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BOV orientation

Hey kids, I'm about to embark on dissecting my stock boost recirculation piston manifold assembly, and using the various parts to link up with my shortneck IC. I have a nice BOV/.bypass valve (popular piston type, not diaphram) and whilst researching various installation options, I came across the following post regarding vertical vs. horizontal mounting of the BOV and supposed benefits of feeding the boost to the bottom of the BOV vs. the side port. Keith has been preaching this for a long time and I finally understand what he was trying to tell us. See the link:

Inlet Overrun Valve Modification? (BOV, lag)

Benefits include allowing the turbo to free-wheel (i.e., spin freely without producing any boost) when at idle as well as at steady state no-load cruise...the theory being that a turbo spinning fast under no load will spool up quicker when the throttle is opened and the BOV valve closes.

I've ran this around in my mind, recalling the old physics classes from too many years ago, and a good healthy dose of logic, and it would appear that this makes sense. But the spring tension in the BOV would have to be something considerably less than the max boost you're running, in order to allow the valve to open under the relatively lesser amount of vacuum seen when at partial throttle vs. a completely closed throttle valve. The premise is that with a weak spring and very little boost pushing on the piston, vacuum alone will overcome the spring tension and open it enough to redirect (to the intake side of the turbo) the small amount of boost generated under cruise conditions. Thus, when the pressure on both the in and out sides of the compressor are identical, it will spin as fast as it can with no load.

Also, you need to visualize four forces controlling the BOV....the boost pressure trying to push the piston open, the spring pressure trying to keep it closed, the vacuum (when present) at the top of the piston trying to open it, and the vacuum-line-turned-boost source when the throttle plate is opened fully and that small vacuum hose is now pressurized as boost builds in the intake manifold. With all these forces, the key lies in the fact that whatever boost pressure is applied to the bottom of the piston to force it open will be exactly the same boost pressure that's being applied to the top of the piston..pushing down. Net-net, the piston does not move under boost, even with a weak spring in place. But as soon as the differential pressure between the top and bottom of the piston changes (creating negative air pressure above the piston), it will open (as in when the throttle plate is suddenly closed at shifting, or when the throttle is held at partial steady-state cruise and a small amount of vacuum is created).

With this understanding, one need only determine how soft the spring needs to be. My BOV came with a stiff spring to hold against about 1.0 bar, and shims to make it even stiffer if wanted....because most people depend on the spring tension only to keep it closed. Theoretically, the spring tension in the above configuration would need to be only slightly less (inversely) as the amount of vacuum available from the throttle body at steady state cruise...so that small amount of vaccum can overcome the tension and open the valve.

Just thought I would spend the evening rambling. Does anyone know how many inches of mercury vacuum is generated in closed throttle plate situations vs. partially open steady cruise throttle positions? Or does anyone think this is all bogus and not worth the effort of fine-tuning?
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Old 12-07-2009, 07:40 PM
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Quote:
Originally Posted by mark houghton View Post
With this understanding, one need only determine how soft the spring needs to be. BJ: As soft as possible. My BOV came with a stiff spring to hold against about 1.0 bar, and shims to make it even stiffer if wanted....because most people depend on the spring tension only to keep it closed. Theoretically, the spring tension in the above configuration would need to be only slightly less (inversely) as the amount of vacuum available from the throttle body at steady state cruise...so that small amount of vaccum can overcome the tension and open the valve.

Just thought I would spend the evening rambling. Does anyone know how many inches of mercury vacuum is generated in closed throttle plate situations vs. partially open steady cruise throttle positions? Or does anyone think this is all bogus and not worth the effort of fine-tuning?
I recently modified my BOV after following the thread you referred to.
I set it up with as little spring pressure as possible so that the slightest vacuum on the top side of piston would keep the valve open. When driving, I couldn't tell the difference between having more or less spring pressure or having the valve upright or horizontal!
Post 28 in that thread is what it all boils down to ... these little 20mm BOVs can't pass enough air to make a noticeable difference IMHO. I can fart more powerfully

As far as vaccum readings are concerned, the red line is what I usually see at light throttle cruise and the green line is throttle closed on over-run.

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Old 12-07-2009, 08:01 PM
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Mark,

I've heard you can get .2 bar of boost as soon as you accelerate if you set your BOV up that way. Might consider a adjustable BOV so you can play with different opening settings.

Cole
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Old 12-07-2009, 08:47 PM
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I remounted my BOV sideways, but I did a couple other things that improve drivability at the same time so it's hard to say if it made a difference.
Seems to work about the same, but my 25mm TurboXS "bosch replacement" BOV doesn't have a real light spring.
It does have some spring tension adjustment and it's easy to screw the top off to try lighter springs.

A BOV that has more surface area on the back side or vacuum line side of the piston would work better this way with a light spring because when boost comes on there would be more boost pressure pushing the piston shut from the back side.
The brass piston in the one I have has the same surface area on the both sides so mine needs a fairly strong spring to hold it shut while accelerating with boost.
There are so many aftermarket blow off valves to choose from these days... who knows which one would work best.

Seems the Synapse BOV is well liked by those that have tried it.
Old 12-08-2009, 08:14 AM
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Quote:
Originally Posted by JFairman View Post
I remounted my BOV sideways, but I did a couple other things that improve drivability at the same time so it's hard to say if it made a difference.
Seems to work about the same, but my 25mm TurboXS "bosch replacement" BOV doesn't have a real light spring.
It does have some spring tension adjustment and it's easy to screw the top off to try lighter springs.

A BOV that has more surface area on the back side or vacuum line side of the piston would work better this way with a light spring because when boost comes on there would be more boost pressure pushing the piston shut from the back side.
The brass piston in the one I have has the same surface area on the both sides so mine needs a fairly strong spring to hold it shut while accelerating with boost.
There are so many aftermarket blow off valves to choose from these days... who knows which one would work best.

Seems the Synapse BOV is well liked by those that have tried it.
The BOV I purchased is from a company called Tuning Alliance. Never heard of them, just got a good deal on Evilbay. 34mm piston, the top being larger than the bottom where it seals, screw top and stiff spring. Would be easy enough to modify as fully adjustable. See the BOV/bypass valve on the left in the picture:



I'm not entirely sold on the whole concept of my post. Rethinking everything, the turbo may spin maybe a little faster with no load on it (when not compressing air), so perhaps there would be a little less lag in getting the turbo up to speed when the BOV closes. Unless I hear more than one success story, I'll install the valve however it best fits functionally and esthetically.

More concerning to me are the occasional comments about these small BOV's not being large enough to pass enough air to really accomplish what we think. Maybe that's why Porsche used the boat anchor instead, to really move some air and prevent turbo stall. Are we fooling ourselves, and are our turbos stalling more with the aftermarket BOV's we install, and the only real benefit of this retrofit is in reducing the quantity of air that needs to be compressed?
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Old 12-08-2009, 09:03 AM
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Quote:
Originally Posted by billjam View Post

As far as vaccum readings are concerned, the red line is what I usually see at light throttle cruise and the green line is throttle closed on over-run.
Thanks Bill. Your gauge looks like 23.6 inches of vacuum when the throttle plate is closed, and about 7.9 inches at cruise. Where is your boost line connected...directly beneath the throttle body plate or further downstream in the intake manifold?

Assuming I will see similar vacuum at cruise, then I would want to calibrate my spring tension to an equal and opposite force, which would be 3.88 Psi to counteract the vacuum and keep the piston closed. Pretty light spring tension, yes.
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Old 12-08-2009, 09:19 AM
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The one on the left of your pic from your post above looks alot like mine, accept mine doesn't have interchangeable fittings on the one end.

The black rings on the top part are orings in the grooves for cosmetics and getting a grip on it to unscrew the top or adjust spring tension.
There's another important oring on the inside to seal the top to the frame even if it is partially unscrewed to lighten spring pressure.

The brass piston in mine is hollow and about 2" long and the majority of the spring is compressed inside it.
The piston doesn't get wider at the vacuum line end so surface area that boost pressure will push against is the same on both sides of the piston but there is a chamber 360 degrees around the sides of the piston just above where it seats so that would improve air flow through it when the piston is off it's seat and the BOV is open.

Here mine installed sideways:

The blue hose goes to a fitting on the back of the intercooler.

When Porsche first made the 930 in '75 I don't think there were any small lightweight blow off valves available like there are today, It was early street car turbo development so they made what they did and it is probably bigger than it has to be.

edit:
I don't have a vacuum gauge in the car but I've hooked one up at idle and my car idles at about 15" vacuum.

I also reinstalled my deceleration valve on the hose fitting made for it on the back of my intercooler and have it venting metered turbo boost air pressure to atmosphere when letting off the gas along with the BOV recirculating air into the big rubber hose.

The deceleration valve opens at 5" higher manifold vacuum than the TurboXS BOV so it opens after the BOV and not completely unless you're decelerating at speed in gear.
The decel valve's exit hose connection is only 7/16" inside diameter so it doesn't flow any where near as much air as the BOV.

Last edited by JFairman; 12-08-2009 at 09:42 AM..
Old 12-08-2009, 09:30 AM
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Quote:
Originally Posted by JFairman View Post

The black rings on the top part are orings in the grooves for cosmetics and getting a grip on it to unscrew the top or adjust spring tension.
There's another important oring on the inside to seal the top to the frame even if it is partially unscrewed to lighten spring pressure.

The brass piston in mine is hollow and about 2" long and the majority of the spring is compressed inside it.
The piston doesn't get wider at the vacuum line end so surface area that boost pressure will push against is the same on both sides of the piston but there is a chamber 360 degrees around the sides of the piston just above where it seats so that would improve air flow through it when the piston is off it's seat and the BOV is open.
Internally mine is built exactly the same as yours. I took it all apart the other day to lap the valve seat just because I could. I think I'll be happy with it once installed. Great price at $36 brand new.
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Old 12-08-2009, 09:44 AM
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It worked well for me. Again I would get instantaneous .2 to .3 bar off idle and .5 to .6 bar off curse, and maintain full bost between sport shifts w boost measured after the throttle plate. This was one of several mods I did to increase pre-boost response including a low restriction exhaust, dramatic lowering of control pressure upon acceleration to reduce restriction of the metering plate, and ideal AFRs near 13/1 off idle and cruse with acceleration instead of the typical CIS lean surge. I was running a K27-7200 which may show better at idle than a 7006 but a HF based on the 7200 hot side should show about the same or better.

In the pic of the BOV above the difference between the top and bottom surface area is much more equal than mine. I think my surface area was about 3 to 1 (Forge externally adjustable?).

The Synapse BOV figured it all out for everyone. Might be easier to just look them up and get the recirculation style. They come if different sizes I belive.
Old 12-08-2009, 10:43 AM
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"It worked well for me. Again I would get instantaneous .2 to .3 bar off idle and .5 to .6 bar off curse"

If it was instantaneous .2 - .3 bar off idle like you have said, it seems like a street car would lurch forward right off idle and be difficult to drive and dangerous in traffic.

I can see how it would keep a turbo spinning faster at idle and cruise when a normal BOV may be closed, but instant .2 - .3 bar is alot and seems like it would be impossible to drive the car smoothly at low speed.

Also if you were "maintaining full bost between sport shifts w boost measured after the throttle plate" than the car would never slow down and keep accelerating because full boost after the throttle plate is the same full boost in the intake ports.

I like your stuff, but I'd have to hear this from more than one source or experience it myself to believe it.
Old 12-08-2009, 11:15 AM
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No lurching but interesting thought. Just a more responsive fealing or fealing of less lag.

Thinking out loud along those lines, if we are making 20hp off idle, .2 bar is about 5% more or one more HP. Even a stock turbo is makeing some boost outside the TP at idle. This just reduces the drop in pressure as the intake maifold is pressurized and the turbo has to respond to the new demand. If we are up to 50hp by 1500, .3 bar is about 15% more hp or about 57hp. It just comes in faster with less lag.

If at cruse if you are making say 30hp and drop the throtle there is a sec of lag as the turbo has to respond to equalize pressure. This reduces that lag time. Anyway .6 bar might jump you to 45hp a sec faster and quickly build from there. At least that is my guess.

It is not like trigering NOS and getting a quick 50 or 100hp, just less lag time and more NA like response.

I may not have stated my point well or accuratly as to shifts and agree with what you are saying. However I recall I did not have to wate for boost to build back between shifts and it seemed like my boost guage stayed pegged.

The improvement was not as dramatic as when the factory BOV had failed and it is fixed. However before the improvement it seems like it took some one or even two sec to fully rebuild boost between shifts. With the change it seemed a significant improvement to where I stayed on full boost between shifts instead of wating for boost to come back. There was no longer any detectable lag between quick shifts.

I was mosty trying to make my point well enough for others to want to try it and for them to try to develop a base of referance to work from. It more or less gives one a little head start and reduces inital lag. After a sec or two of WOT there is probably no differance and it dose not add any peak HP.

CIS is know for its lag and many spend a lot on exhausts, turbos, light weight flywheels and other goodies to try to minimize it. This is just one of many things that can be done. When I got done developing my 91 Turbo with this and a couple of other goodies the differance was night and day in lag reduction and I already had B&B's, ports, cams, larger IC... to start with.

I do not know if this is any validation but the "Synapse BOV" is well received and I think took a top product award at SEMA one year. It dose the same thing. If I do not have the credibility, see what they and others have discovered.

There is another step up from this to an anti-lag stragy where the turbo blead between shifts can be sent to the exhaust primary tubes to further energize and spin up the turbo in reserve. Some of the early non Porsche street turbos were even delivered with the plumbing in place so it could be use in racing. Could hook it up to the exhaust air injection manfold.

Fun stuff. Another fun thought is most guys send a lot of time wanting to improve turbo lag. However, they pull there air pump to save a couple of pounds and 5 hp. However, doing so effects turbo lag. Same with removeing the de-accel valve. Doing so has some impact on turbo recovery time and removes the only safety margn if the anti surge valve fails. Probably another "Belive it or not" moment I guess.

I love CIS.

Last edited by 911st; 12-08-2009 at 12:46 PM..
Old 12-08-2009, 12:44 PM
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Quote:
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There is another step up from this to an anti-lag stragy where the turbo blead between shifts can be sent to the exhaust primary tubes to further energize and spin up the turbo in reserve. Some of the early non Porsche street turbos were even delivered with the plumbing in place so it could be use in racing. Could hook it up to the exhaust air injection manfold.

Another fun thought is most guys send a lot of time wanting to improve turbo lag. However, they pull there air pump to save a couple of pounds and 5 hp. However, doing so effects turbo lag.
I'm one of those guys that no longer has the air pump on the car...though not by my choice as it had been removed before I bought the car. I like the concept of directing some of the boost to the exhaust primary tubes. Since I still have the injection manifolds and injectors in place, that may be worth investigating. Win-win situation, save the weight and HP needed to run the pump by removing it, but still get the antilag benefit by injecting boost air. Interesting.

So, the question now becomes....does one direct 100% of the boost when shifting to the air injection, or just a portion and let the rest get redirected to the front of the turbo? I vote for all of it.
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Old 12-08-2009, 01:19 PM
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I am not totally up on this but I would expect it would require a later style C2 style BOV and all the bleed off boost would be directed to the exhaust ports.

One issue is with the CIS, any air that is bled off the intake has been measured which is why we recycle it back into the intake before the turbo on over run.

Thus, this will create a rich condition. This may or may not be an advantage or problem as we send both fuel and air down stream to the exhaust manfold. It might spin it up like a turbine.

Might look at the ignition to insure one gets the 10 deg of retard to send a bit more heat down the exhaust between shifts at the same time to.

Some potential for someone wild enough to try it.

Sorry, got carried away again.
Old 12-08-2009, 02:34 PM
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Mark,

The first referance is where I learned about the air injection anti lag.


Last edited by 911st; 12-08-2009 at 04:09 PM..
Old 12-08-2009, 02:41 PM
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Originally Posted by mark houghton View Post
Thanks Bill. Your gauge looks like 23.6 inches of vacuum when the throttle plate is closed, and about 7.9 inches at cruise. Where is your boost line connected...directly beneath the throttle body plate or further downstream in the intake manifold?

Assuming I will see similar vacuum at cruise, then I would want to calibrate my spring tension to an equal and opposite force, which would be 3.88 Psi to counteract the vacuum and keep the piston closed. Pretty light spring tension, yes.
My boost/vacuum connection is at the throttle body and below the butterfly (port III in the diagram).

I have tried a few locations (including brake booster) but this gives best results for me. When tee'd into the brake booster hose at the engine, gauge indicated 0.2 to 0.3 bar LESS boost for some reason. Port III boost readings are also very similar to standard boost gauge, which is some degree of comfort.

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Old 12-08-2009, 02:55 PM
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I am not totally up on this but I would expect it would require a later style C2 style BOV and all the bleed off boost would be directed to the exhaust ports.

One issue is with the CIS, any air that is bled off the intake has been measured which is why we recycle it back into the intake before the turbo on over run.

Thus, this will create a rich condition. This may or may not be an advantage or problem as we send both fuel and air down stream to the exhaust manfold. It might spin it up like a turbine.
Might look at the ignition to insure one gets the 10 deg of retard to send a bit more heat down the exhaust between shifts at the same time to.

Some potential for someone wild enough to try it.

Sorry, got carried away again.
You may be correct, and I may just be twisted enough to try it. So what if she goes a little rich, if the result is turning your turbo into a jet turbin for a second. It would be real simple to connect the bypass end of the BOV to the air injection spider.
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Old 12-08-2009, 03:04 PM
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One dose not want to use I or IV as they are modified vac connections w one just inside and one just outside the throttle plate and they move to the opposite side of the plate with just a little throttle. Thus, they will give some wild signals. Something like one will read vac at idle and boost w lite throttle. The other boost at idle and vac at cruse and boost at WOT. Fun stuff.

Not sure about the others but III & IV look like they are below the TP and should be ok.
Old 12-08-2009, 03:11 PM
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Someone posted those anti-lag links farther back in the timing thread.

It mentioned retarding the timing around 40deg after TDC during gear shifts so the mixture is still burning when the exhaust valve opens.
Venting metered air to atmosphere would increase the afterburner effect and secondary combustion in the exhaust headers.
It also said they had 1.2bar at idle in the Subaru rally car from all the combustion going on in the headers.
Sounds crazy for anything other than racing and trying to be competitive within the rules.

The intense heat kill turbos and exhaust headers in a short time, and if it wasn't a factory engineered system in a water cooled engine with thick wall exhaust headers it would probably be an endless source of very expensive repetitive problems.

In my opinion setting up the 930 emissions air injection diverter valves to inject more air during deceleration is another bad idea. Especially from a turbo making around 14.5psi into an exhaust manifold that would have 25psi or more from all the combustion going on to spin up a turbo while the engine decelerates.

I don't think the small volume of air you can get through the factory spider and air injector nozzles would help anything and if you've ever removed those steel air injectors on a few 930 engines you'll see that the tips of them are often melted or burned away.
Add more air through them and they'll get even hotter, so where do the melted burnt pieces of steel air injector go.. just where you don't want them - right into the spinning turbine wheel.
Old 12-08-2009, 03:13 PM
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Quote:
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There is another step up from this to an anti-lag stragy where the turbo blead between shifts can be sent to the exhaust primary tubes to further energize and spin up the turbo in reserve. Some of the early non Porsche street turbos were even delivered with the plumbing in place so it could be use in racing. Could hook it up to the exhaust air injection manfold.
I concur with JFairman on this.
The volume of air from this system wouldn't be enough to make a noticeable difference. If you could burn some fuel and air during this period, the expanding gases would indeed drive the turbo.
That's what the factory did on the 965 LM car to keep the turbo spolled up. It wasn't incorporated into the road cars because of the negative impact on fuel consumption.
As for the heat killing headers, I would bet the Suburu headers were made from Inconel (as are a lot of high end racing headers are), which is a nickel superalloy which will withstand a lot of heat.
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Last edited by 911nut; 12-08-2009 at 03:17 PM..
Old 12-08-2009, 03:13 PM
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You may be correct, and I may just be twisted enough to try it. So what if she goes a little rich, if the result is turning your turbo into a jet turbine for a second. It would be real simple to connect the bypass end of the BOV to the air injection spider.
Would love to see ya try it.

Others have vented BOV to atmosphere and say it is a non issue. Running to rich is only creats drivabilit issue under power, not so much on throttle lift. It will probably increase the fuel making it to the turbo and muffler for the fun crackle some seem to like.

Go for it!

Maybe the wast-gate will act as be your safety valve against excess pressure.
Old 12-08-2009, 03:17 PM
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