BOV - Help me out here
 

Ok, scenario... you're at idle, the BOV is open to atmosphere, and your BOV is seeing vacuum (so is your boost gauge). Isn't the BOV open when it has vacuum on it?

With the BOV open at idle (say, a stoplight or something), you're pulling non-filtered air into the intake. Am I missing something here?

The only time the BOV will open is when there is positive pressure in the intercooler line and when there is vacuum at the BOV nipple. It will not be sucking in unfiltered air.

ok, excuse me. I got it now. Blame the Heineken.

correct!

Hmm.. Not entirely true but.
It depends on the cracking pressure of the bov itself, if you are pulling 18" of vacuum, and bov has a 16" spring it will open at idle, sometimes you have to run a weak spring to combat surge at low boost conditions, like just driving up a small incline, you don't build enough to blow the valve open, but you build enough boost to experience a bit of surge when you back off the gas. When you have to run a weak spring, the trick is to get the idle air from another source, such as the bac valve with its own filter on it, so that the motor draws from it when the bov is open a bit, that way the motor only sees filtered air, then once the throttle plates are open again, the vacuum drops and the valve closes. This is more true for low vacuum motors than anything else, on a v-8 with a tight duration cam that can pull 22" at idle you could run a strong spring and not worry about it, but on a rotary they don't make enough vacuum to always pull open the bov with a strong spring even with boost in the charge air pipe.
I have the Tial bov in my car right now, it comes with 2 available springs, the heavier being a 22"in of hg spring and the other an 18" hg spring, both of these were to heavy for my motor that pulls a typical 12" of vac at idle, and 16 under deceleration, so I ended up using a spring from a transmission shift kit, that I cut down to give a 14" cracking point to. When the bov does float open a tad, the motor is on the bac so it draws nothing through the bov. For reference the stock tII by pass valve opens at 10-12" of vac..Max

Unless your BOV is mounted downstream of the throttle body (i.e. on the intake manifold) you are not pulling significant vacuum on it. Maybe a couple of inches but nowhere near what your vac/boost gage shows (most gages are plumbed in downstream of the throttle body).

Lay off the heine!!

it makes perfect sense now. We were just in a discussion last night and neither of us could understand what was going on :)

Anyhow, i know for a fact that the vacuum/pressure port on my BOV sees the same pressure, or lack there of, as my boost gauge sees, its coming from the same port on the back of the 20B UIM. My engine isn't broken in yet, so when its idling, i only see around 12-13" of vacuum, I'm expecting 15 or so when its broken in.

I have the greddy Type R BOV plumbed in to the intake, after the intercooler, before the TB, just like it should be. Something a bit off topic, but this is what got us into talking about how the BOV actually works... we noticed a threaded hole on the back side of the BOV and were trying to figure out what it was for. Greddy's site doesn't really explain it (actually, they don't explain anything) but we think it maybe a sort of breather for the BOV. The vacuum nipple on the BOV is obviously on the outside of the diaphragm, and this "vent" is also on the ouside of the diaphragm... we were trying to figure out what to do with this, but since they didn't send any threaded adapters for it, we figure its unnecessary for my application. Also, why the hell does the BOV has a giant threaded port on it where it blows the air off? I've heard of different "Horns" you could put on it, but I've never seen one for sale, on a car, in a catalog, just rumors... i suppose its for plumbing the BOV back into the air box or something (for use with a MAF system), but since I'm using a MAP sensor with the Haltech, i don't need to do that....

any thoughts?

Is that threaded port perhaps for a missing adjustment screw? I am not familiar with the greddy bov, but some do have a screw for preloading the valve spring for different opening pressures...Max

So on a stock T2 BOV can i have it open? i mean do i have to have it hooked into the TID?

yes, it has to go back into the TID. It is metered air and the ECU has already accounted for it in the system. If you vent it to atmosphere, your car will run rough.

i didn't want to start a new thread and this is the closest i could find.

anyway, on adjusting the blow-off valve, how do you do it? i mean, i know you'll have to turn the adjustment screw but what am i looking for to know when it's right.

i've got the HKS SSBOV (i just changed my turbo), and it sounds different than it used to, so i'm thinking it may need to be adjusted.

anybody know what to do? i've never done it before and i didn't want to touch it until i asked someone.

thanks