BOV for Pettit supercharger
 

Maybe this sounds like a dumb question but.. is it possible to use a BOV for the pettit supercharger? and it works?

yes and no.

on superchargers you use them in a recirculating fashion as a valve to bleed air back around the compressor at partial throttles. although if you have the throttle body in front of the supercharger they aren't necessary.

you can vent them to atmosphere but you never get a short 'pissh' sound between gears like a turbo; they hiss continuously at partial throttle and can be very annoying.

Ditto and BTW the Pettit Racing Super Charger does have a BOV. :eyetwitch

No, the Pettit system does NOT have a BOV.
It has a recirculating valve.

thats pretty interesting to know. so they use a blow off valve but as a recirculating valve to serve a different purpose? i would have never known :)

any BOV can be plumbed back to the inlet as most car manufactures do for noise reasons.

Actually, they are quite different.
A BOV uses differential air pressure to open.
A recirculation valve is just a servo valve - it opens when it sees a signal at its vacuum port.

BOV (blow off valve) vs DV (diverter valve) - what's the deal, anyway?
 

"A BOV and a DV more or less the same, and perform the same function. The difference in terminology is pretty much meaningless. Where people get their undies in a bundle and/or burned is the issue about venting to atmosphere or not.
With a closed, MAF-based system (such as my current motor, a 1.8 litre turbocharged Audi), venting to atmosphere will cause the motor to run rich. Period. Venting a BOV/DV to atmosphere simply to make more noise is one of a number of definitions which puts the owner of that car into the category of "ricer". ( flames > /dev/null )
The ECU meters incoming air via the MAF, and using inputs from the MAF, TPS, MAP (if so equipped), RPM, O2 sensor(s) and who knows what else determines how long to fire the injectors. If any of that metered volume of air is vented to atmosphere, the ECU has no idea that it's lost some air and will fire the injectors long enough to match the volume or pre-metered air. Result? Rich condition.
And just for a minute, consider what happens a non-recirculated valve is open during normal driving. When not making boost, turbocharged motors live in vacuum. Maybe even enough vacuum to hold the BOV/DV open. Ingesting unmetered, and in 99% of the cases which I've seen, unfiltered air. Sound like a good idea?
Some BOVs are engineered to only vent to atmos, some are engineered to recirculate the vented charge. See attached a picture of 2 valves sitting side by side. One is vent to atmos only, the other is designed to recirculate. Are the BOVs? Are they DVs? Does it really make any difference? (They are BOVs, for what it's worth...)
Ok. With that out of the way.
As was stated abuve, DVs and BOVs are in essence the same. It's simply the nomenclature which is different. They both perform the same function, which is to protect the compressor from 'bad things' when the throttle plate snaps shut during the creation of boost pressure.
The charge in the IC plumbing can travel well in excess of 100 mph, and when the throttle plate closes the charge has nowhere to go. A shockwave can bounce off of the throttle plate and travel back through the IC plumbing to the compressor, and if the impact is great enough can stall the compressor, or perhaps even spin it backwards. Which is 'a bad thing' as far as the health of your turbocharger goes. The wheels are spinning anywhere between 100k and 200k RPM. From 100k to 0 rpm in an instant? No thanks."

More...

"What a Diverter/Blow-Off/Bypass Valve is and what it Does:

There are numerous names given to this part, and it should be understood that they are all interchangeable terms used in different ways by different people all to describe the same basic thing.

Diverter Valve
Blow-Off Valve
Dump Valve
(Compressor) Bypass Valve
Pop-Off Valve
Discharge Valve
Boost Valve
Hooter Valve (Yes, I have even seen people call them "hooter" valves)
Etc.

Whether the valve is venting or recirculating, it is still a “bypass valve”, and since this term gives no implication to recirculating or venting function, it is the best term to use when talking about all of these valves in general terminology.

Basically, a bypass valve exists to relieve a residual amount of boost pressure in a pressurized application when the throttle on the application is abruptly closed preventing the air from “backing up” into the compressor wheel of the turbo, slowing it down, thus creating “lag” when the throttle is reapplied.

Whether the air is vented or recirculated makes NO difference to the amount of “lag” created.

"Lag" would only be created if there was no bypass valve in place at all, and the residual charge air inside the intercooler piping "backed up" into the compressor wheel at throttle lift. Whether an atmospheric or a recirculating valve is used, the valve is still able to bypass the residual charge pressure at throttle lift allowing the compressor wheel to maintain it's rate of speed, thus reducing “lag”."

and more...

"BOVs and DVs serve the same purpose, they vent excess boost from the intake tract. The difference being that a BOV vents the extra boost pressure out into the atmosphere so you are loosing that boost, a diverter valve however blows the extra boost back into the intake before the turbo so it is used to keep the turbo spinning reducing lag. Some people will argue that you don't want the boost recirculating because it is blowing hot air back into the turbo and you want cool air from the outside. Most cars that originaly were equiped with a diverter valve seem to perform better when you stay with the diverter valve. It seems most peoples argument for a BOV is they like the noise it makes.

A wastegate is similar to a BOV or DV except it is between teh exhaust manifold and the turbo. It opens to allow exhaust gasses to bypass the turbo. it does this to keep the turbo from spinning beyond its intended RPM maximum."

Got it? :eyetwitch

Thanks so much guys.

Except, the device actually included in the Pettit kit does not fit that description.

Once again - a BOV, whether it recirculates or not, works on a pressure differential between the air in the charge pipe and the signal line, which operates on the other side of the piston/diaphragm.
The valve included in the Pettit kit is designed to stay closed under pressure unless it is commanded open by the signal line.
In other words, a BOV that sees pressure on both sides will stay shut. If there is a lower pressure on the signal line, it will open at a ratio equal to the differential.
A BPV (like the one included in the Pettit kit) will stay completely closed until the signal line is under a sufficient, fixed point vacuum, regardless of the pressure or vacuum in the plenum.

Charles,

I did not like the "Wikipedia, the free encyclopedia" definition but I came across a better one. See below: :eyetwitch

"Compressor bypass valve (CBV)
In the CBV case, pressurised air is returned to the turbo compressor air inlet for re-use, instead of being dumped to atmosphere. The bypass valve is open (kept open by internal soft spring) under normal low-load engine running conditions, and closes firmly (overcoming spring resistance) when positive (boost) pressure is present in the inlet manifold (or plenum chamber).
When on-boost and changing gear (throttle lift-off), a sudden lower pressure condition is created in the manifold and the valve pulls open again (spring assisted), directing pressurised air from your pipework and intercooler back to the turbo air inlet.
Instead of venting the pressurised air into the engine bay for noise, your directing it back for the turbo inlet to use again.

Blow-off valve (BOV) / Dump Valve / Vent valve
This valve type features an adjustable spring design (soft spring held in compression by preload) to keep the valve closed under idle, cruise and boost conditions (eg 0.5 bar). (If the valve was open like the CBV sort, dust and dirt would be sucked in.)
Only at sudden throttle lift-off (eg 0.8 bar) does the valve momentarily open to vent / dump the pressurised air directly to atmosphere (your engine bay) through a specially designed orifice (and wooshtssh).

How do they work? A small diameter hose connects between the inlet manifold and the relief valve - changing pressure conditions (vacuum or pressure) will exceed the internal spring, and the valve will open or close accordingly.
A CBV or BOV relief valve is forced firmly closed under pressure conditions (turbocharger producing boost ), until such time as a lower pressure condition occurs with throttle lift-off or between gear changes. An instant lower pressure condition or * vacuum inside the inlet manifold will then pull the valve open, momentarily venting boost pressure to atmosphere BOV, or recycled CBV.
* Vacuum (pressure less than atmospheric) is formed in the cylinder bore/s when a piston decends (on intake stroke). Vacuum transfers through the opening and closing inlet valves back into the inlet manifold, and rises when the throttle plate is fully closed.

Bosch style relief valves are compressor bypass valves CBV, and are open most of the time under engine vacuum (idle, cruise, throttle lift-off), but close firmly under positive (boost) pressure. This CBV style returns (recycles) the air quietly back to the turbo compressor inlet or air cleaner area.

Cars and trucks without a vent valve emit a high shrill-whirr sound, as the compressor back cuts the air. Listen to a turbo truck between gearchanges.

Why vent the pressure ?

- Primary reason is to reduce strain on the compressor turbine wheel due to compressor surge. Surge occurs when flow is restricted and hence a risk of abrupt reversal of airflow over the compressor wheel,. Between gearshifts and sudden throttle lift-off, the turbine is still spinning very fast (but slowing) and pumping air at the closed throttle plate, as well as placing strain on the intercooler, hoses and fittings. -> This is why its a good idea to momentarily vent the pressure. A CBV / BOV allows the turbine to continue spinning freely. When back on the throttle, boost pressure quickly rises again with little lag. A suitable valve location is just before the throttle plate.

A compressor bypass CBV must be used with Bosch K-Jet injection, as being a closed system, any loss of air for which fuel has already been metered by the movement of the airflow sensor plate, will result in an over-rich condition and possible backfire. A CBV is found on many "OEM" original engine manufactured EFI turbo systems.

A blow-off BOV is suited to an EFI system that uses a MAP (manifold absolute pressure) sensor. As the MAP sensor source is inside the inlet manifold's plenum chamber, relieving air before this with a closed throttle butterfly (throttle lift-off) will not affect running.

Whether you decide to vent/wooosh to atmosphere with a BOV, or quietly back to the turbo compressor with a CBV depends on your injection system. Magazine test results over the years have shown little performance difference between the two, other than an interesting aural sound. The reason for fitting one is to reduce strain on the compressor turbine wheel, the intercooler, hoses and fittings."

Bingo! Nice picture btw.

OK. I guess you guys just aren't paying attention.
Go out to your Pettit kits and disconnect the signal line to your bypass valve and report back.
You will notice that it will not open. It will stay closed no matter how much you rev (unless it blows apart as some have experienced - it wasn't designed for this function as Ray noted above).
Now, find someone with a BOV and have them disconnect the signal line similarly.
Have them rev. Notice what happens? As soon as pressure is a couple of PSI over atmo, the valve will start to lift.

You can keep regurgitating definitions for things that are not part of this discussion all day long.
It doesn't change the function of the actual bypass valve on the Pettit kit.

found a bigger picture:

http://www.autopartswarehouse.com/im...1618098BOS.JPG