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Some more info to chew on. As posted recently, I was able to mount the Forge valve in the stock position on my '00 9-3 S. This means the piston face is facing the input side of the turbo compressor. As a number of us have speculated, while this is the stock orientation, it may not be the best option. So using a National Instruments card, Labview software and my trusty G3 Powerbook/500, I made the following measurements.
I recorded a signal proportional to boost off the pre-throttle side MAP and the voltage from the throttle switch that is closed when the throttle plate is shut.
I accelerated to full boost in 2nd gear on flat country roads, lifted off on the throttle for ~1 second and then floored it again. I measured how long the boost took to get back to where is was (ambient temp -10 C, lots of time to re-establish cooling before the next run). I only know the voltage from the MAP sensor, not the actual boost level, but it doesn't matter for this measurement. I did this for the piston in the stock position and facing the 'other' way, i.e. piston facing the pressurized side of the manifold.
Firstly, the electronic throttle on the T7 does not allow the throttle plate to 'snap' shut. It essentially does what the old throttle dashpot on C900 and 9000's did, for emission purposes, but perhaps turbo lag reduction too. Takes almost 1 second to completely close after you let up on the gas. this is qualitative, I didn't measure it. I was only interested in keeping the throttled closed for 1 sec.
Anyways, the boost comes back to its max value about 320±50 ms faster with the piston of the Forge valve facing the pressurized side of the manifold (i.e. opposite to the Saab stock arrangement) (10 runs for each Forge valve orientation). This suggests to me that the manifold pressure helps push the valve open faster and dump the excess air onto the input side of the turbo. I have to believe that this would save several tenths of a second on the 0-60 times for each shift.
So now you know. Keep the piston facing the pressurized side of the manifold for best response. I assume Saab doesn't do this because the Bosch valve is pretty flimsy and may not stay seated with pressure on it over the service life of the car.
, Mon, 12 Feb 2001 09:38:13
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