Think of your engine as a Vanagon trying to push air out of the way going
down the road. Now think for your throttle body as the wind resistance of the
air into your engine. Of course the engine has to work harder to push your
Vanagon than it would a Jetta.

In your Vanagon you are pretty much at full throttle all the time. The valve
body creates resistance to the air going into the engine. The engine WANTS to
be at full throttle all the time but it couldn't because of the 75% opening
of the secondary. So it has to WORK harder against the 75% opening. Now that
you have 100% opening it doesn't have to work so hard and it uses less fuel.

The Fuel Curve in a CIS system would remain constant. It really is pretty
dumb. It will only deliver the amount of fuel that corresponds to the amount
of air flow that goes through the sensor plate. If you had the CIS-E system
it may have compensated for this.

I hope you are thoroughly confused now. I am not a fluid dynamics engineer
but know enough to get into trouble. :)

John Wessels
European Motorsport
Livermore, CA
http://www.euromog.com