The stock fuel sending unit is still used to feed the road race unit. There is no pressure against the stock unit, so it's flow will increase significantly. But we do not have a flow chart on the stock pump to know how much it actually will flow. Will it be able to keep up with a walbro 255 that's pressuring the rails? Hard to know for sure.
Even if it wasn't keeping up, the surge can has about 2 liters of buffer for you. So once the stock sending unit is either not keeping up, or in the process of starving, the displacement of the road race unit will start being utilized. How quickly you deplete it and starve again is anyone's guess as the situation is entirely dynamic based on fuel level, g-forces from turning or accelerating, and fuel consumption.
The road race unit has an additional siphon to add its only fuel from underneath it on the drivers side, which will supplement the stock sending units incoming fuel.
At what point would the stock sending unit need a pump upgrade? This is something we will be finding out in time as different people try it different ways. Surely once the road race pump has an output exceeding 255lph (whether with a single larger pump, or dual pumps in it) I don't think I would hesitate to upgrade the stock sending unit to a 255. Just FYI, the latest update to the road race pump install kit includes the fuel pump install kit for the stock sending unit... So if you do upgrade it, you only need the pump.
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