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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,274
You're right.

The fuel pressure in the 6 lower differential pressure chambers in the bottom half of the fuel head will be higher if the pulse vale is unplugged (not the oxygen sensor) or removed and that will make the diaphram that seperates the upper and lower half of the fuel head raise upward a real tiny amount from the increased lower chamber pressure and that will reduce the amount of fuel that can get by the small clearance between the top side of the diaphram and the 6 orifices in the 6 upper differential pressure chamber leading upwards toward the banjo fittings for the injector lines.

The fuel going to those lower chambers has to go through a very small orifice just like the fuel going to the control pressure area on top of the control plunger cylinder so system pressure still remains relatively steady and higher, and the fuel pressure in those seperate areas can be varied by the external devices: the control pressure regulator and the frequency valve.
The fuel used for control pressure and lower differential chamber pressure is being used as hydraulic fluid not fuel, and once it goes through the orifces leading to those areas it never goes near being injected into the engine on that trip from the gas tank to the fuel head. Instead it is returned to the gas tank for the next time.

Because of this people that know what they are doing when removing the frequency valve or unplugging it increase the spring tension on the top side of the diaphram to push it down with the 6 individual spring tension adjuster screws I mentioned in an earlier post up the page to compensate or over compensate, and increase the system fuel pressure if it needs it by shimming the fuel pressure regulator spring.

Hope all that makes sense...
CIS is complicated and two of the 3 seperate fuel pressures: control pressure and lower differential chamber pressure seem to be the opposite of what you would think they are until you learn that the fuel pressure in those areas is used as hydraulic pressure and not for fueling the motor.

System pressure is supposed to stay the same and steady at all times as long as the pumps are running but I hear from Keith/911st that according to Larry at CIS Flowtech it does drop a little at high rpms with the stock 930 fuel pumps.

It's difficult to expalin whats going on without fuel flow diagrams through the internal passageways of the fuel head.
Old 06-10-2010, 04:19 PM
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