I am more familiar with S/C than with turbos. Can anyone tell me the benefit of having a BOV. Also, what could happen if you don't have one. As far as I know the BOV is used for relief of exhaust gas during deceleration? Am I on the right track?

 

A "Waste Gate" is placed is used to bypass exhaust gas around the "hot" side of a turbo. They are not used on SCs. The WG opens (bypasses exhaust around the impeller) to regulate the boost that the compressor (cold side) can produce. Basically once the turbo spools up to speed it acts like pressure regulator (ie clips the boost to xx psi). Witout a WG the system nad the typical turbo would become unstable and destroy the enginevery quickly.

A "BOV" is placed in the inlet tract to releive the instant back surge that occurs when the throttle plate is closed abrubtly (ie. when shifting) . Without a BOV, the Turbo or SC will be pushing boost towards the throttle body with no ideal path of escape (so back thru the compressor it goes). A BOV nearly eliminates this unwanted surge.

At higher boost levels this surge can damage the turbo and is noisy. At light boost levels a BOV is not essential. At higher levels it is a must. BOVS can vent pressure either into the atmosphere or back into the intake trac pre compressor (or both). the former is the "woosh" sound you typically hear on turbo cars.

If you want to know more, Corkey Bells book "Maximum boost" is a good read

 

When you lift off the throttle, due to gear change or just decelleration, the throttle plate shuts. Now, if you are in a state of positive boost pressure at that point, the positive pressure is trapped between the discharge of the turbo/SC and the closed throttle plate. If this pressure is high enough, it can cause surge in the turbine (where you get a backflow the wrong way through the unit), this can lead to extra stress on the bearings of the turbine, be it a turbo or supercharger.
A BOV is mounted between the discharge of the turbine and the throttle plate. When you come off the loud pedal, vacuum pressure operates the BOV, which is really just a fancy daiphragm relief valve. This either vents back upstream of the compressor (which is what most factory turbo cars etc have) or the aftermarket types which dump to atmos, and make the cool wooosh.
This prevents compressor stall/surge and allows the turbo to keep spinning in a more uniform manner.

Hope this helps some.

 

Aha, someone types quicker than me!

 

Thanks for the info. Do you know at what psi a blow off valve should be put it? Most TT kits seem to be from 5-7 psi. Is a BOV needed? Would adding a BOV at a low boost level harm anything?

 

I don't know about the TT kits, but the superchargers come with a bypass valve, which is the same thing, except it vents back to the intake of the supercharger. It doesn't make the cool sound, but it does it's job.

About putting a BOV on a supercharger setup that vents to atmosphere. I don't know, it should work fine except for one thing which is stopping me from doing it. I'm worried stomping on the pedal at low RPM with a supercharger. I don't know if the SC makes enough pressure to supply the intake with enough air to stop it from sucking through the BOV. If it sucks air through a BOV that is open to the atmosphere, you'll be sucking in unfiltered air.

 

Just to clear up my first post. I was asking about a BOV in regards to a TT setup. I had a SC on my old mustang and did not have to worry about such things. I did not mean to ask about a BOV with a SC. I can see now that my initial posting was unclear. Sorry.

 

Again, I do not agree with the statement that a bov is not essential for a turbo system running 'low boost'. Low boost is 1 psi. Boost it boost, pressure is pressure, that volume of air needs to go somewhere. The first time I took my Z out to test it with the greddy kit I did not have the bov flange welded in yet. The surge was really bad. Sure it would be worse with more boost, but it was still unacceptable.

I am using a stock bov off the mitsu 3000gt. You can find them for ~$20. Nothing fancy, it works great for anything under 10 psi. I had it laying around so I used that. It works great.

 

I have one for an obvious reason, and the most important reason of all.............it sounds really really cool. :-)

 

Is there really a difference between BOV's from different companies like Greddy or HKS or any other company?

 

The BOV is a very simple and elegant device. There are minor differences between brands, but most work exactly the same way. Some are designed for hi boost applications, and some for lower boost applications...are sequential. I don't think you can go wrong with either Greddy, HKS, or APEXi.

I agree with ETX above. I feel a BOV is very benificial for all turbo'ed cars...even if only boosting 5-6psi. The turbine is spinning at 50-100K RPM, and I wouldn't want anything abruptly attempting to reverse the natural inertia of those spinning blades. I think Greddy would have included it in their kit, but it would never pass CARB exemption if they included it...hence...it was omitted from the kit. CARB exemption is pending on the Greddy kit.

Here is a pic of mine installed on the Greddy Kit.

http://www.savepic.com/is.php?i=3744&img=IMG_1867.jpg

 

I agree with the poster as to the "desirability" for a BOV at even low bost PSI. It is ;howver, quite possible to make a reliable setup without one at <~5psi. Any setup ,that I would consider , if it didn't come with one , would quickley get one.

If a BOV is vented back into the inlet trac (pre-compressor) it is easier to get a CARB EO. If it is vented into the atmosphere it is more difficult . Must prove that there is "no way" crankcase breather gas can escape the BOV sytem (crankcase typically vents into the pre-comressor inlet, post ari filter , pre TB. Crankcase fumes are "noxious" and venting into the atmosphere is very non-CARB-friendly.

Jesse, as for a "BOV into atmosphere on an SC", the Vortech setup does this already, so we'll see how their CARB EO goes.

Looks like HKS makes a cool BOV which allow you to adjust how much "blow-off" is vented vs. plumbed back into the inlet. So the "woosh" levels can be tuned. Anybody on this thread ever used ine of these?

 

So if the BOV does not come with the TT kit, the plumbing has to be cut to fit the BOV?

 

Yep.
Two basic methods.

a) cut a few inch section out of each pre and post pipes and
use Hoses , clamps and Tees Easy but looks hacked.

b) weld flange for BOV between the IC and TB
and (optionaly) a return Tee between air filter and Compressor.
A return hose useually >1" diameter is then run between the two.

The recommendend method is b) with return Tee if you intend to be CARB legal (at least by appearance). Otherwise woosh away.

 

Plain and simple. If you have a turbo engine and you remove the BOV from the system, you are f*cked. The reason Y you have a BOV is to prolong the life of your turbos.

 

I'm not sure if you or anyone else would know this, but (1) once GReddy's kit is CARB exempt, would it still be CARB exempt or street legal if a blowoff valve is added by the owner? and (2) could a cop pull you over if he hears the whooshing sound?

 

legally if you ad a BOV (or Bypass) to a Greddy system and the non-BOV system has a CARB EO number, the sytem will no longer be legit. And in Ca. ,yeas a cop could give you a fix it ticket if he heard your woosh and he was very knowledeable of "your" particular setup. However; unless an officer just wants to be a real D-ck or he's bored, this most likely woudln't happen. It is more liley that you would be sited for speeding as you enjoy you new powerful toy.

Were you would most likely have issues with the upgrade is during the inspection portion of the emissions test (when you eventually need to get one). Personally I would put the BOV in a non-obvious place and vent it back into intake. It is likely that the inspector won't notice.

 

Ealrier in the thread I mentioned an HKS BOV.
Actually I mean the Stealth BOV offered by these guys.

http://www.gofastbits.com

I'll be considering this option, seems to be best of both worlds
and another **** to play with. Anyone reading this thread have any experience with these guys?