Piston lift pumps. 2 questions.

[JBradley500]

I'll try and make this as simple as possible. 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 so that's not my problem)

I took my low pressure pumps low tension spring and installed it into a stock 94-98 pump with the bigger bore piston/bore, hoping that the added 30% displacement of the big piston/bore would help me keep 8+psi under WOT. My results are kind of disappointing. 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. Under WOT it still goes to 5psi (same as the small piston) but not quite as fast.

My questions are-
1. Why are my results like this? Im not an expert in hydraulics so maybe im missing something. Basically i lost pressure with a bigger piston.

2. Do I need to run a spacer with the piston lift pump? (I am now) If not, wouldn't removing the spacer add tension to the spring and add pressure also? Kind of like the posts about shimming it from back side but youd actually be doing it from the pistons side. Id like to gain 4-5psi of pressure at WOT and can still gain 4-5psi at idle before seals are a worry.


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-

 

[CaptainChrysler]

1. The way I understand it the spring pressure forces the fuel out of the pump, determining pressure. So your outlet pressure is going to be your spring pressure divided by surface area of the piston. When you put in the bigger piston, you reduced the pounds per sq. inch.

2. I think you need to keep the spacer, but don't know for 100%.

 

[JBradley500]

The spring pressure thing i understand. It was kind of what i was thinking. Im not upset if thats the case. I can always increase the spring pressure but i cant decrease it.

As for the spacer, ive read about people running without it and not having issues but id like to hear from people who actually know whats going on inside of the pump, not just some joe-schmo who forgot to install it and said it worked for him.