Yet another n00b Supercharger question

[m4vr1k]

Well, i have finally managed to get all the money i need to get my supercharged S13 project underway, and i have another n00b question, im sorry.
Blow off valves. Needed on a supercharged system?
the throttlebody is on the front of the plenum chamber, so a long way from the charger itself.
Again, all help is appreciated, and if anyone wants to put their hand up to give me assistance when the time comes to install everything, im sure i can make it worth your while.
Cheers guys
Zac

[Ae92typeX]

bov not needed

[m4vr1k]

Can anyone explain why a bov is not needed?

[Norty]

As i understand it (from looking around the internet when i got my GTZ)

A BOV is simply a bypass valve that gets vented to the atmosphere. They do the same thing, only one vents to the atmosphere the other back to the intake tract. You can put a blowoff valve on a supercharged car so long as the mass air meter is setup for blow through, that way you don't vent off metered air.

is this correct guys?

I wouldn't waste my time if you want that wank factor then do what i did and get a turbo car as well

[sergei]

As i understand it (from looking around when i got my GTZ)

A BOV is simply a bypass valve that gets vented to the atmosphere. They do the same thing, only one vents to the atmosphere the other back to the intake tract. You can put a blowoff valve on a supercharged car so long as the mass air meter is setup for blow through, that way you don't vent off metered air.

is this correct guys?

I wouldn't waste my time if you want that wank factor then do what i did and get a turbo as well

Not really correct.
The blow off valve is to vent pressure build up when the throttle is closed but compressor is still pumping (inertia) air, if the pressure is not vented the air tries to stop the compressor (typical za-ta-ta sound) which is not good for compressor (well bearing - a lot of side load, I guess). With 4AGZE setup the throttle is before the supercharger so with closes throttle the piping between the intercooler and supercharger (and between supercharger and inlet manifold as well) is under vacuum (opposite to throttle after comperssor setup) also when you let go of the throttle supercharger stops pumping - the ECU deactivates the supercharger clutch. Even with the supercharger before throttle the extra boost that will be created on lift off (which will not be created due to disengaged clutch) will not do any harm to the supercharger.
If the BOV is made to work with stock 4AGZE setup it will be sucking in due to vacuum and all the crap from engine bay - which is not very good for piston bores and such.

[m4vr1k]

Okay, on my system, unless i figure out a way to replicate the 4AGZE system, the supercharger will still be going when the throttle is closed, so it seems to me that i would in fact need a bov on my system, yes?

[Adamal]

As the S/C is run off the engine, its revs are directly proportional to the revs of the engine, unlike a turbo which will still try to create a positive pressure when the engine revs drop.
Result is compressor surge as the pressure keeps increasing as less of it is being consumed by the engine. That causes the turbo to suddenly halt and be forced to spin the other way (Which is compressor surge). That or its gonna put a huge hole in your intercooler

My paragraph construction is pretty damn messy there, but I think you get the idea

[no_8wire]

As the S/C is run off the engine, its revs are directly proportional to the revs of the engine, unlike a turbo which will still try to create a positive pressure when the engine revs drop.
Result is compressor surge as the pressure keeps increasing as less of it is being consumed by the engine. That causes the turbo to suddenly halt and be forced to spin the other way (Which is compressor surge). That or its gonna put a huge hole in your intercooler

My paragraph construction is pretty damn messy there, but I think you get the idea

And plus, the throttle on SC car is before the SC...so you cant get surging etc...unlike on a turbo where the turbo ( or +ve pressure source) is before the throttle so a build up of unvented pressure can result in surge etc...

[Crampy]

Okay, on my system, unless i figure out a way to replicate the 4AGZE system, the supercharger will still be going when the throttle is closed, so it seems to me that i would in fact need a bov on my system, yes?

You can do it the way you have mentioned in your first post (Put the supercharger blowing into the throttle body). So, yes you can put a BOV on it.

There is a Yellow Ford Capri V6 which goes to the drags fairly often, which uses a similar setup to what you are suggesting. He runs two Toyota S/Cs (unsure if they are SC12 or SC14s) and the compressed air goes through an intercooler and into the throttle. He uses a BOV (can't remember if it's one or two BOVs) and it sounds pretty insane when he's staging at the drags.

[Crampy]

Also, if you are worried if the BOV will let air in with vacuum, then use BOVs that have the filters on them to be sure you're not going to suck in cr@p.

[Ae92typeX]

you can put it on, but I still say you dont need to.

what type of s/c are you planning on using?

[m4vr1k]

Im going to be using an SC14 off a 1G-GZE.
Okay, from what i can see, i CAN put a BOV on, but its not really necessary?

[vvega]

ift the tbody is before the s/c its not nesacerry
if the tbody is after the s/c a bypass valve is required(read plumback bov)

[Crampy]

ift the tbody is before the s/c its not nesacerry
if the tbody is after the s/c a bypass valve is required(read plumback bov)

Yep plump back is a good idea. Do it man.
If you don't have a BOV, it'll pop intake pipes off when off throttle, on a high load gear change. The s/c is postive displacement, so at say 7000rpm, you come off throttle to change gear. The throttle snaps shut, the s/c is still pumping air at the rate of the turning motor (well you have to take into account the pulley ratio). So, with the throttle now closed, where does all the air go? The BOV will circulate it back to the intake of the s/c and thus relieves the pressure on the s/c outlet/intercooler/throttle body section.

[Crampy]

Also, a turbo without a BOV causes the turbo to compressor stall, so the pressure forces the turbo to stop spinning, which stops it producing boost....

So, with a positive displacement s/c which is driven straight off the motor, it can't be stalled, unless it starts making the belt slip like a b*tch. You could hook up the electromagnetic clutch mechanism on the sc14 to disengage when the clutch pedal is in, but I wouldn't bother. Just chuck it on a switch (through a relay obviously), so you can turn it on and off like mad max .

[Ae92typeX]

I just re-read througheverything and realise I have not been reading propery last few days.

bipass/bov for charger-throttle-engine would be best bet.
Some s/c's have integral bipass setups for idle, does the sc14 not?
Early carb blow through sc setups didnt all use a form of bipass - perhaps also lower boost pressure appliactions were used here?

would be easy as to rig up an microswitch from throttle to the electro clutch on supercharger, switching offer the charger with dropped throttle applications.

[m4vr1k]

That is what i had thought, but wasnt sure.
While a plumb-back BOV would be ideal, i do want the +10 wank factor that an atmosphere BOV adds. so i will most likely go that way.
Thanks for all your help guys.

[Adydas]

Sounds like this thread has been nothing but justification for you to use rice boy toys.. If you want a Atmos BOV get a turbo car..

Correct me if im wrong but if your in need of a plumb back simply putting a atmo unit one will create problems with the ecu and fueling etc.

[SUBARUCONVERT]

As the S/C is run off the engine, its revs are directly proportional to the revs of the engine, unlike a turbo which will still try to create a positive pressure when the engine revs drop.
Result is compressor surge as the pressure keeps increasing as less of it is being consumed by the engine. That causes the turbo to suddenly halt and be forced to spin the other way (Which is compressor surge). That or its gonna put a huge hole in your intercooler

My paragraph construction is pretty damn messy there, but I think you get the idea

The compressor does NOT spin backwards during compressor surge, air just cavotates (sp?) in the compressor housing making the ta ta ta sound. I have run no BOV on my subaru for the last year or so, no problems with lag between gears

[bluemaumau]

are you sure i thought it was reverse spinning, which is why it compressor surge is bad, because it wears the seals out quicker.

i.e no BOV = shorter turbo life (probley not standard boost, but any thing after the factory intended.)