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What does it do:
The purpose of a blow off valve or compressor bypass valve is to prevent large pressure spikes in the intake pipes when
the throttle plate is closed while boosting, preventing the turbo from surging. The BOV sits between the turbo and the throttle
body and has a vacuum line that is attached to both the other side of the BOV valve and the intake manifold. When you are
boosting the pressure on both sides of the valve is essentially equal because the pressure in the intake manifold is the same
as in the intake pipes. When you close the throttle plate while boosting all that air slams into the plate and pressure spikes.
At the same time the intake manifold pressure drops to a vacuum, and thus so does the pressure on the other side of the valve.
The result is the valve opens wide to allow the pressure to vent out. Also included in a good BOV is some form of spring
to aid in keeping the valve closed during modulating part throttle situations so that the BOV doesn't open and cause drivability
problems.
Why upgrade:
The stock bypass valve in the 2g's is a very weak. The result is that it leaks at boost levels above about 12 psi. The
fix then, obviously, is to install a BOV that doesn't leak until higher boost levels. If you don't, anything you do to increase
boost will be fruitless. You even run the risk of damaging your turbo by stressing it trying to get higher boost.
Here is what happens, if you do any mods that increase boost (intake, exhaust, modding stock boost controller, installing
after market boost controller, installing larger turbo etc) before upgrading the BOV. Air is sucked into turbo, compressed,
sent to the intercooler, then past the BOV, to the throttle body, into the intake manifold, and into the pistons. The BOV
vents into the intake system BEFORE the turbo which is effectively like venting it outside in this case because it is allowed
to expand back to atmospheric pressure.
Now as the turbo spins faster to increase the pressure in the intake pipes beyond the leak point of the stock BOV, pressurized
air in the intake pipes leaks past the BOV. The pressure in the intake pipes thus can't reach the point where the boost controller
will keep the turbo from spinning faster and limit boost. So the turbo spins even faster to flow more air to try and get
pressure up to the point where the boost controller will start regulating the turbo's flow, but air is still leaking out of
the BOV and in fact as pressure increases the BOV opens more and leaks even more. The end result to this cycle is that either:
a) The turbo will eventually spin fast enough to overcome the leak and get the boost pressure up. At this point the turbo
is running way outside its efficiency curve and is doing little else than pushing very hot air and working hard to do it.
Hot air knocks, detonates, and contains less air mass than the same pressure/volume of air at a cooler temp (remember your
physics PV=nRT) and you'll be pissed that your car is slower than a Honda Insight
b) The turbo never really can flow enough air to overcome the leaking BOV, all the steps in a) occur, and your turbo dies
an early death.
Now let me repeat that again:
BEFORE YOU DO ANYTHING WHICH CAN INCREASE BOOST LEVELS YOU MUST INSTALL AN UPGRADED BLOW OFF VALVE.
What to upgrade to:
There are several different alternatives which may be considered. The most popular is to utilize the stock pre-95 BOV
(aka 1g BOV). This unit is usually good for up to 22 psi or higher when crushed. To use it you will need either an adaptor
to install it on the stock upper intercooler pipe BOV mount, or you will need a new upper IC pipe with a 1g BOV flange. If
you are planning on getting a new upper IC pipe (not including the radiator hose mod mentioned in the Upper IC section) then
you should get it at the same time you buy the 1g BOV. Otherwise if you get the BOV now you will need an adaptor which will
be useless with new upper IC pipe.
Replacing the stock Blow Off Valve (BOV)/Compressor Bypass Valve (CBV) with a better one is really a no brainer . . .
once you've done it. To be honest there really is no need to have a BOV FAQ, however like myself, many new members to Club
DSM are nervous about doing mods without getting a handle on what is involved. With this in mind I have decided to write this
FAQ so that you newbies can get a feel for what is involved, and to point out a few of the minor yet annoying things that
can go wrong and cause you to curse the day you ever thought about modding your car.
Here it goes, I'm assuming that you already have the Dave Brode UIC Pipe, without it, you can't install the BOV.
First thing you'll need to do is remove your stock BOV:
* Remove the hose running from the top of the BOV to the throttle body
* Loosen the clamp holding the base of the BOV to the Upper IC Pipe
* Loosen the clamp holding the BOV outlet and return hose together.
* Now slip the BOV out and set it aside.
Now all you need to do is put your new BOV on. I'm assuming you already have a Dave Brode UIC Pipe, otherwise you did all
this work for nothing.
* Install the BOV into the Upper IC Pipe.
* Connect the hose connecting the BOV outlet to the return hose. Don't forget the clamp :)
* Connect the vacuum line from the throttle body to the port at the top of the BOV
* As mentioned earlier, the vacuum port is angled differently than the stock one. This is OK and you should be able
to pull the hose that connects to it to make it reach. Make sure that you don't cause it to kink or wise you'll have problems.
AGAIN, THESE DIRECTIONS ARE FOR THE INSTALLATION OF THE BOV ON THE DAVE BRODE UIC PIPE, NOT THE STOCK UIC PIPE.
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