is not physically possible for it to leak enough air to cause a measurable boost pressure drop. It’s either working or it’s not.


So the smoke tester is a very useful tool, however the results of such testing should simply be regarded as highlighting areas of interest for further investigation, rather than the definitive answer to a problem.


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Learn stuff - Turbo lag, how to beat it. Part I

The following is part 1 of an extract from a discussion paper put together by our engineers called "Turbo lag and the TMS solution". Part 2 follows in the next thread.


Hope this explains a few myths and misconceptions about this issue.



Engines with turbochargers invariably suffer from poorer throttle response than their normally aspirated counterparts, because of the time it takes for the turbo to build boost pressure when the throttle is opened. This is known as turbo lag, and this paper looks at the role a diverter valve plays in managing boost and turbo lag.


Definitions and explanations of terms used


It is important to clarify the following terms, as they are all too often used incorrectly or without proper understanding of their meaning.


Turbo lag:

A measure of the delay between when a turbocharged engine’s throttle is opened, and a “significant portion” of the maximum boost pressure is available.


Boost threshold:

Turbo lag should not be confused with boost threshold, which for simplicity’s sake we’ll say is the engine RPM above which the turbo is capable of producing a “significant portion” of its maximum boost pressure. For example, stamping the throttle open at 1500RPM and having to wait until 3000RPM for boost is not so much lag as it is a function of the engine and turbo system’s boost threshold. Once the engine is operating above the boost threshold however, then the delay when the throttle is opened can be assumed to be turbo lag.


Diverter valve, or “DV” (a.k.a. BOV, bypass, dump, recirculation & blow-off valve):A valve fitted to the intercooler piping between the turbo’s compressor and the engine’s throttle body. A signal hose typically connects the top of the valve to the intake manifold after the throttle.


In its simplest form, when the throttle is closed and boost is present in the intercooler, the BOV opens up (as a result of the signal hose switching from boost pressure to vacuum) and provides a relief path for air to escape.


The air can be vented straight out to atmosphere, or it can be recirculated back to the turbo’s intake. Generally, the term blow-off or dump valve is used to describe a valve that vents to atmosphere, whilst recirculating (or recirc), bypass or diverter valve is applied when the air is vented back to the turbo’s intake. In this article we are not concerned with what is done with the vented air.


TMS (Turbo Management Solution) valve:

A term used by GFB to describe a valve designed to enhance or supplement the basic factory DV functionality through the ability to adjust aspects of the valve’s operation critical to its performance. It may be either atmosphere or recirc venting, being differentiated from a typical DV by the fact that its operation is designed to maximise throttle response and boost for improved performance outcomes.



Typically defined as the process of speeding up the turbo until full boost is achieved.


Compressor Surge:

The nature and causes of compressor surge could cover an entire topic on their own, but for the purposes of this discussion, compressor surge is a condition that can occur when the throttle is closed on a turbo car where no DV is present. The pressurised air in the intercooler and pipes has nowhere to go and exits back out through the turbo’s compressor.


This causes a characteristic fluttering sound (that is often incorrectly mistaken for a certain type of aftermarket BOV or “wastegate chatter”), and also results in pressure spikes and pulses in the intercooler piping.


Compressor surge varies in severity – at low RPM/boost it is generally nothing to be concerned about, but in highly stressed turbos it has the potential to accelerate wear or even cause damage to the compressor. Additionally, the turbo will slow rapidly as air escapes back out through the compressor.


It is a task of a DV to provide a relief path for air to escape in order to prevent compressor surge.


Evolution of the blow-off valve


When turbos were in their infancy in motorsport, race engineers were always looking for ways to reduce lag, and the DV was one of the methods that was developed and is still in use today. By eliminating compressor surge, the turbo would not slow as rapidly when the throttle was lifted. Since turbos in that era were slow to spool up, maintaining shaft speed was a high priority.



During the early ‘90s, it became common practice for car manufacturers to fit DVs to their turbocharged models to prevent compressor surge. Here’s where the purpose of the DV changed however, and why there are still improvements to be found.


As far as car manufacturers are concerned, the process of preventing compressor surge has the benefit of eliminating the associated fluttering noise – strange noises, no matter how cool they may sound to an enthusiast, are generally to be avoided in production vehicles.


To achieve this task, factory fitted DVs tend to work on a common principle, which is to open the valve readily and fully, usually as soon as the manifold pressure drops into vacuum, effectively relieving the maximum amount of air regardless of whether it is necessary or not. This design is like a blanket solution to ensure compressor surge is completely eliminated.


Using this principle, it is common for a factory DV to be wide open at all times except when the engine is on boost. This effectively prevents the turbo from building any amount of boost pressure until the valve closes, to the detriment of throttle response. Fitting a larger intercooler can actually make the problem worse since the turbo has a much larger volume to fill when the throttle is opened.


Because factory DV have worked this way for so long, it is commonly believed that the principle behind them is to allow the turbo to “freewheel” whilst the throttle is shut, thus resulting in higher turbo RPM when the throttle is re-opened. Using this argument, it is also assumed that venting as much air as quickly as possible must surely aid the turbo in “freewheeling”, and therefore any aftermarket DV that replaces the factory one should flow more to be considered an improvement.


Whilst the above may seem a sound argument, there is a flaw. The idea that a turbo will “freewheel” is not correct – it is still pumping air and, in the absence of any exhaust gas energy (when the throttle is closed) to keep the turbine spinning, will still slow down rapidly. It must be mentioned that the same lightweight material technology used to improve turbo spool-up also means it will slow down more readily when the throttle closes.


Factory turbo cars are usually fitted with relatively small, lightweight and therefore fast-spooling turbos, and intercoolers with small volumes. These features alone play a significant role in reducing lag, and therefore an increase in lag from the DV is acceptable in order to ensure compressor surge never occurs.


So to summarize:

• Venting no air during throttle lift results in compressor surge where the turbo rapidly slows and potentially damaging pressure spikes and pulses result.

• Venting lots of air eliminates compressor surge, but increases the turbo system’s lag by evacuating all pressure from the intercooler piping.


The best throttle response and lag reduction is to be found somewhere in between the two extremes of no DV and a factory-style DV. This is the basis of GFB’s TMS principle.



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Learn stuff - Turbo lag, how to beat it. Part II

The following is part 2 of an extract from a discussion paper put together by our engineers called "Turbo lag and the TMS solution". Part 1 precedes the post.


TMS principle


By designing and adjusting a TMS valve to specifically suit the application, GFB has made it possible to reduce turbo lag during gearshifts or when modulating the throttle by holding some pressure in the intercooler and piping. By venting only enough air to prevent compressor surge, pressure in the intercooler can be maintained for as long as the turbo’s inertia will allow.


During a high RPM/full throttle gearshift for example, boost pressure in the intercooler with a factory valve will typically drop to zero (atmospheric) before the throttle is re-opened. With a GFB TMS however, the rate at which the boost pressure drops can be reduced, so that it is possible to have positive pressure in the intercooler when the throttle re-opens. This gives more power immediately as pressure is higher, and also reduces the time taken to reach maximum boost.




To demonstrate this, our test bed was a 1.8L turbo engine, with a large front-mount intercooler running 12psi peak boost. A data logger measured the throttle position and the manifold pressure.


Since this engine had no factory valve fitted, we used one from a Mitsubishi EVO IX to begin with for a base run.


A full boost 2nd-3rd gearshift was performed at 6000RPM and the results logged. From the data we can measure the actual lag time from when the throttle first starts to re-open, to when full boost is reached.


The factory valve was then replaced with a GFB TMS valve, and the spring was adjusted as firm as possible without incurring compressor surge and the test repeated.





The graph opposite (refer TMS_lag.jpg) shows manifold pressure and throttle position, starting from part-way through a gearshift, since it’s what happens when the throttle re-opens that we’re interested in.


Importantly, the manifold pressure trace shows two distinct zones: Initial pressure rise and turbo lag. The initial pressure rise is similar to the behaviour of a normally aspirated engine in that as the throttle opens, the manifold pressure rapidly rises from vacuum to the supply pressure (i.e. the pressure upstream of the throttle – on an NA engine this will usually be atmospheric pressure).


The pressure increase then abruptly reduces in rate as the manifold equalizes with the supply pressure, and the turbo is still spooling up. The rate of pressure increase during spool up is determined by the engine and turbo system itself, and the time taken doing so is defined as turbo lag.


This graph (refer TMS_boost.jpg) shows the two test runs overlaid, with the red line representing the manifold pressure with the GFB TMS fitted, and the black line is with the factory valve fitted.


It can be seen that with the GFB TMS, the initial manifold pressure rise doesn’t stop at zero (atmospheric) as it does with the factory valve, but continues to 2.5psi. This means that the supply pressure in the intercooler when the throttle is re-opened is 2.5psi rather than zero, and as a consequence, peak boost is achieved 0.21 seconds (30%) sooner than with the factory valve.


Peak numbers aside, it can be seen on the graph that higher boost pressure (and therefore engine power) is available during the entire spool up process.




A GFB TMS valve can help maintain boost pressure in the intercooler and pipes whenever the throttle is closed with the following benefits:

• Higher boost available upon re-opening the throttle for instant response

• More boost throughout the spool-up process (up to 40%)

• Shorter lag time (reduced by up to 30%)

• No compressor surge


When driving, a factory diverter valve will typically create a slight hesitation immediately upon re-opening the throttle, followed by a noticeable turbo lag. Fitting a GFB TMS will sharpen the response by eliminating the hesitation and then reducing time to peak boost. During a quarter mile race (where 3-4 gearshifts can occur) the time saving is quite significant, and on the circuit the power delivery upon corner exit is sharper and more responsive to the throttle opening.

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Learn stuff - Diverter valve leaks, how relevant to engine performance?

The following is an extract from "Diverter valve leaks & the effects on

engine performance and TMS solutions" discussion paper put together by our engineers.


It provides the results summary for tests conducted by our engineers to clarify this contentious issue and dispel the myths. A full copy of the paper is available as an attachment.




Test 1

Pressure_______________ 10 psi / 15 psi / 20 psi / 25 psi / 30 psi

Tight tolerance_________ 136 sec / 119 sec / 97 sec / 71 sec / 58 sec

Loose tolerance_________ 93 sec / 84 sec / 60 sec / 37 sec / 35 sec


A further two tests were performed at 30psi, both with pressure fed only to the top of the valve.


Test 2

Pressure___________ 30psi

Tight tolerance_____ 58 sec

Loose tolerance_____ 35 sec


The purpose of test 2 is to demonstrate that the leak is confined only to the amount of air that can leak through the piston-to-bore clearance, and that no leak occurs on the piston seat side. The importance of this will become apparent in the discussion to follow.




To work out the size of the leak in CFM, a few simple conversion formulae are applied:


Displacement volume = 500mL = 500cc


1 cubic foot = 28,316cc


Therefore displacement volume = 500 / 28,316 = 0.0176 cubic feet (CF)


To convert the test results to CFM:


CFM = (displacement volume (CF) / time (s)) x 60


So the leak results table from test 1 converted to CFM becomes:


Test 1

Pressure___________ 10 psi / 15 psi / 20 psi / 25 psi / 30 psi

Tight tolerance____ 0.0077 / 0.0089 / 0.0108 / 0.0149 / 0.0182

Loose tolerance___ 0.0114 / 0.0126 / 0.0176 / 0.0286 / 0.0302




If we take an example of a 2.5L engine at 7000RPM and 10psi boost, using an engine airflow calculator the maximum airflow is approximately 386 CFM.


With an airflow of 386 CFM into the engine, the GFB valve with the loose tolerance on leaks 0.00295% of the total airflow.


If the same engine were modified and running high boost, at 7000RPM and 25psi it will be flowing approximately 621 CFM, of which the loosest GFB valve would be leaking 0.00461% of the total airflow.


Other boost leak sources


As an interesting comparison, there is a device fitted to almost every turbo car that produces a leak far larger than the results shown above, but is often overlooked.


The boost control solenoid, or in fact any boost control device, must leak air in order to function. This leak is almost never noticed however, as a factory boost control solenoid is a closed system that won’t show a leak during a smoke test, nor will it even operate until the engine is running and on boost.


A common boost control solenoid valve was connected to the test apparatus and driven at 50% duty cycle. The actual duty cycle depends entirely on the turbo system and boost level, but 50% is a common figure for a mild boost increase on a stock car.


Test 3

Pressure______ 10 psi / 15 psi / 20 psi / 25 psi / 30 psi

Boost control__ 0.0960 / 0.151 / 0.176 / 0.211 / 0.264



From these results we can see that at 50% duty cycle the boost control solenoid is leaking up to 17 times more air than the GFB TMS valve.




From the results above, it can be seen that the expected leak from a GFB TMS valve represents such a tiny percentage of an engine’s total airflow that it would be impossible to for any engine performance measuring equipment (such as a dyno, boost or air/fuel ratio gauge) to detect the slightest change in boost level, torque or air/fuel ratio.


When boost leaks are suspected and tested for on a car, this is usually because a loss of boost pressure or performance has been noted. Smoke testing or pressure testing the intake tract will often discover a leak from the BOV, but the size of the leak is not taken into consideration by such testing. More can be read on this topic in the "Turbo lag" discussion paper available from GFB and reference sites


What if…?


As surely as the results of this test will answer many questions and put things into perspective, it will also undoubtedly raise a few more questions such as “What if my valve is old and worn”, or “what if my valve is leaking more than the tests results above”?


The results from test 2 show that the leak measured is all coming from the top chamber of the GFB TMS valve. The air source to the top chamber is via a nipple connected to the intake manifold, usually using a length of 3/16” or similar vacuum hose.


It makes sense than, that the largest leak possible from the top chamber of a TMS valve is limited by the vacuum hose – i.e. the vacuum hose will only flow up to a certain amount at a given pressure, even if the leak at the valve were larger. Even though a leak this large would render the GFB valve ineffective and would be immediately noticeable, it’s worth performing the test for perspective.


The pressure regulator was connected directly to the test apparatus using a 400mm length of 3/16” vacuum hose.


The vacuum hose displaced the 500cc of water in an average of 0.21 seconds at 10psi, which was difficult to measure accurately but is still indicative nonetheless. Since the time was so rapid and the test equipment not big enough to test a larger volume of water, higher boost pressures were not tested.


So a 400mm length of 3/16” vacuum hose at 10psi leaks at a rate of approximately 5 CFM. Again, using the example of the 2L engine at 10psi, this represents 1.29% of maximum airflow.


So although this amount of air loss is a grossly exaggerated example and would not actually occur, it is still not large enough to be noticeable when driving, and would be difficult if not impossible to measure at the engine.


Further testing


As stated initially, any amount of air escaping from a TMS valve can be correctly referred to as a leak. However, from the results above it can be safely concluded that even the theoretical maximum leak at the top of a GFB TMS valve will not cause a drop in boost pressure or engine power large enough to be noticeable, let alone reliably measurable.


So the top of the valve can be eliminated as a potential source of power loss. The only way it can leak enough air to cause a power loss is if the piston were to actually open under boost.


The test procedure used in test 1 simulates WOT conditions up to 30psi, and test 2 proves that no leak from the bottom of the valve occurs under these conditions.


It is often thought that BOVs will eventually begin to open given enough pressure. This is true in many factory fitted valves as they are actually designed to do this to limit boost pressure. However, a GFB valve’s design means that the opposing pressures on the piston cancel each other out, meaning that no amount of pressure at WOT will open the valve.


In fact, because the diameter of the Mach 1’s piston seat is slightly smaller than the outside diameter of the piston, there is a slightly larger area on top for the pressure to push down than there is on the bottom pushing up. Therefore the spring pre-load is irrelevant – it is not possible for the GFB valve to open under boost.


As final proof of this, test 4 was a repeat of test 1 at 30psi with the spring removed entirely from the Mach 1 valve. The piston did not open and the results were again exactly the same.





So technically, GFB TMS valves can be said to “leak”. Importantly however, when put into context the following has been demonstrated:


• on a 2L engine at 10psi the leak would represent 0.00295% of the maximum engine airflow


• the largest expected leak at 30psi is 0.0302 CFM


• all boost control devices also leak air, and at an average setting of 50% duty cycle a boost control solenoid leaks up to 17 times more air than a GFB TMS valve


• The amount of air lost to these leaks is so small that it would not be possible to detect or measure a performance loss as a result


• It is a simple matter to prevent the leak altogether with the use of an O-ring, but we choose not to (even though it would be cheaper) for valve performance reasons


To summarise, a GFB TMS valve shown to leak as per the tests above indicate that it is not large enough to cause a performance drop or even be measurable at the engine.


The only way a GFB valve can cause a loss of air large enough to be felt as a drop in performance, or measured as a lower boost reading, power drop or AFR change would be for the valve to open under boost.


However, as all of the tests demonstrate (in particular test 4), the GFB valve remains shut regardless of the boost pressure or the spring pre-load, so the possibility of a GFB TMS opening under boost pressure can also be safely eliminated as a source of power loss.

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Learn stuff - Lightweight & underdrive pulleys add wheel power

A potentially controversial question that our guys have decided to tackle with some useful testing and results that will surprise some.


The following is an extract summary of the whole GFB Discussion Paper on the subject titled "Lightweight Under-Drive Pulley Kit Performance Testing" which is available for download below.




GFB’s range of Lightweight Under-Drive Pulley Kits are designed to improve

acceleration by reduce the rotational mass (inertia) on the crankshaft as well as parasitic loads from the driven accessories.


The pulleys are manufactured from 6061 T6 billet aluminium on the latest precision CNC machines, and typically save up to 2.5kg of weight over the factory pulleys.


Under-driving is achieved through the use of a smaller diameter crank pulley, which reduces the amount of drag from the accessories, particularly at high RPM. Note that GFB take care to ensure that accessory performance is not affected.




The results shown above (refer article) are actually the pair of passes that show the smallest gains. Other tests could have been combined to show larger and better results, but this pair has the closest-matched conditions and represents the most honest results.


Gains or losses resulting from variations in boost pressure or ECU-related

adjustments are usually seen only in part of the rev range, as a bump or a dip, or a different shaped power/torque curve. The pair of runs chosen for comparison however, are very similar in shape, indicating that the engine is operating in as close to identical conditions as could be expected – the resulting improvement can therefore be said to be from the installation of the pulley kit only and not from external variations.




It can be seen in these results that fitting a GFB Lightweight Under-Drive Pulley Kit does in fact produce an honest, measurable and repeatable improvement in acceleration throughout the entire engine rev range.


The tests revealed that the minimum recorded improvements after replacing the factory pulleys with the GFB Lightweight Under-Drive Pulleys were:


· 3% faster (0.1 seconds) from 30-90 km/h

· 4% higher average G-Force (0.02g) from 30-90 km/h

· 7.2% higher G-Force (0.03g) at 40km/h (2800 RPM)

· 5kW peak power increase (173.7kW to 178.7kW)


The results charts all tell the same story, which is a consistent improvement in GForce throughout the 30-90 km/h test, which results in more power and consequently a faster time to 90km/h.


Importantly (especially on turbo engines), the G-Force improvement off-boost and during spool-up are enhanced by a much larger percentage because of the limited engine torque available at low RPM. This gives the car what can be best described as a noticeably “livelier” feel in the lower rev range, and a greater willingness to rev during acceleration.


This is unlike many other engine modifications that improve power through increased airflow, since such gains are usually only available for a portion of the rev range, and often the ECU is required to be re-tuned to suit. The GFB pulleys however offer their benefits independently of the engine tuning, and regardless of how much power it makes.

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Learn stuff - The truth about compressor surge. Part I

The following is an extract from a discussion paper put together by our engineers called "The truth about Compressor Surge -Part 1. Part 2 follows in the next thread.


Hopefully this should placate some fears and provide a reality check for turbo enthusiasts.


What’s in a name?


First of all, let’s get the terminology straight. Compressor surge often goes by many other names (usually in reference to the distinct sound made when surge occurs) such as turkey, pigeon, dove, dose, wastegate chatter, sequential BOV and undoubtedly many others I am not aware of or have yet to be invented!


Whilst I can’t argue that compressor surge noise does share a similarity to the noise made by certain avian species, the remainder of these names are either misleading or flat out incorrect. For example:




I have no idea how this name was derived, but it is most commonly associated with the Holden VL Turbo, since no factory diverter valve is fitted and it compressor surges readily. Going further, a “Dose Pipe” typically refers to any type of pod filter arrangement that makes the fluttering sound louder than with a factory airbox, therefore a “Dose Pipe” is not a device that creates the fluttering sound, but rather a means of making it louder.


Wastegate chatter:


This is a term that has found its way into common turbo vocabulary, and is often incorrectly used to describe compressor surge. Whilst it is possible for a wastegate to make a small clattering or rattling noise if it is just at the point of opening (exhaust pulses can make it vibrate on its seat before it lifts clear), it has nothing to do with compressor surge.


Sequential BOV:


Compressor surge is often thought to be the sound made by a specific type of “sequential” BOV. In actual fact, no BOV makes the compressor surge fluttering sound, the noise actually comes from the turbo. This certain “sequential” BOV however is often responsible for causing compressor surge, which is how this misconception propagates.



What is compressor surge?


To the driver, compressor surge is apparent as a fluttering or repeated “choofing” sound, typically when closing the throttle.


Here's a good YouTube link showing classic compressor surge caused by the absence of a BOV [ame=



To the turbo, compressor surge is a condition that occurs whenever turbo boost pressure is high and airflow is low.





With an understanding of the mechanics of compressor surge and the conditions that cause it, it can be seen that in the majority of cases it will not cause significant turbo wear or damage, however there is the potential if it is severe enough.


There is however good reason for preventing compressor surge, and the fitment of a diverter valve that addresses the issues discussed is the best way to do so. The GFB TMS valve's are specifically designed as Turbo Management Solution.



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  • 3 weeks later...

so, what has been diagnosed in my car as a wastegate flutter might actually be BPV flutter?


Stock BPV and stock wastegate in my Perrin TMIC, Gimmick afta maf and inlet tube, CoBB lite pulley, COBB UP, CNT catted DP to X02 exhaust, K&N panel filter all on stock VF-40. 19psi tune.


** By the way, ran zero re-circ for a week on my 5EAT and it was an utter disaster at anything other than full throttle. So much stutter it was crazy at partial throttle and a huge jerking motion when letting off the throttle. That's what I get for destroying the re-circ hose when I installed the new turbo inlet tube.**


Summary: do I need an AVO 15psi solid boost actuator to solve current stutter or an upgraded BPV? Or both?



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haha are you australian mate?? i see you mention a VL turbo in the spiel above but i know they are an Australia only car?


No, the info comes from GFB Pete. he is Australian as GFB is an Australian company.



so, what has been diagnosed in my car as a wastegate flutter might actually be BPV flutter?


Stock BPV and stock wastegate in my Perrin TMIC, Gimmick afta maf and inlet tube, CoBB lite pulley, COBB UP, CNT catted DP to X02 exhaust, K&N panel filter all on stock VF-40. 19psi tune.


** By the way, ran zero re-circ for a week on my 5EAT and it was an utter disaster at anything other than full throttle. So much stutter it was crazy at partial throttle and a huge jerking motion when letting off the throttle. That's what I get for destroying the re-circ hose when I installed the new turbo inlet tube.**


Summary: do I need an AVO 15psi solid boost actuator to solve current stutter or an upgraded BPV? Or both?




Well, 19psi is high for a VF-40. Honestly. You are right on the edge of that turbo.


Stock BPV should only be run 100% recirc. Stock BPV should hold your boost and work for your app. But it could be giving out too.


Upgraded wastegate actuator might also help. Not sure. It is easier if I saw the car myself, or at least some data.

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so what i'm getting from this thread is that a GFB bov will solve my problems, because other than that I am not sure how else to get rid of the surge. I really dont need my turbo blowing up from surge and its bad even at any boost. I thought my 300 dollar BOV would have been good but i guess not compared to GFB.
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No. Take out the GFB and just insert "your brand here". The SSQ works slightly differently, but principle is the same.


Like I said in my original post, there is propaganda here. But plenty of great info too. Look past the propaganda and take in the data and instructions.

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No. Take out the GFB and just insert "your brand here". The SSQ works slightly differently, but principle is the same.


Like I said in my original post, there is propaganda here. But plenty of great info too. Look past the propaganda and take in the data and instructions.


ok so it seems to me like the pressure from the turbo is there but the volume is not there... so would that mean the vf40 isn't big enough for this set up? Also I am going to recirculate my BOV and see if that helps.

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What set up do you have? Sorry if you already posted. If you have a FMIC, chances are teh VF-40 is struggling. You could try a smaller restrictor pill (if not using an EBCS). Maybe a 0.8 even. It will make the turbo work harder, but could get you the charge air you need.


We have seen VF-40's have issues with the pressure drop from FMIC's. It takes a lot of work to get the boost back after sometimes. The VF-40 is undersized for most FMIC applications (IMHO).

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