fixing to order a pump and i'm not sure if I need the high or low pressure pump. Whats the differance????

 

In very general terms, carb systems require low pressure pumps in the range of 7-10psi. Fuel injection systems require high pressure pumps, in the range of 35-85psi. A stock LT1 fuel system needs a pump capable of flowing about 110 liter/hour at 43.5psi. Higher HP applications need more capacity. A 200 l/h pump is good for up to 500HP, as long as it can produce that flow at 43.5psi.

 

The standard 255 lph Walbro pump deliveries 255 lph at about 40 psi; as the fuel pressure increase, the delivery amount will drop. The high pressure version will not loose as much flow as the pressure increases. If you're not running an elevated fuel pressure, you won't need the high pressure pump.

 

Copied from a vendor's site:

The Walbro high output in-tank electric fuel pumps are available in flow ratings of 255 liters of fuel per hour. These particular pumps flow significantly more fuel at higher pressure. For example, at 80 PSI the standard 255 lph pump will flow around 132 liters (35 gallons) per hour. At that same 80 PSI the equivalent HP (high pressure) fuel pump will flow over 210 liters (50 gallons) per hour.