4586

Just thinking out loud about actual fuel pressure throughout an EFI fuel line system.

If I install a high pressure inline fuel pump for a LT1 swap wouldn't the actual pressure be the highest at the point between the fuel pump and regulator?

Would assume that the pressure would be the lowest between the tank and fuel pump and again low pressure on the open ended return line?

Injuneer

The pressure between the inline booster pump and the fuel pressure regulator will be the pressure set in the fuel pressure regulator (minus any vacuum compensation). The regulator is after the fuel rails, and regulates the return flow to the tank to control the pressure in the rails. You will just be returning a lot more fuel to the tank at idle and low engine load.

The intent of adding the inline booster is to increase flow. It does this by splitting the pressure increase between the tank pump and the inline pump. The lower the pressure differential across each pump, the higher the possible flow rate.

I've run a stock in-tank pump with a 205 LPH inline booster, and the stock fuel pressure regulator, and had no problems with the fuel pressure at the rails holding 43.5psi (without the vacuum compensation).

If you choose to run the fuel system at a higher pressure (I now run mine at 58psi) using an adjustable fuel pressure regulator, it will still work fine.

4586

Fred..thanks for the information...I was thinking more about potential fuel line leaks related to pressure in a hot rod with inline pump on frame (no pump in tank). I would think the highest pressure area would be from the pump to regulator.

Wouldn't pressure be minimal via the tank pick up tube to pump and the open ended tank return line?

Ed

Injuneer

The pressure at the pump will equal the fuel pressure regulator setting, plus the frictional pressure loss in the rails (near zero) plus the frictional pressure loss in the 3/8" line between the pump discharge and the fuel rails. Pressure at the pump discharge will only be very slightly higher than the pressure at the fuel pressure regulator.

I did a quick calculation using an online program, and for 200 LPH through 10-ft of 3/8" steel line, the pressure drop would be a bit less than 0.5 psi. Meaning the max pressure at the pump would be about 43.5 psi + 0.5 psi - 44 psi.

Don't worry about it.

Line pressure between the tank pickup and the pump can be negative (depending on vapor pressure in the tank), referenced to gauge pressure. Too much negative, and the gasoline in the suction line will flash to vapor and the system will vapor lock. I have an outboard pump on my tank (in addition to the one in the tank) and it uses a -10AN line from the tank to the pump. But my line from the merge of the outlet of the tank pump and the outboard pump, to the fuel rails, is only 3/8" and the combined flow can reach 410 LPH.