Should downhill distance be considered for head pressure of sump pump discharge ?

Looking to get a backup sump pump and noticed all the manufacturers rate GPM based on how many feet of "Lift" is required. I have a 1.5 discharge line that goes up about 8 feet and then travels downhill about 100 feet. (It's 1.5" the whole way) When the pump stops I can hear air sucking from the pump until all the water drains from the line. To estimate the GPM a backup pump will give me do I only need to consider the "Lift" or is there friction loss for the downward travel ? Or would it actually achieve better performance because a "siphoen" effect is created ?

Why do you want another pump when you don't need any pump at all? Once the water is over the 8' lift and down 10 feet its all a free ride thereafter. In fact you could put a pelton wheel on the end of the 100' drop and make a few pennies generating power.

Perhaps you mean 100' of slight drop in distance, not elevation. In any case, you have no head pressure, you have a positive pressure, natural suction. You do not need a pump at all if you utilize the siphon, but how to do that automatically is going to require some careful engineering. Some old manuals on septic systems show automatic siphons, quite interesting and essentially lost technology.

Your head pressure is what you need to lift the water to its highest point, initially. Once that is achieved, then it appears the water descends below the level of your sump pit which then creates a siphon to assist the pump.

I just read my post over and understand the confusion of what I described. The discharge pipe goes about 8 feet high to the outside of the house, from there it travels about 100 Feet grandually downhill to a elevation below the sump pit. From the responses I got it sounds like once it gets pumped up the 8 feet it's all downhill from there !! pun intended.. Thanks for the help