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Which BOV stays open when no throttle

I know the BOV type and location has been beat to death but I am interested in the type that stay open when not on the throttle since that makes sense to me. Can someone tell me which brand or model has this feature or what the feature is called?
Old 03-23-2010, 07:49 PM
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Well surely someone knows what this feature is called. Is this now standard on BOVs? I would like to know what to search for please.
Old 03-24-2010, 07:04 PM
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Originally Posted by forwheeler View Post
Well surely someone knows what this feature is called. Is this now standard on BOVs? I would like to know what to search for please.
Talk to 911ST. He has some experience with this adaptation.
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Old 03-24-2010, 07:26 PM
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911ST, I hope you chime in here...
Old 03-25-2010, 09:47 AM
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This will get you started.

Inlet Overrun Valve Modification? (BOV, lag)
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Old 03-25-2010, 10:46 AM
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There is not a type of BOV that is made to operate that way. 911st simply re-plumbed a 964 style BOV to operate that way. Keith is a great resource of information on 930's and I'm sure he would be glad to tell you how he did it.

Cole
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Old 03-25-2010, 12:13 PM
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Quote:
Originally Posted by cole930 View Post
There is not a type of BOV that is made to operate that way. 911st simply re-plumbed a 964 style BOV to operate that way. Keith is a great resource of information on 930's and I'm sure he would be glad to tell you how he did it.

Cole
O'l Cole,

Mr. Flintsone here (aka Mr. Gimpy). I can't resist...let me prime the thought pump.

Take a "modern" piston-style (not diaphram) BOV as many of us use. With the vacuum line attached to the intake manifold post-TB, and swapping out the stock 14 lb. spring with something in the 2 lb. range, and given that the intake manifold will produce something like 200 millibars of vacuum when at steady no-load cruise, that's just enough vacuum to overcome the spring tension and open the valve...allowing the turbo to freewheel. Now, stomp in the throttle and the vacuum pretty much goes away with the throttle plate wide open, and the 2 lb spring closes the piston. And as boost begins to build, that vacuum line to the intake manifold now becomes pressurized, adding to the spring tenson on top of the piston to keep it closed until you take your foot off and the vacuum spikes back up to open the valve and vent the boost pressure that is residing in the IC/pre-throttle body intake.

All theory of course, and since my BOV is buried beyond easy access I'm not inclined to pull the spring out and stuff in a 2 lb. spring to test this. But hey....could actually work.
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Old 03-25-2010, 01:24 PM
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How does routing airflow around the turbin during off-boost increase turbine response/threshold? I must be missing something. Seems that the opposite would be of benefit.
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Old 03-25-2010, 01:37 PM
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Quote:
Originally Posted by RarlyL8 View Post
How does routing airflow around the turbin during off-boost increase turbine response/threshold? I must be missing something. Seems that the opposite would be of benefit.
This is where Keith needs to jump in. I think his premise was that with no boost being built (valve open and recirculating) there would be no drag normally associated as the compresssor tries to compress air. Thus, the turbo would just free-wheel and spin up in rpms. Net-net, less rotational mass to overcome when you do want to start generating boost = faster boost response.
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Old 03-25-2010, 01:56 PM
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McGiver,

That's basically what Keith did but his BOV had an adjustable spring so he could just dial the spring pressure in. He says it worked really well. I can understand what he was trying to do but also don't know if there is any significant
change or advantage, Now, you have a healthy working unit and that would be prefect for clinical trials.

What a great idea !!!!!

Cole
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Last edited by cole930; 03-25-2010 at 08:07 PM..
Old 03-25-2010, 03:48 PM
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Synchronic makes a BOV that dose what I did by reconfiguring an aftermarket BOV.

Here is a Vid on the Sycchronic BOV: YouTube - Synchronic Blow Off Valve Turbo S2000

What I believed was keeping the BOV open at low load did was allow the turbo to spin faster in reserve as with the BOV loop around the turbo being kept open this would allow compressor pressure to be bled off under low load conditions and the turbo would keep spinning faster.

Then with acceleration the faster spinning turbo's potential could be captured.

What I did not think of and what seems to be the original intent of the Synchronic BOV was to reduce exhaust drag at cruse in search of better cruse MPG. Latter they discovered such a set up had performance benefits.

What I did find was doing this and setting my control pressure so it would dramatically lower immediately with acceleration (not at 5 psi boost) to get the metering plate moving quickly with first acceleration, combined to make a significant reduction in lag on my CIS turbo.

Last edited by 911st; 03-25-2010 at 07:00 PM..
Old 03-25-2010, 06:40 PM
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Quote:
Originally Posted by 911st View Post
Synchronic makes a BOV that dose what I did by reconfiguring an aftermarket BOV.

Here is a Vid on the Sycchronic BOV: YouTube - Synchronic Blow Off Valve Turbo S2000

What I believed was keeping the BOV open at low load did was allow the turbo to spin faster in reserve as with the BOV loop around the turbo being kept open this would allow compressor pressure to be bled off under low load conditions and the turbo would keep spinning faster.

Then with acceleration the faster spinning turbo's potential could be captured.

What I did not think of and what seems to be the original intent of the Synchronic BOV was to reduce exhaust drag at cruse in search of better cruse MPG. Latter they discovered such a set up had performance benefits.

What I did find was doing this and setting my control pressure so it would dramatically lower immediately with acceleration (not at 5 psi boost) to get the metering plate moving quickly with first acceleration, combined to make a significant reduction in lag on my CIS turbo.
Yes thats the one, thanks. Maybe they are the only ones doing that right now huh. It makes sense to me that lowering the back pressure by recirculating through an open BOV would help to keep the turbo spinning at small throttle plate openings. I'm just thinking out loud though...
Old 03-25-2010, 07:48 PM
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Again, I made my own but would go this way if doing it today as they seem to be the only other gear heads to have figured this out.

In my case it seemed like I got an instant .5 bar off cruse. Could swear I got instant .2 bar off idle.

I do not know if they make a 1" size.
Old 03-25-2010, 07:53 PM
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Brian,

Jump in here, I can't get my old head around this.

And the BOV is a Synapse not Sychronic. I have one and it is just a very well designed and very fast response BOV that recirculates just like the bosch. I got it because I've moved my fuel distributor and no longer have the 965 elbow. The Synapse gives me a lot of mounting and installation flexability.

Cole
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Old 03-25-2010, 08:34 PM
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I found this video while researching Tial BOVs.

YouTube - Filtering a Blow-Off Valve (BOV)
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Old 03-26-2010, 04:19 AM
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Quote:
Originally Posted by RarlyL8 View Post
How does routing airflow around the turbin during off-boost increase turbine response/threshold? I must be missing something. Seems that the opposite would be of benefit.
Quote:
Originally Posted by 911st View Post
Again, I made my own but would go this way if doing it today as they seem to be the only other gear heads to have figured this out.

In my case it seemed like I got an instant .5 bar off cruse. Could swear I got instant .2 bar off idle.

I do not know if they make a 1" size.
Brian,
Sounds like a Perpetual Motion Machine to me.
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Old 03-26-2010, 05:05 AM
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Brian,
Sounds like a Perpetual Motion Machine to me.
This would assume that the turbo will spin almost indefinitely when the engine is not running, which of course is not the case.

This is just my thinking at this time but I am not an engineer.

Assuming the turbo shaft is not put or kept in motion by the intake, most if not all the motion is made by the exhaust side. The compressor side is then a drag on the spinning motion and reducing that will make it spin sooner, faster, longer etc. When the throttle plate is wide open there is low pressure/resistance to the impeller. When the plate has a smaller opening during slow cruise then there is more pressure/resistance to the impeller.

I think the BOV being open during smaller plate openings will help keep the impeller spinning. I may be wrong but it just makes sense to me right now.
Old 03-26-2010, 07:04 AM
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THAT makes sense. Thank you!
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Old 03-26-2010, 07:24 AM
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Fourwheeler is correct. Energy can be stored in anything rotating according to the formula E=1/2IW^2
E = energy kW
I = rotor inertia Nm/s^2
It is just the flywheel effect. When the BOV closes the stored energy produces boost, which produces exhaust, which produces boost... and so on.

At some point though it would better to have the intercooler and everything to the throttle pressurized so that boost is available as soon as the throttle is open. That point depends on the engine speed and volume of the system.
Old 03-26-2010, 08:39 AM
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A normal BOV only opens on over run when the throttle is closed.

The Synapse BOV only closes with acceleration.

In theory I do not know which is better. With a stock style BOV the intercooler might be pressurized to a small degree on cruse (probably not at idle) and there would be vacuum in the intake manifold at cruse.


With the Synapse BOV the turbo seems to spin faster and in reserve so it can increase or establish pressure faster in the gross area between the turbo and intake valve. It also should reduce drag on the exhaust at cruse.

With my reconfigured BOV that functioned like the Synapse it felt like I had better throttle response off idle, off cruse, and between shifts (stayed at full boost with sport shifts).

I do not know if my system totally depressurized the turbo to throttle body or intercooler section. It might have maintained some resident pressure in the IC and still allowed the turbo to spin at a faster rate in reserve. I believe my BOV was based on a Forge adjustable BOV which flowed more than the stock Bosch plastic C2 BOV but was still only a 1" unit.

This was on top of several tricks I played with that got my CIS to act more like an EFI than CIS as to throttle response. The other significant one was lowering Control Pressure immediately and aggressively with any throttle input. This lowered the restriction to the turbo's suction side to reduce drag from the metering plate and get it moving faster. It also reduced the 930 CIS lean surge and produced an AFR more conducive to acceleration.

I also did not have to run a fat CO and I could still maintain a fuel efficient AFR at idle and cruse. Many run a 3 to 3.5% CO or about 13/1 AFR on part load to get the fueling they need for better off idle acceleration.

Then there is the ignition side...
Old 03-26-2010, 09:40 AM
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