Sorting Out Our Fuel Pressure

Update by Per Schroeder to the Saab 99 project car
May 16, 2008

With a system pressure of 71 psi, we went so far as to lean it out with a diff pressure of 68 psi.
Rob bolts on the warm-up regulator.

We dial in our air/fuel curves by changing the fuel pressure in this Bosch CIS injection system.

When we put the Saab on the dyno in Atlanta, it was pretty clear that there was a lot of work to be done on our air/fuel curve. The oxygen sensor was seeing values of less than 10:1 after 6000 rpm, and despite that fact, we were still putting out 111 horsepower. Clearly we needed to lean things out. Adjusting the idle/CO set-screw would change the curve somewhat, but not enough to make an appreciable difference.

With the help of Rob Freund of Action Street and Performance here in Daytona, we replaced the warm-up regulator with one that has been modified. The new component is designed to be manually set for the optimal amount of differential fuel pressure. This is a common trick with Improved Touring Volkswagen race cars. The differential pressure is used to control the movement of the air flow sensor plate. The closer this differential pressure is to the system pressure put out by the fuel pump, the leaner the car will run. We played with these levels a bit, and we’re still dialing in the proper settings with the help of an Innovate LM1 wideband oxygen sensor.

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