Hey all, Just a thought...
So we all know that you can't/shouldn't install a BOV on a Trionic 7 system, it just won't work right. I found this article though that seemed really interesting, and also seemed to provide some solutions for those with VW's who really wanted the atmospheric BOV effect.
Does anybody think any of the solutions in the following article would actually work on a SAAB??
Here's the link BTW :http://www.vagsport.com/articles.php?artid=26
The Wastegate,Diverter Valve,and the Blow Off Valve
courtesy of Ian Frechett (reproduced with direct permission)I shall start with the wastegate because it has little to do with the others.
The turbo consists of a turbine in the exhaust stream and a compressor attached via a shaft in the intake. The wastegate is just an exhaust bypass around the turbine. The actual turbo boost is regulated at all times by opening and closing the wastegate, otherwise the turbine can spin too fast creating too much pressure (more heat, more detonation, more stress on the compressor).
In a car like the WRX you can force the wastegate to not divert the gasses as early, causing higher boost, with a cheap little spring loaded valve regulating the vaccuum (aka Manual Boost Controller). On the 1.8T it's not so simple and generally the only reliable way to raise the boost is with a chip that tells the wastegate to open later, thus raising boost.
Ok, back to the intake side. The air flow is as follows..
Atmposheric air -> Air Filter -> MAF -> X -> Turbo -> Intercooler -> Y -> Throttle Plate -> intake manifold.
When the throttle is open and the engine making power the air is compressed by the compresor
wheel and forced through the intake manifold into the cylinders and then the expanding exhaust
gases turn the turbine which drives the compressor, and so on. The car's ECU measures the air flow through the Mass Air Flow (MAF) meter and provides the appropriate amount of fuel to use up all that air in combustion. But when the car is already making boost and let off the gas, the throttle plate closes, but the compressor is still spinning at 100+ thousand rpms trying to move a lot of air. If it is not allowed to escape a huge pressure spike occurs between the compressor and throttle plate and the backpressure from that can slow or even stop the compressor wheel, also creating a nasty thrust load on the floating oil bearings the turbo uses. It's a bad thing.
A Blow Off Valve [BOV ] (with twin horns venting to the atmosphere)
To relieve that pressure you put a valve in above where I put the letter "Y".
If it's a BOV it opens when theres a vacuum in the intake manifold (meaning the throttle plate has closed), and all the excess pressure just vents to atmosphere. The problem is, all that air has already been metered by the MAF and more air will flow through it as you stay off or at part throttle. The engine will continue to pump fuel into the cylinders to match the air it thinks it's getting and you get fuel that doesn't get burned until it gets to the cat. You either burn up the cat, or remove it and spit serious flames out the tailpipe everytime you let off the gas.
To solve this problem you take the output of the BOV and run it back to the spot marked X, just downstream of the MAF and upstream of the compressor.
This is a diverter valve.
When the throttle plate closes, the pressure in the intake is released by the DV and just flows back around through the compressor, over and over, and the net flow of air through the MAF stops completely. The turbo is now happy because there's no boost to speak of so it can spin freely and the ECU is happy because there's no flow through the MAF.
A Diverter Valve [DV] (one pressure inlet and one outlet to pre-turbo section along inlet tract)
People do put BOV's on the 1.8t, and there seems to be two ways to handle the problems they cause.
1. Unplug the MAF. Ignore it. The 1.8t has another sensor downstream (in the exhaust manifold I think. Correct me if I'm wrong) called the MAP sensor. It basically measures the air/fuel ratio post combustion and then the ECU adjusts the amount of fuel into the engine up or down to compensate, much the way it normally uses the MAF. If it can't find the MAF it'll use the MAP signal exclusively. Course you'll trip a CEL and it'll stay on.
2. Crank the spring tension down on the BOV so that it only opens at zero throttle and high boost. The stock DV opens any time there's any vaccuum in the manifold and more boost in the intake so it may be open at idle, and part throttle. A BOV set up in this high preload setup will only open at higher rpms, higher pressure, and zero throttle. The reason this works is that the 1.8t, like most modern fuel injected cars cuts fuel through the injectors completely when you take your foot off the gas and the revs are above idle and dropping, so even though there's airflow through the MAF, the ECU shouldn't be supplying any fuel until you get back on the gas a little, and with the high preload the BOV should close up about then.
Oh.. there is one other way. If the MAF were put downstream of both the turbo
and the BOV then you can vent pressure to atmosphere without the MAF ever seeing it.
This is not uncommon on non-stock turbos (turbo or S/C added to a NA car).