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jimberg · 06-07-2002, 10:03 AM

Chris,

Okay, I'm gonna stick my neck out and offer and idea as to what might be happening. This is just a guess since I don't have any expertise with superchargers. You may want to re-title your message so that SC guys will be drawn to it.

The pressure at which the fuel flows through the injector is the combination of the pressure inside the cylinder and the fuel rail. Think of someone pulling on a big desk and someone pushing at the same time. If you want to move the desk at a constant rate by using a specific amount of force, say 100 lbs, you can have someone push with 80 lbs of force and someone else pull with 20 lbs of force. If one of the people stops pulling, the other can push with the full 100 lbs of force. The fuel pressure regulator is what works to maintain that constant force. This is why the vacuum line is hooked to it. When vacuum increases the pressure in the rails has to drop to maintain the fuel pressure that was set with no vacuum applied. I hope this makes sense, and if you already knew it, other people may hopefully get something out of it.

A supercharged engine will not only have vacuum, it will have a positive pressure inside the cylinder which will be like having someone pushing on the desk in the opposite direction of where you want to go with 20 lbs of force which would mean that the guy pushing in the right direction would have to push with 120 lbs of force. You didn't mention what injectors you have, so I will guess that they are still 19#ers. Since boost isn't constant and increases with RPMS, your injectors may not be able to deliver the proper amount of fuel for the increased air flow so the FMU allows the pressure in the fuel rails to increase at a greater rate than the pressure inside the cylinder so that the injectors will provide more fuel when needed. Going back to the desk moving example, the FMU needs more work to be done so the total force applied to the desk is increased to 130 lbs. This may explain why your fuel pressure is going so high.

My guess is that, if the car seems to be running fine, everything is probably working as it should.

I hope this helps.

06-07-2002, 10:03 AM	#6
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Chris, Okay, I'm gonna stick my neck out and offer and idea as to what might be happening. This is just a guess since I don't have any expertise with superchargers. You may want to re-title your message so that SC guys will be drawn to it. The pressure at which the fuel flows through the injector is the combination of the pressure inside the cylinder and the fuel rail. Think of someone pulling on a big desk and someone pushing at the same time. If you want to move the desk at a constant rate by using a specific amount of force, say 100 lbs, you can have someone push with 80 lbs of force and someone else pull with 20 lbs of force. If one of the people stops pulling, the other can push with the full 100 lbs of force. The fuel pressure regulator is what works to maintain that constant force. This is why the vacuum line is hooked to it. When vacuum increases the pressure in the rails has to drop to maintain the fuel pressure that was set with no vacuum applied. I hope this makes sense, and if you already knew it, other people may hopefully get something out of it. A supercharged engine will not only have vacuum, it will have a positive pressure inside the cylinder which will be like having someone pushing on the desk in the opposite direction of where you want to go with 20 lbs of force which would mean that the guy pushing in the right direction would have to push with 120 lbs of force. You didn't mention what injectors you have, so I will guess that they are still 19#ers. Since boost isn't constant and increases with RPMS, your injectors may not be able to deliver the proper amount of fuel for the increased air flow so the FMU allows the pressure in the fuel rails to increase at a greater rate than the pressure inside the cylinder so that the injectors will provide more fuel when needed. Going back to the desk moving example, the FMU needs more work to be done so the total force applied to the desk is increased to 130 lbs. This may explain why your fuel pressure is going so high. My guess is that, if the car seems to be running fine, everything is probably working as it should. I hope this helps. __________________ 351W 89 Mustang GT Convertible