Calhoun

I got this information from the i-Club forum. How does this information pertain to our cars?

Here is the meat of the information. The whole thread is below.

Andrew: It'd be good to explain exactly why, perhaps by detailing what a BOV does and how proximity to the throttle body enhances its performance...newbies won't know what compressor surge is, usually, even if they've heard of a BOV.

What a Blow off Valve (BOV) does is vents the air coming from the turbo to the engine. The reason it does this is becuase once you let off the gas (close the throttle plate) the air that was on its way to the engine hits the plate and goes BACK to the turbo. this causes major strain on the turbo and is known as compressor surge. (I believe, if i remember correctly, its a fluttering/pigeon/butterfly type sound). You can avoid this by putting your BOV closest to the throttle plate as possible (hence the idea of putting it on the intercooler itself). Some things you need to watch for, you need to have the proper spring inside the BOV or else it might not vent the built up pressure properly.


Let me see if I can expand on this a bit to explain why things work the way they do.

The reason the BOV is most effective near the throttle body is because that is where the compression wave is initiated when the throttle snaps shut when the driver lets off the gas to shift. By placing the BOV near the throttle body, the purpose of which is to release that excess pressure of the compression wave, the excess pressure is ejected as early as possible.

The reason proper spring tension in the BOV is important is that the spring tension determines the pressure at which the BOV begins ejecting air from the intake tract. If the spring pressure is too high, the BOV will fail to open, or fail to open enough, and compressor surge will continue on its way back to the compressor wheel in the turbo.

Now, what's so bad about compressor surge? If your turbo has a 360 degree thrust bearing, nothing, according to a certain UK rally driver and mechanic. However, most turbos do not have that feature, so the compressor surge can unbalance the compressor wheel, potentially overloading the shaft. Snap the shaft, dead turbo.

There are 2 primary types of BOVs: atmospheric (also called dump valves) and recirculating. An atmospheric BOV vents the air directly out (making that "pfffffttt!" sound that some people love so much). A recirculating BOV has a tube running from it to a point on the intake tract _before_ the turbo. A recirculating BOV is significantly less noisy than an atmospheric valve and injecting the excess air back into the intake tract can improve performance in cars with certain air flow sensors (MAS? MAF? help me out?).<ul><li><a href="http://www.i-club.com/forums/showthread.php?threadid=115053&referrerid=0">The whole thread.</a></li></ul>

Andy Hedin

Stick with a BPV (diverter valve) for best performance on the 2.7T

4kq

Lesssee, this has only been discussed for years.

ChaoticS4

<center><img src="http://www.funkadelic.org/albums/chaotic/aac.jpg"></center><p>Hybrids. So far so good. They are working properly with no fault codes.

I am communicating with the vendors to customize some for the S4 so that there are no clearance problems with the valves.