RAD951

I tried today to use the stock BOV to vent into the atmosphere. I unplugged the bov valve from the intake boot and left it in the air. And I plugged up the hole left in the intake boot.
However, when I started the car up. It wants to die on me. Whats up??? I thought that you could vent into the atmosphere with these cars?

adrial

Not with the factory BOV (Compressor bypass Valve really...CBV) because it is slightly open at idle and causes a nasty vac leak...

RAD951

Oh, I think thats the problem. so I guess I need to buy a greddy or hks blow off valve. Is there any real performance gaing from this other than a cool sound?

Ahmet

I don't think you would see much gain from an aftermarket BOV before you start moving a lot of air. The only thing I find annoying is the (IMO too extreme) surge between off/on power at higher rpms. It's very hard to keep the car smooth if you're deep in a gear trying to manage a turn. It could help in this department but it would also throw off the mixture some. I thought about doing two factory BOVs, one vented to the atmosphere with a one way check valve on it so that it wouldn't cause vacuum leaks at idle. (This would help move more air out between shifts yet still open early unlike aftermarket BOVs).
Ahmet

Flight_951

[quote]Originally posted by RAD951:
<strong>Oh, I think thats the problem. so I guess I need to buy a greddy or hks blow off valve. Is there any real performance gaing from this other than a cool sound?</strong><hr></blockquote>

I have had a Turbonetics BOV venting to atmosphere on my car since last year. I just replaced it with the upgraded 993 BOV (not venting) because I was tired of the whooshing sound, the car would drop in idle to 500rpm if I quickly let off the gas, and shifts weren't very smooth. I have noticed no performance difference either way, but the car is more enjoyable to drive now without the BOV venting to atmosphere.

David Salama

Ahmet,

I am still waiting for my HR monster blow off valve horn to arrive. I am still wondering if it has a check valve in it. When I experimented venting my Monster BOV to air, I also had a vacuum leak a idle, and upon initial throttle. Where can you buy one way check valves for this purpose??
<img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> <img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> <img src="graemlins/jumper.gif" border="0" alt="[jumper]" />

Edman951

[quote] Is there any real performance gaing from this other than a cool sound? <hr></blockquote>
No,but from my experience with my atmospheric bov,i did not feel a loss of reaction time between shift because of venting. I think it's even help my car a bit because i did have a small dead spot between shift with my 993 valve and i think it's smaller now.Maybe i was the stock valve not working right...

Here's my opinion on the matter.(hope i can write what i'm thinking....)
So when you shift gears and your stock valve recirculat's the air back into the turbo does it actually help!!! i think it does'nt because when you are at 6500rpm and about to shift gear, the turbo is spooling at apromx.i guess +or-80 000rpm and during the shift the diverter valve recirculates the excess boost back into the turbo but does it keep it spooled or does it slow it down???
Because the stock valve is so small and i think it restric's the air and slow the turbo down.Because you are making the compressor wheel compress the incoming air but there's not exhaust pressure to keep the turbine spooling,so i think it slow the turbo down.
Also when you shift and you have a good aftermarket bov venting to atmosphere then there's no air to slow the turbo from keeping a good rpm.How much rpm could it loose?And since you shift fast the turbo just pick up a few rpm lower and spools again.
Maybe with a big high flowing diverter valve like the Turbonetic's Godzilla,then it would probably keep the turbo at the same rpm +or-.
Maybe some others can give some input
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Danno

"Because the stock valve is so small and i think it restric's the air and slow the turbo down.Because you are making the compressor wheel compress the incoming air but there's not exhaust pressure to keep the turbine spooling,so i think it slow the turbo down.

Also when you shift and you have a good aftermarket bov venting to atmosphere then there's no air to slow the turbo from keeping a good rpm.How much rpm could it loose?"

Ok, I think the mistaken assumption is this one:

"Because the stock valve is so small and i think it restric's the air and slow the turbo down."

This is assuming that the turbo is sucking ALL of its incoming air from the bypass valve?

"Because you are making the compressor wheel compress the incoming air but there's not exhaust pressure to keep the turbine spooling,so i think it slow the turbo down."

"a good aftermarket bov venting to atmosphere then there's no air to slow the turbo from keeping a good rpm."

With NO air coming into the turbo, it would stop almost instantly!

Here's a picture of what's really going on:



In the stock configuration, the bypass valve routes air that would be piled up against the throttle-plate back around to the turbo-inlet. This air is added to the volume that's already coming into the turbo. However, this air is PRESSURIZED versus the raw incoming air which is underVACUUM (due to intake-track restrictions). Now adding recirculated pressurized air lowers the pressure-differential between the turbo's inlet vs. outlet. The lower this differential, the longer the turbo can keep spooled up.

So let's say the bypass-valve only recirculates 100cfm of air @ 15psi, while the turbo is compressing 200cfm, the turbo still has to compress 100cfm of fresh incoming-air at 0psi or a little vacumn. This extra air at vacuum is what slows the turbo down (it doesn't have to compress air that's recirculated @ 15psi). If you can recirculate ALL of the turbo's outlet air @ 15psi around to the intake, it can pretty much keep on going almost indefinitely with the only thing slowing it down being bearing and seal drags.

In the last case of the blow-off valve venting to atmosphere however, ALL of the air ingested by the turbo is fresh incoming air at a little vacuum. So the turbo has to compress ALL 200cfm @ vacuum up to 15psi. Thus, it will slow down faster between shifts if you vent to the atmosphere.

However, the larger BOV venting to atmosphere CAN reduce boost-spikes if your stock bypass-valve isn't venting the air between the turbo-outlet and throttle plate fast enough (usually happens with upgraded turbos at higher boost than stock). In this case, when you close the throttle, the outlet pressure can creep up from 15psi to 16-17-18psi, then when you re-open the throttle after the shift, you'll have an 18psi spike. In this case, the extra build-up of boost will also cause increased turbo-lag. So the real solution to this boost-spike is to get a larger bypass-valve. Just happens that a larger BOV venting to atmosphere has the same effect (solving boost-spike) at the cost of extra turbo-lag vs. recirculating.

I agree with Flight951, after the novelty wears off the sound and stumbling idle/closed-throttle operation will get annoying, and Edman951 will put his BOV back to recirculation mode.

Edman951

Danno, you are right,i will probably go back to diverter in the future but with something bigger.I think the size of the stock valve is the problem. If you could get a bigger diverter valve then i think it would out do the bov.
But i did feel a reaction gane when shifting,and my 993 valve does not leak.
Also i never get the stumbling idle???
The only i'll effect i found was when boosting at 3/4 trottle on the highway and letting off just a bit,the bov opens too long and creats a big rich condition.

Danno, you should offer a bigger diverter valve as an option with your MAP kit....

Stuttgart951

Always great opinions on these boards...

I think Im going to scratch the BOV that vents to atmosphere off my "to do" list.

Ahmet

[quote]Originally posted by David Salama:
<strong>Ahmet,

I am still waiting for my HR monster blow off valve horn to arrive. I am still wondering if it has a check valve in it. When I experimented venting my Monster BOV to air, I also had a vacuum leak a idle, and upon initial throttle. Where can you buy one way check valves for this purpose??
<img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> <img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> <img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> </strong><hr></blockquote>

I'm planning to use one off another car's brake booster that's suitable in size. This would ofcourse be free... I'm sure you could find one at an auto-parts or hardware/plumbing store if you like shiny things.
Ahmet

Edman951

Because of this discution, i did a little test today. I re-installed my stock non leaking 993 valve insteed of my Apexi.

With the stock valve the rpm drops slower when i give small shots of gas and between shifts.
There's a much bigger kind of lag or delay between shift for the power to get back on,compared to the Apexi venting to the atmosphere.With the Apexi i can almost shirp second gear(because of weak clutch) and with the 993 valve, there is no way in hell i could shirp second.
The only + side i think is the part trottle boosting difference.
With the Apexi when i boost on the highway at part trottle and i reach the desired speed if i get off the gas just a bit to hold my speed,the bov discharges and i get a big rich condition and it obligates me to let of the gas completly to dump the boost.
With the Apexi venting to atmospere i do not have any idle drop or eratic idle problem.
So the next thing i will try is my friend Forged valve to see if there a difference.
i'll keep you guys posted...

Ahmet

It's this same surge that I find a little annoying with recurcilating valve (stock set-up). Part throttle is OK, but keeping the car stable at high rpm between on/off throttle I find is very hard. However I don't like the idea of venting all to atmosphere (and I don't see how it could be any faster this way at all, you do have the wrong mixture). This is why I want to hook up a second (factory)blow off valve venting to atmosphere, with a limiting/check valve. I think that'd be a descent compromise. I did hook up two bovs at some point but both recurcilated and I saw only a very slight improvement. BTW to my experience recirculation is definately a lot more responsive at part throttle especially if you're coming off from high boost.
Ahmet

Edman951

Over1g, two valves is a good idea,ì've sean many suppras with 2 valve and it must work good to remove all unused boost.
check out this link for a hybrid BOV, it the best of both worlds.vent and recirculate at the same time.
<a href="http://www.newdimensions.com/products/page_11.html" target="_blank">http://www.newdimensions.com/products/page_11.html</a>