I have Zex Dry kit I am planning to install on my LT1. I wanted to be as safe as possible so I have window switch and an Adjustable FPSS. My plan was to do a dyno run with AFR readings (to ensure I am not to lean) and note the fuel pressure at the rail (i.e. after the FPR). Lets say it ends up being 75lb when the juice is flowing and AFR looks good. Then I figured I would just adjust the FPSS to "disable" at a few pounds less than that (lets say 65lb). Sounds simple right? Here is the problem I now foresee. If I hook it up like this it won't work because when I activate the N20 the Rail pressure will be somewhere around stock lets say 40lb (anyway no where near 65lb) and so the system will never "activate". I now figure I need some kind of delay so that the "disabling" functionality of the FPSS is not introduced into the circuit until the juice is actually flowing (and rail pressure is up). Hope that makes sense. It's kinda of a which came first chicken/egg problem. (i.e. I don't want to activate nitrous unless fuel pressure is good but fuel pressure won't be increased until nitrous is activated... AHHHHhhhh. Its a Paradox... AHhhhh! Any electrical gurus have ideas for this one?
Kris
P.S.
I know I could setup the FPSS before the FPR but then I am at risk if the FPR fails or if the vacuum line to it falls off or or or etc. etc. I really do want to be safe. Thanks guys, open to ideas

 

First, fuel pressure is the same before and after the fpr. our fpr works by 'bleeding' off pressure constantly, so the entire line at WOT stays at 3bar (43.5), and idle is around 2bar, from the intank-pump to the rails.

So you basically don't want the n2o to spray until the fuel pressure is up right? For that, you only wire the n2o to the fpss. The inline fuel pump (which boosts the pressure) is only wired to WOT. That way when you go WOT, the pressure can jump up, which kicks on your n2o. The fpss can be anywhere in the fuel supply line, since it is constant pressure throughout.

 

Can I assume your ZEX dry kit is similar to the NOS dry kit, and uses a slip-stream from the main nitrous line to boost the fuel pressure by applying a pressure to the fuel pressure regulator? In the NOS kit, there are dual nitrous solenoids in series. The takeoff to the fuel pressure regulator is taken from in between them. You wire the interlock to the fuel pressure safety switch ONLY to the "downstream" nitrous solenoid. That way, ONLY the first solenoid opens.... the pressure is applied to the FPR, the fuel pressure rises, the circuit for the downstream solenoid is completed, and nitrous flows only after fuel pressure has been established.

As far as fuel pressure, the regulator is on the outlet side of the rails, so you have to check the pressure on the inlet side of the rails, typically at the Schrader valve. You will not see full fuel pressure on the downstream side of the FPR, since that is the flow going back to the tank, and will have a reduced pressure as it flows through the FPR.

 

Hey juiced_lt1, thanks for straightening me out re the FPR bleeds off to regulate pressure. With the Zex dry kit the in line Fuel pump is always "on" (there is no wire to enable it at WOT). As Fred guessed the Zex Dry kit has a "vacuum" line which runs from the ZEX Nitrous Management Unit (basically the nitrous noid, WOTS, vacuum line and electronics all crammed into one small box) to the FPR to up the Fuel Pressure when the system is activated. I suppose I can buy another Nitrous Noid and install it after the Zex NMU and wire it to be activated by the FPSS as Fred described (re how it works on a NOS Dry kit). Not quite the clean "all in one box" Zex install I was hoping for but better safe than sorry. I think I have a plan now. Thanks very much for you help guys. I will post back when I get it all installed.
P.S.
It could be a while as I have got a lot of crap to install. On top of the Zex kit I have; FPSS, Window Switch, Remote Bottle opener, Bottle Heater, Remote Nitrous bottle pressure gauge kit, have a purge noid on order, still need to get a switch panel, and of course now another Nitrous Noid... (and since I am going all out on safety I figure I might as well get a nitrous filter as well (who knows if there is one in the Zex"magic box")...dam that seems like a lot!... did I miss anything?)

 

Just thought of something. Zex claims that its NMU will try and regulate Fuel Pressure based on the amount of Nitrous is "senses" flowing. I guess it would do this buy adjusting the vacuum to the FPR based on the Nitrous Flow. Sounds like BS to me, but since this kit has nitrous jets for 75 100 and 125 HP (and no way of making any changes to affect FP when a nitrous jet is changed) I guess it must do something to read flow so it can adjust FP. Anyway I know realize that if this is the case and I implement the solution of having a 2nd nitrous noid activated by the FPSS it still may not work because the Zex NMU might sense zero nitrous flow and not apply any vacuum to the FPR and then the FPSS would not activate the 2nd noid. Dam. Has anyone opened up the Zex "magic box" to see if it really does do something to measure Nitrous flow?

 

it's not really based on nitrous flow, it's based on nitrous pressure, which is directly related to nitrous flow. If the first solenoid opens, it should bump the fuel pressure up. BTW, the zex noids are good to 180-185hp max. Anyway, go ahead and hook that extra n2o solenoid after the zex NMU (n2o line of course), and you should be fine.

The way you have it set up in the next to last post sounds correct.

n2o bottle --> nmu --> extra n2o noid --> nozzle

Make sure you have some colder plugs like tr6's, autolite 103/104, etc... If you start spraying a lot, you'll need them. No platinum plugs either. But you prolly knew all that.

Now go whoop some c5's!!