Inlet Overrun Valve Modification? (BOV, lag)
 

........ Dose anyone have a factory digram of the Inlet-Overrun Valve?


Over the years I have read several times about some European tuners modifying the Inlet Over Run Valve on 930's to increase throttle response. The most recent was in Total 911, The Porsche Magazine -- March 2008 Issue 34.

Dose anyone know how this is done?

I once modified my C2 turbo so the BOV would stay open at idle and cruse to allowed the turbo to be spinning in reserve. When I accelerated and manifold vac went away, the BOV would instantly close. The result was faster boost response off idle and cruse. I saw an instance .2-3 bar off idle and .5 bar off cruse with this strategy so I truly believe there is opportunity here for 930's.

It would be great if we could find out or figure out how to accomplish this.

If anyone has any diagrams or pictures to post of the Overrun Valve it would be helpful.

Thx.

The artical noted above that the mods were by John Greasley / Dage Sport.

Based in Aylesbury, Buchkinghamshire in England. Could not find them on the internet but it sounds like there company name was changed to G Force Motorsport. Also of Blue Coral racing fame I believe.

I was going to try to email to see if they would share info on the mod but could not find contact info on the net.

I guess my hope being someone here might have some contact with them and might find out how the mod is done for the common good. ;)

I'm interested! I'll do some looking!

GForce got taken over by DS Automotives The Company - DS Automotives
Don't think you'll get much from them now as they mainly specialize in Jap stuff and the old Porsche chaps aren't there anymore, it's in the same location but that's about it

Keith, I'm not seeing how this would be possible. Allow me to be devil's advocate here:

(1) The compressor discharge is not a nozzle like the turbine inlet is so any reverse flow would most likely have little to no effect.

(2) The turbo spinning in reverse, even if it could, would not yield instant boost. It would be quite the opposite, in fact, creating tremendous backpressure in the exhaust tract.

(3) I'm not sure what effect the bov remaining open during steady-state cruise would have on the turbo speed but my instincts say it would be nil effect.

On the 965 Le Mans Turbo, some fuel over and above what was required to idle the engine was injected to create more exhaust gases and keep the turbo spinning faster between shifts. However, Porsche did not believe this method to be practical for road cars due to the additional fuel consumption.

It seems to me that, for snappier boost response, we would want the turbo to lose as little speed as possible between shifts. Garret has data to this effect when comparing their ball bearing turbo to the conventional mandrel bearing turbos like the KKK ones.

More food for thought from the air injection delete post.
Sparrot writes: Has anyone considering keeping the air injection for between shift anti-lag? My motorsports background is rallying. Many of Lancer Evo 5 & 6's (Toyota celica all-trac) had an air injection manifolds with a one way check valve connected to the dump valve. When the anti-lag was armed the ecu would use the dump valve connected to air injectors to dump all excess boost pressure to the hot of side of the turbo between shifts or throttle lift. The excess air would help burn excess fuel and keep the turbo spooled. Of course the rally car would also retard the ignition also so that unburned fuel would enter the manifold but just dumping the excess air works fairly well without destroying the exhaust like a full-on anti-lag does. I guess this might require EFI conversion, CIS may dump excess fuel.

Paul,

Thanks for the info. I think I said reserve, not reverse.

I have read about the Rally anti-lag strategy in the past. Cool stuff. They basically turn the hot side into a little turbine jet motor when engaged at the start and between shifts. Kind of shortens the turbos life span I would think.

---

If we vent the section after the compressor (and feed it back between the metering plate and the turbo intake) this reduces the pressure build up in the area outside the throttle body.

This allows the turbo to spin faster at idle and at cruse. Then when we accelerate the turbo will be spinning faster (in reserve) then if we had not and we start off with a jump in boost when we accelerate. Again, I did it with my C2 turbo by flipping my adjustable BOV's orientation and adjusting the spring tension so it stayed open at idle from the turbo's pressure pushing the BOV open and the suction pulling on the top of the BOV's piston. Then when I accelerated the vacuum went away and the spring tension would start to close the BOV. With boost, the top of the BOV would fill w boost pressure. As the top had much more surface area the boost was sufficient to keep the BOV closed against the pressure from the turbo trying to push it open. Worked pretty well for me.

However, not sure how we could mod the factory 930 overrun valve to operate that way or if it already may work that way.

PS, I think I remember there was something I read here that noted that turbo lag was increased with the removal of the stock 930 air injection system. This might even be in the Porsche factory manuals some where. Make sense as it increases the pressure and volume in the exhaust manifold from the mechanical compression and from it helping any unburnt fuel to burn further in the exhaust with the addition of oxygen. The cost is about 5-10 hp in HP on boost per one point of referance I have read.

Yes, here it the referance I saw on the air injection actually helps reduce lag. His point is that air injection helps reduce turbo lag.

A little off point but still very interesting.

The 965 air injection is directed to the exhaust ports for up to one minute after start-up; thereafter a diverter valve switches the flow over to the cat.
So for better spool-up on a 965, the simplest solution would be one of those nice Garrett ball bearing turbos. I'm planning on installing one in the future.

That works!

Sounds like an Anti-lag system like in rally cars would be more efficient for earlier boost. However this would require electronic timing control. Typically done by retarding timing by about 20 degrees. Destroys turbos but can idle at up to 1 bar, but typically less.

Dose any one have a digram of how the over-run-valve is set up and plumbed?

Again, it is not as sexy as turning ones turbo in to a jet but by releasing the pressure in the inter-cooler section of the plumbing when not accelerating it lets the turbo spin up faster because there is less drag on the compressor wheel.

Then when one accelerate if the bypass is closed one gets some instant boost.

On my car by manipulating the BOV I got an instant .2 to .3 bar boost off idle and .5 bar off cruse. This helped to wake up my car.

There is a company that has figured this out to and makes a new style BOV that dose what I am describing. If one has an aftermarket IC that uses the C2 or TT turbo style BOV you can just use that.

I believe others have done it on a normal 930 by modifying how the Inlet-Over Run-Valve which if we can figure it out would be a benefit for most of the Turbo owners here. ;)

[QUOTE]There is a company that has figured this out to and makes a new style BOV that dose what I am describing[QUOTE]

Which company?

For the early arrangement ('76 to '89) the compressor bypass is already open at idle isn't it? The piston is drawn back by intake vacuum, and this connects the upper manifold (including intercooler) to the compressor inlet.

The diagrams below are how I have my BOV installed. When I replaced my standard IC with a B6 long neck IC, a Forge BOV was included in the kit because the OE by-pass housing is deleted.
Normally, the BOV is installed upright, but as I understand it, when it is mounted sideways and if the spring is light enough, at idle and light throttle the vacuum on the top side of the piston will overcome the spring and hold the valve open. When there is manifold vacuum, this allows some air to by-pass the turbo compressor which reduces the load on the turbo and allows it to run at a higher speed off boost. As soon as any boost appears, the BOV is closed by boost pressure on the top side of the piston.

The effectiveness of this arrangement will depend upon the strength of the spring.
Don't assume that mounting the BOV sideways will automatically allow it to open at idle. The spring must be light enough to allow the valve to be opened by a moderate amount of vacuum, otherwise it will always remain closed until it gets a combined dose of full vacuum on the top side and boost on the bottom side (i.e. when lifting off throttle under boost).
As far as I can figure, mounting the BOV sideways effectively switches the inlet and outlet ports and the pressure that each port is exposed to. Because the areas of the piston exposed at each port are different, the operating characteristics are changed accordingly.

I am not convinced that my BOV is actually opening up under idle and I haven't figured out how to check it under operating conditions. I suspect that I might need a lighter spring, but haven't gotten around to following up on that yet.

I hope this is of some help in understanding how the BOV can work as a by-pass valve at idle/cruise.

http://forums.pelicanparts.com/uploa...1258125651.jpg

http://forums.pelicanparts.com/uploa...1258125666.jpg

http://forums.pelicanparts.com/uploa...1258125682.jpg

Bill,

You your digram is great (correction) and shows the BOV fliped. That is exacly how I set mine up. When I did mine some 6 years ago, the spring tension was adjustable and I used a hand vac to calibrate it.

I also flipped the BOV so boost was pushing on the bottom of the piston and vac was pulling on it. This ensured it was open on idle and boost.

On boost the piston surface area ON TOP was about 3 times that of the piston the boost was pushing against. With this when the vac went away to spring tension would help close the piston but it was the boost entering the top of the BOV with three times the effort that closed it and kept is closed on boost.

A second reason I flipped my BOV was I feared that side load might alow the piston to bind. With the change I thought there was less chance of this happening. There was twin turbo guy here that had there BOV stick and he sheared a Turbo Shaft.

Again, w a K27-7200 I had an instant .2-.3 bar boost off idle and .5 off cruse with this arrangement.

I added to the a way to lower control pressure at the WUR with acceleration. Thous two things together make a significant difference in my throttle response. This can be done with the D-WUR.

Roland,

Synapse makes the BOV that can act as an intake deverter. There is a link to a U tube video that showed it somewhere in this thread. http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/461279-advanced-cis-turbo-tuning-discussion-thread-ultimate.html

Speedy,

I do not believe the stock overrun valve stays open at idle but not 100% sure. Wish we could get a factory digram and or description to verify. Again, from articles on 930 tuners over the years I have seen hints as to modifying the valve to achieve increased throttle response which suggests that it is only used as an anti stall valve between shifts, not a bypass valve under low load. Also, with Porsches redesign of the 3.3 CIS motor for the 91 turbos the Bosh BOV was designed to function as only a BOV I belive.

Wow, that could work.
Keith, what vacuum setting did you use to set up your bov. I have a Forge hybrid bov with an adjustable spring. If you remember, I'll set mine up that way tomorrow and report back with results. The Forge bov is half the price of the Synapse, by the way.
Cool stuff.

It was 6 years ago so I am not 100%sure.

I think I measured the manifold vac at idle and cruse (via connection with my custom electronic-WUR's MAP sensor.) and used that as reference points.

I used a hand vac on the BOV with it off the car where I could watch the piston. You want it open at the lowest vac you expect to see and to close when that drops as you start to accelerate. Probably comming in on the less spring tension the better.

For this to work the BOV has to have larger surface area in the upper chamber that is hooked to the intake manifold for Vac-boost so it can push against the boost that will be trying to push it open from the pressurized / inter-cooler end.

Before you do this please get a base line feeling for how fast your boost responds by watching your boost gage off curse and idle. Also try blipping your motor from idle to see how it responds. Then after the conversion you will have something to compare against.

Good luck.

billjam,

Sounds like you are not sure if your BOV is acting as also acting as a turbo-deverter.

You might try taking the spring out for a test to see what happens. With that the vac and pressure from the I/C will definitely open is. Not sure but it should also close as boost starts to build but there may be a lag with out the spring untill the upper chamber fills with boost enought to close it.

You might also try putting something on the BOV to see if you can hear if it snaps shut when you blip the throttle.

Cool helpful thread.

I've got my recirculation valve in sideways now.
Mine is a TurboXS 1" C2 replacement valve.
The top is part is threaded and o ringed so you can unscrew it a few turns releasing spring pressure on the piston before it screws all the way off but the top side of the hollow brass piston is the same size as the bottom unlike the Synapse blow off valve Keith mentions and I think Cole has.

The side inlet that is the outlet when installed sideways has a 360 deg chamber around the piston so there is no chance of sideloading the piston and causing possible binding when installed upright.

A couple pics of it when things were partially installed and then with the hoses hooked up. You can see the brass piston in the first pic.
http://forums.pelicanparts.com/uploa...1258153079.jpg

http://forums.pelicanparts.com/uploa...1258153138.jpg

Then for the fun of it I decided to reinstall my deceleration valve like a blow off valve so it would let off a little boost pressure to atmosphere while the recirculation valve does it's thing when closing the throttle quickly between shifts under boost or letting off on the throttle while under boost.
I had the pieces old hose and hose clamps sitting around along with the unused decel vlave so it didn't cost me anything to try this.

It makes a nice little phooosh when letting off the throttle at around .6 - 1bar like an EFI blow off valve that I kinda like.
As expected dumping a small amount of the metered air to atmosphere during deceleration makes the AFR go rich to around 10:1 but I can't see a problem with that other than wasting some gas. There is a little less popping in the exhaust which surpised me, I thought it would pop more from doing this.
Here's a pic of it:
http://forums.pelicanparts.com/uploa...1258154143.jpg

The recirculationg valve is still there and it's 1" while the ID of the Decel valve is 7/16" ID for the outlet hose so most of the metered air is being recirculated.

I also soldered a small brass valve I had sitting around on to the T fitting in the vacuum line that opens the turboxs recirculation valve and the decel valve so I can close the valve and make the deceleration valve inoperable if I want while the recirculation valve is always functional.
http://forums.pelicanparts.com/uploa...1258154387.jpg

If I had a zork on the car maybe I would have some flames during high speed deceleration... I don't know, I like the sound of my partially gutted B&B single out muffler.

935's and 962's could have shut off the fuel during shifts but they didn't because the rich mixture in the exhaust during shifts cools the turbo even with the flames in the zork pipe where the hot exhaust mixes with air.
It's called turbo overun and you would see flames out the exhaust during shifts and throttle lifts during races.

and the car...
http://forums.pelicanparts.com/uploa...1258154435.jpg

J,

What an absolutely stunning car !!!!!!! Nice work thanks for the pics. very helpful when relating to what you have done. Very cool.

Cole.

Thanks man...
ahhh,, gonna go photograph a lingerie fashion show now.

could be worse:)

JF,

Very, very nice car. Hope you post your new pics from the show when you get back!

Saw you are getting better off idle response from the timing advance on another post. That is exciting!

There has been debate as to the value of the de-accel valve. I believe keeping it helps to keep the boost up between shifts and it acts is added protection in the case the BOV fails. (one twin turbo post here sheared a turbo when there BOV failed.)

If the air being bypassed is sufficient that you can hear it, it should be a benefit to keep it. I know of one 930 tuner that made sure it functioned as well as possible as part of his 930 program.

Looking at your C2 replacement BOV I am wondering if the size of the piston in the top chamber is much larger than the bottom piston as shown in your picture. There has to be a larger surface area of the piston that connected to the boost coming from the intake manifold to overpower the boost pushing from the inter-cooler section for it to close when mounted this way. On my BOV it was about a 3/1 ratio of surface areas.

I believe running as low of back pressure after the turbo as possible, more timing advance off idle, relieving pressure from the turbo under low load, and reducing control pressure to the metering plate with first loss of vac -- all together can transform a CIS car.

Any chance you have a digram from the factory lit on the stock 930 Inlet Overrun Valve and any writing's Porsche might have written to describe its function?

The best.

Forge BOV
 

I removed my Forge BOV today and checked its opening pressure.
With 5" Hg vacuum, the valve just starts to open and it is fully open at 9" Hg.
While this is not much as far as vacuum goes, I thought I'd do a bit of tuning on it to reduce the opening vacuum even further.
The best way to do this would be to use a lighter spring, but I don't have one.
When assembled and the valve is closed, the spring is compressed by about 6mm so I machined a total of 4mm from the spring seats inside the piston and the cap so that the spring now has only about 2mm of preload when the valve is closed.
I also machined 2mm off the length of the piston so that it opens up 2mm further.
Now the valve starts to open at only 2" Hg and is fully open with 7" Hg vacuum.

I haven't had a chance to test drive today, but if there's any improvement, I'll let you know. I don't expect to see a lot of difference, but I guess every little bit helps :)

http://forums.pelicanparts.com/uploa...1258213844.jpg

Bill,

You've got skills! Hope it works out. Your valve looks a lot like mine did except my top allowed adjustment.

I have to say that this mod makes an improvement but did not seem way dramatic at first but I did find it noticeable. It was when I noted my boost response looking at my gage that I know it made an improvement. If I think about it. If boost is raised .2 bar off idle that is about a 10% increase in power at a very low power area.

Most will still have the lean surge that comes with CIS for most. I further modified my C2T so I did not have the lean surge so the two together might have added 10-20% more of an increase in first response power. Add to that optimization of timing and minimization of back pressure and it starts to become significant.

Saw a flow test on BOV's once that compared I believe 5 different units and a ported unit. There was someone that ported them and there was a difference between valves and between a stock and ported BOV. The more flowed the less the turbo is going to stall between shifts. If you think about it the 993TT and later have two BOV's so twice the venting. Thus, a larger BOV or twin BOV's could be something to consider.

Size is everything
 

Dead right!
Looking at my BOV yesterday, it dawned on me that regardless of when it opens, the air flow passage through the BOV (with 20mm inlet and outlet) is really quite small compared to the size of the inlet tract that we are trying to by-pass. I did think about "flowing" the valve but there isn't much meat to work with so I opted to just increase the piston stroke a little, but a much bigger one (or two) would be so much more effective.

Ya, the 993TT has two Bosch style BOV's on a 400hp car. The 996TT at 420 hp has two...

Hey Keith, you said you wanted to see a few from last night so...
babes and blow off valveshttp://forums.pelicanparts.com/uploa...1258260474.jpghttp://forums.pelicanparts.com/uploa...1258260533.jpg
http://forums.pelicanparts.com/uploa...1258260718.jpghttp://forums.pelicanparts.com/uploa...1258260877.jpg

You are living the life my friend!

hey nice wheels last time i saw your car you had turbo twists. These look better. nice shots from the show, a great thing about south Florida all the beautiful women. I havent dated anyone older than 30 since my divorce, great place to be single.

But you have two intake tracts, remote from each other, to vent. Adding a second bov to a single turbo car gains nothing, IMO.

Paul,

Yes, they do run two probably because of the split turbo section however all the factory turbos joining to a joined track before the throtle body and before hitting the intake manifold so a single could be done.

Still, they effectively end up with twice the anti surge capacity for a motor that is withing 10% of the size of a 930's.

I do not know if we would benefit in terms of surge protection however increasing the capacity of that circuit is proven to reduce RPM drop between shifts.

That is part of the sale of aftermarket BOV's, increased capacity. There was at least one company that ported them.

My interest is more in keeping the turbos spinning faster in reserve at idle and cruse by keeping these open. More flow capacity meant less drag on the turbos on part load (increased MPG to?).

I was searching on BOV orientation and found this.
My BOV is piped as Bill's is in #16. It has always been this way, as it was the only way that made sense to me. I think it is the same kind as Bill's also. I bought it used, and it has easily changeable springs and is really shiny.
Pat

JFairman, nice car, BTW.