Ahmet

I'd like the ability to to move a little bit more air than one stock (or one 993) valve can move. I'm pretty sure as is most of our cars would get pretty violent pressure spikes between the turbo and the intake manny during shifts. I'd like to hook up a pressure gauge there and watch between shifts but I doubt I'll ever get around to it.

Anyway, I wouldn't mind a slightly rich mixture during shifts, but I don't want it too rich either. So I want to run one to atmosphere and one to recirculate, to get a bit of both and if I can use a second cheap valve I can get for less than $20 (through the shop) all the better.
Ahmet

tecart

can someone tell me if the FORGE valve is stiffer then the SAAB valve spring? if it matters i use the 993 valve and i too run 25 psi of boost to redline, then it drops to 21 or so, what would a stiffer spring do for me in this case? I dont see what the fuss is all over if i get these numbers with a big kkk29 #8 and i use the 993 valve in stock configuration, please show some evidence of fact that the 993 valve isnt up to par, im not saying i wont switch but i need some proof/ dyno test anyone, switch 4 valves and do 4 pulls at 18 psi, do the stock, 993, saab, forge valve test, please someone, ill contribute 25 bucks to anyone who will do this test

white 944 turbo

Ahmet, my car is in NY and I am in CA right now so I cant check it out on my car but I do have a friend out here with the same valve so I will ask him to do this test. I will let you know hopefully tomorrow.

white 944 turbo

Tecart, the best I can do is refer you to a site called audiworld.com/tech then once on that site you click on engine then scroll down to torn bypass valve. This will take you to an experiment that Don Pavlik did comparing the stock audi bypass valve to the Forge valve. Remember that the stock Audi valve used in the experiment is very much inferior to the Saab valve but maybe you can get some info that will be helpfull. By the way your boost going from 25 to 21, could it be your wastegate?

Transaxle

On mild modified streetcars I use the HyperBoost diverter valve from Stratmosphere. It works great and you can change the spring if you need a stiffer one.

<img src="http://www.transaxle.net/ta/Sections/tinfo/umluft/compare-open.jpg" alt=" - " />

There is a new adjustable version available, but I had no time to test it.

white 944 turbo

Ok, go on line and just type in The great diverter valve face off. You will see a big comparison with most of the big guns including the hyperboost that transaxle kindly showed us. Of course in this extensive test the SAAB valve was not compared,darn it, but the porsche valve was. I think you will all find it interesting.

TurboTim

In my experience plastic and rubber is no match for aluminum, stainless and silicon:^) I also know that the stock plastic valve is not really the best thing to use for speed density systems(MAP) since it is open at idle. This causes low and inconsistent KPA readings which can give you grief when you are trying to dial in your idle. The valves we have used with the least amount of problems are: Greddy Type-S, Godzilla, Tial(yes they make a blow-off), HKS, Blitz, to name a few. All of these valves offer some sort of an adjustment too so you can use them on MAF and MAP systems.

TonyG

I don't know where people get the data that the stock blow off valve opens up under high boost.

It doesn't.

I previously, on this list, saw someone post that they tested the valve by pressurizing up the discharge of the valve while it was off the car, and that it opened up at "x" psi boost. This test is flawed and doesn't reflect what happens to the valve on the car under boost.

On the car under boost, the valve has a spring which exerts "x" amount of force on the valve to keep it shut. BUT.... when you're under boost, say at 20 psi, you also have 20psi of air pressure forcing the valve shut PLUS the "x" amount of spring pressure. On the other side of the valve you only have the 20psi of boost on the high pressure side of the valve.

Given this, how can the valve open? It can't. Not until you let off the gas, and the boost drops.

Transaxle

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by TonyG:
<strong>I don't know where people get the data that the stock blow off valve opens up under high boost.

It doesn't.
</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">There is a significant time-delay if the boost changes. Since it is hard for me to descibe it correctly in English, I want to use this drawing to explain our measurements:

<img src="http://www.transaxle.net/ta/Sections/tinfo/umluft/umluft.jpg" alt=" - " />

There is a long and small line leading the boost (p2) to the valve. It takes some time for the pressure to go through the line. That causes situations, where the pressure at p1 is higher than the actual pressure at the top of the valve.

The effect is the same that's used in the APE-style banjo-bolt. The pressure signal is delayed.

In high-boost situations with small throttle-movements the valve may open if the spring is to weak.

We use the hyperboost valve with stiffer springs and a larger pressure-line direct from the intake to the valve to work against the effect.

white 944 turbo

That was nice transaxle. I would like to add that a weak spring can cause problems with the seating of the valve when the boost goes beyond the threshold. That is why a piston design found on the after market valves is superior. However I still maintain that for the average modified 951 the SAAB valve has a stiff enough spring. Many have run the 993 spring with out any problem so if your going to buy another bosch bypass valve get the one with the stiffer spring. Again, I mentioned this valve as an alternative to the weaker 993 valve. You can get better valves like the Forge, etc... but I dont think they will make any difference in performance they are just built better and can be taken apart. Of course this applies to the average modified 951.

white 944 turbo

Ahmet, the car will have a big vacum leak with the valve open to atmosphere. The vacum line will open the valve so if you want this to work as a blow off valve it wont. This is not because of the spring but that this valve was designed as a bypass valve not a BOV.

TonyG

I've run the Turbonetics Godzilla (with many different spring rates, spacers, etc...), the stock 951 valve, the 993 valve, the Stratosphere valve, etc... on 4 944 modified turbos over the years.

None of them performs better than the other with respect to holding boost pressure.

The "time delay" would be so small, that it would be imperceivable.

In fact... the best performing valves with respect to drivability are the stock and 993 valves.

white 944 turbo

Tony, hows that beast of yours coming along? Good job, you moved outside of the mainstream and Im sure will set new standards for the 951 which I think has way more potential then has been shown. Right on!

TonyG

The beast is coming along good. I'm resolving some issues associated with fitting the large turbo on the stock motor mount. We're having to do several things to get the turbo to fit properly (including chopping up the stock motor mount)... it's tough.

I just pulled the turbo, and insulated the hot side, the down pipe, and the cross over pipe, and also installed a bunch of the Thermotec insulating sleeve/sheathhing since we don't have all the heat sheilding as the stock cars run.

I should have it back on the road tomorrow. I'm doing a port match job on the stock intake right now while waiting for the new SFR intake.

I have to tell 'ya'.. nothing like V Band clamps on the turbo feed & down pipe! Pulling the turbo is a piece of cake compared to the stock setup. 2 V Band clamps, and the 2 bolts under the turbo, and it's off! :-)

TonyG

Here's a pic of the insulation <img src="http://www.tonygarcia.org/insulated_turbo.jpg" alt=" - " />