I don't understand why people fit BOVs. My understanding is that is it a longevity measure not a performance measure. As in it reduces compressor stall and hence reduces turbo bearing wear when closing the throttle when full throttle. It produces 0 (zero) measurable increase in HP.

What am I missing?

 

Extra boost has to go somewhere,safety measure.

Ever wonder why rally cars make that sound you hear when you open a bottle of pop? Well, maybe not exactly like that but more or less similar to the sound of air being released (air, not gas!). You usually hear this when you let off the throttle, sorta sounds like a woosh!. In effect, what you are hearing is the sound of built up boost pressure being released from the intake system. The reason for this is that the turbocharger will keep spinning even after you let off the gas. So as you close the throttle plate, allot of pressure builds up in the intake system. This becomes problematic in that this excess pressure can cause the turbines to seize. Ultimately, this would destroy the turbo unit. For this reason, we incorporate BOV's, bypass or diverter valves. These mechanism work because on the other side of the throttle plate, vacuum gets built up in the intake manifold. Blow off valves, diverter and bypass valves all work by detecting this vacuum. Having done so, they use this vacuum to mechanically open a valve in order to relieve unnecessary boost from the other side of the throttle plate.

Forge Diverter/Dump Valve Now let us differentiate BOV's, diverter and bypass valves. First, a blow off valve (seen top right) is common to high performance applications in that it provides the least bit of compromise. A BOV essentially releases this pressure straight out into the atmosphere. Quite often you will find that these units take on particular shapes, making them resemble musical instruments. I guess some people out there really like to flaunt their gadgets. Just wait ti'll you see how much they impress the ol' 5-0. Aside from this legal dilemma, the second problem you encounter with BOV's is that the mass air flow sensor will cause the engine management system to "think" that the air will go into the engine and in turn will release an appropriate amount of fuel for it. However, a BOV system will vent this air out before it gets to the injectors causing your mixture to run lean. In turn, this will result in unburned fuel to escape into the exhaust system. This is why you often see flames and hear those "pop's" in rally cars. It is unburned fuel exploding in the exhaust system. Doesn't take a genius to figure out that this is not the smartest application for the day to day car. For this reason, reasonable tuners will usually employ a bypass or a diverter valve (see left). These units essentially redirect this pressure back behind the compressor causing the net flow of air to remain constant. This in turn slows the turbine down gradually and allows the air flow sensor to work appropriately.

 

They need to be upgraded because the spring inside must be operable for higher boost levels. If the boost is too high for the bov, the bov will leak, and therefore create a vacuum leak.

 

The stock BOV begins leaking at around 25psi so if you run high boost (E85, race gas, meth) then you can achieve better boost hold at high rpms with a replacement.

The other reason that people replace them is for the 'cool' noises that they make. Aftermarket BOVs will often have an option to VTA (vent to atmosphere), either completely or partially. Our BOVs were designed for recirculation so installing a VTA can cause tuning problems. Simply getting rid of the stock airbox makes the sound very audible. I recently went to an intake and I must admit that it is kind of cool, even with the stock BOV.

 

This forum is nuts at times...you go to reply and 3-4 people post first. lol.

 

So when I said they produce 0 extra HP I was right, right?

Rally cars make that weird squeaking sound due to the induction charge escaping back through the turbo. Wastegate chatter is a myth. That would imply that no rally car uses vent-to-atmo BOV.

 

yep BOV is not there to make horsepower, just a necessity when you run a turbo car.

 

It doesn't make HP, but when the BOV starts to leak, you will lose hp. So yes you are right.

 

Huh, well that's me schooled. I didn't know the the Evo had a BOV.

 

You don't NEED a bov but it's a good idea, just like you don't need to use a condom with a hooker but it's a real good idea.

 

Just making sure... this isn't sarcasm, right?

The BOV is located just behind the MAF sensor between the UICP and turbo inlet tubing.

If you don't have a BOV (recirc valve in the Evo's case), then where's the extra pressure go when the throttle plate swings shut? The turbo is still spinning, wanting to push air in one direction, this air can either push back or it can be diverted to the turbo's suction by use of a diverting/recirculating valve, or the "BOV."

You do gain HP with a better (stronger spring, etc) BOV by means of what others have already mentioned, holding boost better... the stock BOV will leak boost from the outlet of the turbo back into the inlet if it's seeing 30 psi (others have seen the same at 26-27 psi, upgrading to a stronger spring only the stock turbo WILL still taper but the usual reports I've seen show that the taper isn't as significant, aka there's more area under the boost curve).

 

Well, technically, we have a "Bypass Valve". He might just mean we don't have, stock, the ricer-riffic "Blowoff Valve" with VTA.

I don't see how he could think we don't have SOME method of getting rid of excess pressure, be it a Bypass Valve, Blowoff Valve, whatever.

 

Bypass/recirc, tomato/toe-mah-toe

They're the same thing, different wording...

 

I hear the OEM recirc valve is pretty good and I actually havnt heard of one that hasnt caused problems was wondering if anyone had any suggestions as to what has had the most success for the X thanks

 

http://www.youtube.com/watch?v=50QLfmRMeP4