JBradley500

On 1st gens you can upgrade to a low pressure piston pump. The only differences between low pressure and the stock p-pump piston pump are the 1. tensions of the spring 2. the diameter of the piston/bore and 3. thread of the suction side of the pump. I kept the same thread size because I used the stock suction line and fitting.

I took my low pressure pumps low tension spring and installed it into a 94-98 pump with the bigger bore piston/bore, hoping that the added 30% displacement of the big piston/bore would help me under WOT. My results- 15psi at idle, which surprised me because it was 19psi with the low pressure pump and I though the spring dictated the pressure. I assumed it would stay at 19psi. CaptainChrysler's explanation of the lower idle PSI is "outlet pressure is going to be your spring pressure divided by surface area of the piston". Makes sense. Under WOT it still goes to the same PSI as the small piston but not as fast. This may be because i'm still using the stock pickup tube and that's my restriction in the fuel system.

Overall i'm fairly happy. Better WOT fuel pressure is always good and lower idle pressure is good too because I don't want to blow any seals. I don't know if it was worth the money, but it was fun to play with the pumps and there are some numbers for people who were wondering. I've seen threads on the same thing but no one who does it ever has a fuel pressure gauge.


Here are some pictures to try and make things as clear as possible. The low pressure stuff is on the left, stock p-pump is on the right.

spring-


piston-


inlet threads-


internal bore-

The_Head

Is this a recent mod for you in the quest for more fuel?

JBradley500

Yeah, I did it this past weekend. Im always on the hunt for more fuel and air. I just cant always afford either of them.

The_Head

So before this you just had a normal LPPLP?

JBradley500

yeah the one that Tennessee Diesel Conversions sells. The "purple dot" pump.

92smokin blacky

So do you know if you can buy the internals of these pumps seperate? or do they have part numbers on them? I got a 2nd gen lift pump but heard that if I changed the spring to the Low pressure one then I wouldn't have to run a regulator. I'm still not sure what I want to do with my fuel supply. My mind changes everyday.

JBradley500

From what ive gathered (which probably isnt much), no you cant buy the low pressure spring. I think their are other parts you can buy separately but that isnt one of them.

BILTIT

What are you getting for pressure at WOT?

JBradley500

5psi seems to be the number id say... my goal is an unregulated piston pump that can hold 8-10psi under wot with injectors my size and a pump thats near runaway.