Pumps in general provide more pressure than suction, i.e. it's easier for the pump to pump up thru the filter rather than provide suction to get fuel out of the tank and up to the filter.
Consider the old problem of pumping water out of a pond up to a higher elevation. If you were to take a perfect pump and put it up top, it would provide, say, 29" of vacuum (equiv to a gauge pressure difference of approx. 15 psi. The height of water equal to about 15 psi is about 34 feet, meaning that this perfect pump could suck up water a max of 34 feet. Put a pump down at the water level, instead, and it can provide enuf head pressure to pump the water much higher.
Regards, George
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"...out to my ol'55, I pulled away slowly, feeling so holy, god knows i was feeling alive"....written by Tom Wait from the Eagles' Live From The Forum
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