A friend of mine and I were having a conversation about the difference between the two. He drives a 2000 Camaro 3.8 with a home built turbo setup and we were discussing what I'll need to do with my supercharged setup when I put the smaller pulley on. A bit of web searching led me to this article and I thought I'd share it here. Long story short, we were both wrong because we were using the wrong terminoligy.

http://wwwrsphysse.anu.edu.au/~amh11..._off_valve.htm

-Pete-

 

Very interesting.

So is he calling the one on the left a bypass, and the red one a blow off?



He also stats, a blow off stays closed at idle, cruse, and part thottle, and only opens at sudden high boost conditions. Is that true and why

---Bill.

 

The one on the left has an inlet from the discharge tube and an outlet on the other side to vent pressure back into the system with a valve between the two. The valve is spring loaded (light load compared to a BOV) and assisted by boost or vacuum from the intake manifold. The BOV on the right has no passage way to vent pressure back in front of the compressor after discharging it. BOVs can only be vented into the atmosphere. BOVs have a much higher spring pressure to keep boost in and keep the valve closed while driving around. A BPV can be used AS a BOV if you vent the pressure into the atmosphere instead of recirculating it back to the front of the compressor inlet.

The Bosch style BPVs have a 10lb spring holding the valve shut. When you're driving around at 15lbs of vacuum, that valve will be open. When you smash the loud petal and the intake manifold pressure (and vacuum lines) briefly goes to 0 (before boost) that 10psi spring will shut the valve to keep pressure in the compressor discharge tube to allow the manifold to build boost. The boost in the vacuum lines will push the valve closed even harder.

In a BOV the spring pressure is strong enough to hold the valve closed under normal driving vacuum conditions and only open when in an intake manifold vacuum condition WITH pressure in the compressor discharge tube. In other words snapping the throttle closed at 15lbs of boost causes pressure in the discharge tube to spike while pressure in the intake manifold goes to vacuum. That pressure will "blow off" the valve while the vacuum is assisting opening it from behind, whereas the vacuum could not do it alone.

-Pete-

 

[QUOTE=That Camaro Guy;4929288In a BOV the spring pressure is strong enough to hold the valve closed under normal driving vacuum conditions
-Pete-[/QUOTE]

Pete, All but that one part makes sense. Why would you want boost at idle and part throttle cruise? Is it just a turbo thing?
I use the red one on the right, since it's open at idle, cruise,..it's a bypass? that's vented to atmosphere.

---Bill.

 

The basic need for either is such to relieve the pressure when the gas pedal is released while boost is being generated to prevent damage to the super/turbocharger. THAT"S IT! They can either re-circulate (BPV) or vent to atmosphere (BOV).

The "bosch" as it's described seem as a "OEM" thing to keep the fuel-economy up when just cruising around and only use the boost during acceleration periods.

All aftermarket setups I've seen or dealt with make boost either all the time (SC'd) or once the turbo's spool and the only time the BPV/BOV is actuated is when the gas pedal is released where there is boost in the system.

 

You wouldn't have intake manifold boost while cruising, you would have pressure in the compressor discharge tube with the valve closed. If you have a weaker spring holding that valve closed it could fail during high boost runs, generally more than 15lbs from what I've read on OEM style bypass valves.

And yes, the only purpose for these valves is to prevent compressor surge right after the throttle plates close on a WOT run, not keep pressure from being in the discharge tube during cruise.

-Pete-

 

[QUOTE=And yes, the only purpose for these valves is to prevent compressor surge right after the throttle plates close on a WOT run, not keep pressure from being in the discharge tube during cruise.

-Pete-[/QUOTE]

Turbos and centrifical superchargers are very different the article you posted in mostly correct but misleading in a few areas. You never want excess pressure in the outlet tubing unless under boost! Thats why centirical superchargers use bypass valves they make boost based on RPM turbos make boost based on compressor maps and its not related to rpm turbos will spool down quickly supercharges won't. You can run a BOV on your supercharger if you want to greatly decrease its life this will most likely cause catastrophic failure at some point and maybe take out your engine with it as it spits the blades into the motor. Please people forget about all this BS about venting to atmosphere or not. you recycle if you have a maf before your BOV or bypass valve. If your MAF is after this then it would be stupid to recycle air as its hot as hell after being compressed by the blower dump that to atmosphere. Bottom line if you have a cetrifical SC maake sure it is open at idle and under cruise!