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kellcats521 kellcats521 is offline
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Join Date: Feb 2005
Location: Atlanta Metro
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Originally Posted by beepbeep View Post
You don't get it. It's a closed system. Everyting that has been drawn into engine passed barn-door. And everything that passed barn door has been accounted for and corresponding fuel injected. Air (=oxygen molecules) annot magically dissapear or get bigger between engine and CIS just beacuse turbo compressed them.

If you pushed 13 g of air into engine, CIS will inject 1 gram of fuel. AFR=13. If turbo starts finding it hard to push that air into engine (beacuse it expanded) it will just inhale less, and CIS will push less fuel into engine. Status Quo.

Example: You have a bakery. There is a stream of dough passing trough a tube and flow-sensor of some sort. For every 14kg/sec of dough it passes out 1kg/sec of sugar. After that device, dough flows trough warmed tube where it expands. Dough cannot escape anywhere else, it must go trough tube. When it comes out of the tube, sugar is added.

Imagine someone turns up the heat in the tube after flow sensor and dough starts swelling more than usual, making it harder for pump to push it trough the tube. What happends is that flow slows below 14kg/sec, device senses it and starts adding less sugger. In the end, the dough is just as sweet as usual, but there is less dough coming out of the pipe.

Trust me on this
I know this is not entirely true, as CIS has real issues when exposed to elevation changes, just as it does when exposed to temperature changes. IMO (this may not be the exact explaination) the negative intake pressure (either from the engine itself or from the compressor inlet) draws air across the CIS plate causing a deflection based MAINLY on the degree of negative pressure applied to the plate. If the air moving across the plate is less dense (i.e. at high altitude or elevated temp), it will have less oxygen, and the motor will run richer.

There may be more to this, but I know for a FACT that CIS needs to be tuned to intake temp and altitude. Porsche designed some margin in the WUR (in my experience in working on other, non-Porsche CIS cars, we called the WUR a control pressure regulator (CPR), which is a MUCH better description of what this device is) to account for some temp and altitude variation, but I have had to 'adjust' the CPR on other cars due to temp AND altitude change.

Pat K
87 930
Old 08-15-2007, 12:12 PM
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