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#2: 12-02-2010, 07:49 AM
 m sprank
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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 , 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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