Flow Divider Operation
Here is what a Flow Divider (spider) is actually for and how it works.
First you have to understand a little about the fuel injection system. The Injector nozzles on these systems (Airflow Performance and Bendix/Precision) use what?s called an air bled injector nozzle. The nozzle has a fixed orifice size that?s sized according to the fuel pressure available to the fuel injection system and the maximum fuel flow through the injector nozzle. Therefore, with the limited fuel pressure available (usually around 20 ?30 PSI) the nozzle orifice is fairly large (usually .028? diameter on 360 and 540 engines). At idle the pressure drop across the injector nozzle is less than 0.02 PSI, so if the nozzles are not level (tail dragger) or the flow divider is above the nozzles the fuel distribution to each nozzle will be poor, and the engine will not idle. Further more at shut down the valve in the flow divider will shut off each nozzle port tending to keep the nozzle lines from draining out, promoting a cleaner shut down.
The flow divider has a precise metering slot at each port. The valve in the flow divider opens and closes this port based on the total fuel being delivered to the flow divider. The valve, which is connected to a diaphragm, has a spring that sets a 1.5 to 2.5 PSI pressure differential across each metering slot. If you look at the division at 2 PSI, which is equivalent to approximately 70 inches of fuel head, the fuel in each nozzle would be divided equally to each nozzle even if the nozzles had a difference in height of 50 inches. As the total flow increases the valve in the flow divider continues to open as nozzle pack pressure increases. At the higher fuel flows the flow divider valve is open so that the area of the flow divider port is somewhat larger (10X in the Airflow flow divider) so that the injector nozzles then divider the fuel to each cylinder.
This is a simple concept but a very demanding component to manufacture. So basically if you didn?t need to idle the engine you could do away with a $500.00 part.
Hope that helps with the understanding. If not we have a FI 101 class on the first week end in November 2010. Still seats available.
Don