Blow-off valve functioning and installation
Today in the Atomic Labs we’ll be going through the installation of an atmospheric blow-off valve (BOV). So what, exactly, is this valve thingy and what does it do?
On a turbocharged engine, when the turbo spools up it pressurises the intake system - from the compressor, past the throttle body and through the inlet manifold, forcing more air into the combustion chambers. This is how a turbo produces more power, but I’ll go into the specifics in a later article.
When you take your foot off the accelerator pedal, the throttle body closes - the stream of pressurised air created by the turbocharger is now cut off from the inlet manifold. The only way it can escape is back up the intake stream, surging into the turbo compressor. This reversal of intake charge pulse can put additional strain on the turbo components, as well as reducing the compressor wheel’s rotational velocity. This means that the turbo will take longer to spool up when the throttle is opened again.
A valve placed before the throttle body cures this problem by allowing the pressurised charge to escape the intake system, keeping the compressor spinning and reducing turbo lag. I won’t go into the specific details of how the valve opens when needed, but if you want to know more then email me. Many turbocharged cars come with such a valve from the factory, but they are recirculator valves rather than BOVs - the compressed air that escapes is plumbed back into the intake before the turbo. BOVs vent this air to the atmosphere instead.
So why install an aftermarket BOV? They provide improved throttle response and can hold much higher boost levels compared to the factory-fitted version, and as a bonus because they vent to the atmosphere they provide that sought-after “psssht” sound when shifting gear or free-revving. Plus they look damn cool, if that’s your thing.
The only drawback to this type of valve occurs on cars with air flow meters. Because the engine is venting air that has already been measured by the MAF, the car’s computer adds a little too much fuel for the air that is left, resulting in a rich mixture for a short period of time. This can sometimes result in backfiring, but results vary from one car to the next. This can usually be addressed by proper tuning. Again, I won’t go into details, cos it’s not necessary at this point.
In most cases, a BOV is fitted to the car’s factory boost hoses, or is attached to a weld-on adaptor plate for upgraded intercooler systems. The choice depends on your car’s level of tune, and what your needs are.
Okay, onto the installation. The BOV in question is a Stealth FX, made by Go Fast Bits in Australia. This is a top of the line model, made from billet aluminium, and is built like a tank. I chose one with a mounting flange specifically for the Nissan S14 200SX, eliminating the need to adapt anything for fitment. While this can save a bit of hassle, a generic-fit BOV is nonetheless fairly easy to install.
[Disclaimer: we accept no responsibility for you breaking your BOV, your car, or yourself, scaring your cat, spilling your beer, or any damage incurred by following this procedure. If you are in any doubt whatsoever, please have your installation carried out by a professional.]
First off, the battery is removed, allowing easy access to the existing factory recirculating valve. (This will vary from car to car). Here you can see it in place. The large lower hose is connected to the boost piping, and the upper one is joined to the intake system just after the airflow meter. The small hose at the top of the valve is a vacuum line, which is connected to the intake after the throttle body. This provides pressure differential required for the valve to actuate.
(Please excuse the crusty battery tray, it’s the only part that doesn’t get cleaned regularly )
The factory valve has been removed in the next picture. Hose clamps were loosened and simply slipped off, but I recommend that you use a screwdriver to push the vacuum line off - it’s on very tight, and simply tugging on it might cause damage. Much squirming and cursing was involved - the space is tight, and some of the screws are damn tricky to reach. You can see the mounting points to the left of the hose.
This is the new BOV attached to the factory flange, a direct snug fit with minimum hassle thanks to the vehicle-specific mount. The silver trumpet thingy is the valve’s atmospheric outlet, and the silver cap can be twisted to adjust the internal spring setting.
Here we have two different outlet trumpets. On the left is the regular one, on the right is a variant that alters the noise produced by the valve when venting. I decided to go with the longer one after comparing the sounds.
A couple of screws and hose clamps later, the new BOV is in place. It’s a tight squeeze to get it under the fuse box, but once in place it sits perfectly.
Once installed, some minor adjustments have to be made to the valve spring. This affects how quickly the valve actuates and how long it stays open for. If left open too long, the engine will stumble, but if if it shuts too quickly not all of the pressurised intake charge will escape. A couple of short drives with some adjustment in between is enough to get things sorted.
Total time to install: 20 minutes and one beer. This will take a bit longer if you are using a generic-fit BOV, but nothing hectic. A six-pack at the most. Overall, this is an effective, rewarding, yet easy to accomplish modification. BOVs range from a few hundred rand for a no-name-brand hunk o’ junk, to a few thousand for big-name high-performance models, with many in between. As always, you get what you pay for - don’t spend too much if you don’t need it, but neither should you skimp on quality. Find the compromise that suits you and your car.
A quick note: if you drive an stock, unmodified turbo car, the benefits from fitting an aftermarket BOV are small, and on factory boost settings you can expect a small amount of engine bog between gears. However, if you’re seeking vroooooompssshhhtness, it’s the price you pay.
Before anyone asks: *no*, there is no point in fitting a BOV to a car without a turbocharger
. And yes, I have been asked this before.
A word of advice to anyone planning on getting hands-on with their car: buy some latex gloves, the kind doctors and dentists use. They’re dirt cheap, and save you the hassle of cleaning your hands - just toss em away when you’re done. They also provide the added bonus of preventing minor scrapes and cuts. Unless you’re uber-careful / experienced, you *will* bleed a little every time you get under the hood.
Until next time, may your boost be high and your beer be cold =).
Subject: Blow Off Valve (also know as 'Dump Valve') - Function and Installation 
Tue Oct 13, 2009 2:09 pm
