What does BOV do?
Imagine that you are accelerating or speeding. You're under boost, the turbo is fully spooled, and life is good - then you shift. That means your foot comes off the gas - and the throttle plate slams shut. Suddenly, instead of flowing in a continuous stream through the engine, the intake air smacks into the closed throttle plate. The turbo, which is still spinning and producing boost because if it's rotational inertia keeps producing pressure, and the intake stream, caught in between a rock and a hard place, jumps in pressure. In fact, you get a high-pressure shockwave that travels from the throttle plate back to the compressor vanes, that once it gets there, is a little like poking a stick into the spokes of a bike wheel.
The repeated shock is hard on the compressor vanes and the shaft bearings, and in any case acts like a brake, slowing the turbo, and requiring it to be spooled up again.
The BOV sits in between the turbo and the throttle plate, and if it detects the shockwave created by a shift, vents into the atmosphere.
Why do you need a Blow-off Valve (BOV)?
- To eliminate compressor surge when changing gears. Excessive backpressure is created when the throttle is closed during gear changes or deceleration, causing turbo cavitation. This is detrimental to the life of your turbocharger; however fitting a BOV will substantially improve this problem.
- By installing a BOV you will significantly reduce turbo spool up time. You will notice a substantial improvement in response from your turbo between gearshifts. Boost will come on earlier giving an increase in torque at lower RPM (which means Reducing Turbo LAG)
- Nice sound … Each time you let off your foot of the gas, after pushing it into high RPM’s, the BOV will make a nice PSSSSSSSSSSSSSSSST. (Fast&Furious )
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.
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asta-i de la profesoru ( teacher) citire...poate sa puna ce vrea el... lu sgmu cred ca era speciala pentru motor de dacie cu set, chiulasa si restul de alpine..a lui era de munte
