I was trying to help a customer our with some info for his new Forge diverter valves he got from us. With his searching (international customer), and mine, we came across some good info I wanted to share. I posted it on our tech board here - http://www.vividracing.com/catalog/pages.php?cID=10

The first is an article found on Audiworld.com which is a comparison between the Forge valves and the factory Bosch ones from a 2.7T Audi S4. Virtually the SAME valve as in the 996TT.

Here it is -

I chose the Forge valves over other possible choices for several reasons:

• Forges are serviceable. Both the piston and the o-rings are easily replaced should the need arise.
• I preferred the black finish over other finishes available.
• Forges were readily available
• They fit into the stock location and look totally stock. Some valves I looked at would not fit in the stock location easily.

The Stealth Forge Valve
click to enlarge images

• Stock valve vs the Forge
• The stock valve uses a rubber diaphragm while the forge uses a solid piston.

• Hard to see, but two o-rings are used as a seal between the valve body and the piston.
• The piston seats against the angle at the bottom of the valve

• The piston
• I don't know what it's made of (some type of plastic or nylon) but it's clearly not going to tear like the flimsy rubber diaphragm of the stock Bosch valve.

• The valves in place. They are just a smidge bigger than the stock valves but fitment is not a problem.
• To the untrained eye they look just like the stock valves.
• The stock vacuum hose for the LH valve was too small. I had to replace it with a larger hose.

Testing
The Forge is available with two spring rates -- a green street spring and a blue race spring. I'm using the street springs. A friend has the blue race springs. So how does the valve compare to the stock valves? The Forge is a hefty well crafted piece that dumps pressure quickly when opened then firmly snaps closed when vacuum is released from the nipple. The stock valve? Well... uhhh, it's uhhh, crap. Sometimes it sealed on release... sometimes it didn't. Simply pathetic for valves with only a few thousand miles running stock boost pressure.


To compare valves I did a few simple tests. First, I checked to see how much vacuum was required to pull the valve open with 1 PSI of pressure on the on the outlet side of the valve. I used slight pressure on the outlet so I could hear air leak through the valve when it started to open. And, I checked how much pressure was needed to push the valve off it's seat to compare spring pressures. The way the valve functions on the S4 boost pressure is applied to the back of the piston (via the nipple) so boost pressure actually holds the valve closed when you're on the throttle. The spring rate does not indicate how much boost pressure the valve will hold but rather how hard it is to pull it off the seat with vacuum. So, when I say it took X amount of pressure to push the valve open that doesn't mean that boost pressure is going to push the valve open. Engine vacuum pulls the valve open. I planned to have another brand valve in this test but discovered they were not in proper working order so I've excluded the results.

Forge vs Stock Bosch Bypass ValveStockForge StreetForge RacePull open (inhg) less than 1"2"6"Push open (PSI)359.5
In the graphs below the red "Requested Boost" line is the boost pressure the engine computer was calling for. The red line abruptly dropping is the result of lifting off the throttle to shift gears. The steeper the positive slope of the red line the sooner I was back on the throttle. I did not stay into 3rd gear very long... I was more interested in seeing how boost recovered (and not getting a ticket). This data was captured with VWTool using data block 115. These graphs represent boost pressure on full throttle 1-2-3 runs.
Note that boost pressure held between gears is influenced by the time I took to shift. I will be doing additional testing without shifting (just chopping and reapplying the throttle) so that boost hold and recovery times are more consistent. Note that "Stock Stock" means: Stock valve, Stock boost. "Stock Chipped" means: Stock valve, chipped boost... and so on. All runs were performed on the same car with the various valve configurations.

 

Now you can fine tune your Blow Off/Diverter Valve to the exact needs of your engine with this complete tuning kit.


Your valve tuning kit you will find four springs and three spacers.


The strongest spring is the red, followed by the blue, yellow and finally the green which is the weakest. The spacers enable you to fine tune the settings between two spring tensions if required.


We normally supply our valves, unless requested differently, with a spring that will match the tension of the manufacturer's original fitment valve. However, as many owners undertake tuning modifications some may wish to re-tune the valve characteristics to suit a given application.


If you are finding that the valve is dumping more than required or you are loosing boost pressure it is normally an indication that a stronger spring is required. Conversely if the valve is failing to dump it may be necessary to install a weaker spring. If during this tuning of your valve you are faced for example with the red spring being to strong and the blue spring to weak, you can add the spacers to the blue spring to increase the tension by small increments, thus achieving a setting between the red and blue springs.

Valve Spring color coding
Green - 5-15 PSI
Yellow - 15-23 PSI
Blue - 23-30 PSI
Red - 30 + PSI
The above figures are estimates only and should only be considered as a guideline

Please note - fitting a stronger spring into a valve will not increase boost pressure unless existing pressure is being lost. Failure to allow the valve to operate by installing a spring which is to strong may cause damage to the turbocharger.

 

If you are using Forge Diverter Valves, you may want to know how to change out the springs or service them to keep their optimal performance. They are very easy to maintain to keep your boosted motor lasting long. Check out this Service Manual by Forge Motorsports on their Bosch replacment diverter valves.

 

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so what do you recommend on a flashed car?

 

Even though each valve comes with a spring and shim kit, I always run the stock spring that is included. I have had good luck with that. I run about 1.1 bar and have no leaks at all.

 

If I were to install a set on my 2005 Turbo S Cab which has the Revo Stage 2 Flash to 1.0 bar which spring color should I use?

 

Again I would use the spring that comes in the valve out of the box. I would not change it. That would be Yellow.