Doing FRx, HPOP, HPx and a few other mods.
Riff Raffs FRx comes with some choices of springs. Which one should I install?

 

The gold spring seems to be the most popular choice.

 

Do you have a fuel pressure gauge?

I went with the black spring when I installed Dieselorings FPR Billet
housing and the KOEO or idle pressure is 70-71 PSI. 65 psi at WOT.

From Riffraff FRx page.

 

I dont have a fuel pressure gauge. "yet"
What are some of the determining factors in deciding how much fuel pressure I need?
Throttle position I mostly drive at? Mods?
This is my first time diving into the diesel fuel system.

 

WOT pressure is the determining factor. That is why it is nice to have a FP gauge you can read while driving. WOT 50+ psi would be good with stock injectors. Like Roland said the gold spring would be a good start until you have a FP gauge. Keep track of that black spring. You may want to use it if/when you upgrade injectors.

Be certain you get the spring in properly. If it is cocked, and that is easy to do, the fuel pressure can reach or exceed 100 PSI. I personally don't like to mess with the FP springs unless I can read the FP to be sure the spring is not cocked.

When installing the FRx is a good time to do an FPR rebuild. LINK

 

Thanks,I installed the gold spring. Seemed to go in pretty easy. A FP gauge will be on the list.
How would I know without a gauge if I put that spring in crooked.
I also rebiult the entire fuel bowl including the FPR.

 

You might be able to tell by how it runs or whether your bowl starts leaking. Here is a low-cost gauge: 7.3l Engine Mounted Fuel Pressure Guage

 

I use a stock spring, but I did drill out my brass head plugs with a 1/4 bit for less restriction while hot. I have 62 - 63 psi's at idle and WOT.