I am just curious why the instructions would differ between the EFI version of the FPSS and the carb version. When I initially read this, I saw that it said to have the FPSS before the regulator, so I did, then reread and saw that on the EFI applications, it needs to be attached to the solenoid.

Anyone have any guesses as to why the two applications differ, even though they would be doing the same job?

http://www.nitrousexpress.com/Instructions/fpss.pdf

 

I dont see the difference, if you are pulling fuel from the rails or pre regulator somewhere in the line. It all gonna be same pressure. Since its a different part number perhaps its just more convenient to mount it to solenoid so you dont have to go cut up lines to install it somewhere else, since high pressure lines are more expensive and more prone to leaks. Just my thoughts and since high pressure, there wont be as much of a drop in fuel pressure when system kicks on. Unlike low pressure carb setups. So you can do it however you want in my opinion it should still function correctly. It just sounds easier to do it on solenoid.

 

It says why right in the document. For low-pressure carb applications "Do not install the FPSS between the fuel pressure regulator and the fuel solenoid. The normal momentary drop in fuel pressure when the nitrous system is activated will activate the switch resulting in erratic nitrous system performance." The higher pressures of EFI don't have such an issue.