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.