Ok, a little background. Recently installed my wet nitrous kit. I used the Robbie the Robot calculator and got jets for 50, 75, 100, 125, 150 and 175 HP settings.

Initially, I jetted the system for 50 HP.

I logged the car with tune #1 without the spray and say o2 readings in the 950-960 range. So I hit it with the 50 shot and got readings in the 930-940 range.

I loaded a bolt-on tune from Madwolf on the car (tune #2) and logged it without spray. Got o2 readings in the 880-900 range. I wanted the car to be richer so the nitrous was safer. I split the difference between the two tunes and added that to the PE vs RPM table with LT1 Edit.

I loaded this new tune (#3) to the car and logged it, got readings in the 930 range. Hit it with the 50 shot and saw readings in the 910-920 range. So I rejetted for the 75 shot and got readings in the 900-910 range. Then rejetted again for the 100 shot and got readings in the 840-860 range. The car had a strange feel, kinda like it was missing. That was when I remembered not pulling any timing from the tune. I was only spraying for a few seconds (5 or 6). I did not know the o2 readings while running, but only after the fact.

I took the tune (#3) and increased the PE vs RPM table to match that of tune #1, and pulled 2 degrees of timing at 95 and 100 kpa from 2800 RPM and up. Loaded that to the car (#4)

With tune #4 loaded, I went to the race track and ran on motor only for 3 passes. I looked at the logs and the readings were still in the 910-920 rance. That confused me considering I added fuel in the tune. So I ran one partial nitrous pass, and one full nitrous pass. I logged both the passes but couldn't see the laptop while making them. At the end of the 2nd pass, the car felt like it did a couple of days before. So I stopped the car and replayed the log files. I saw o2 readings while spraying as low as 300

I immediately closed the bottle and resolved to not run the nitrous any more until I figure out what's going on.

I made two more N/A passes later and the o2 readings were in the 890-910 range.

I drove the car home. On the way I got to thinking about how I guessed at setting the FPSS since I didn't have a pressure gauge. I thought that maybe the fuel pump is weak, letting pressure drop during spraying and the FPSS is set so low that the pressure drop isn't killing to solenoids like it should. I also wondered if maybe the fuel system is maintaining the pressure but not the volume to feed the engine when spraying.

So when I got home, I pulled the fuel line off the nozzle, pulled the vacuum line off the FPR, held the fuel line to a gas can and with the engine idling, I hit the switch to spray. The solenoids started clicking, cycling open and closed. When they were open, the fuel sprayed pretty good (or appeared to for my untrained eye). I assumed at this point that there was a significant enough pressure drop to let the FPSS shut the solenoids down. I put the fuel jet back in the nozzle, hooked the line back up and while holding the throttle open slightly (engine running) I hit the switch. The solenoids stayed open as they should while it was spraying fuel. I could tell that it was spraying as the sound of the engine changed (it bogged slightly). So I stopped spraying and let the engine run a few minutes to burn off that extra fuel.

I ordered the parts to plumb a fuel pressure gauge into the fuel side and received them today. Tomorrow I'm going to go driving and see if I get any pressure drop while running, and how much I get. I discovered that the FPSS is set somewhere in the area of 34-35 psi, BTW. I tried it at idle, and the pressure drops by 2psi at idle. I only tapped it though as I didn't want to send a bunch of raw fuel into the intake at idle.

So, now my question... What's an acceptable amount of pressure drop for the fuel system while spraying?

No drop, 2psi, 4psi, 10psi?

The tune should be running the car fatter than it is, so there has to be something causing it to show leaner than it should. I'm trying to elminate the fuel pump as a suspect if I can, before I go rejetting the fuel side.

Oh, I'm running a .047 nitrous and a .027 fuel jet. Robbie The Robot calls for a .026 jet for the size nitrous jet I'm running.

 

While you may see a very brief dip in pressure, such as 2-3 pounds the recovery should be immediate. You aren't using much fuel and it should not overly tax your fuel system unless its already running at its limit. I would not get too greedy though until you install a higher volume pump, such as an in tank 255.

 

My plan is to stick with the 100 HP shot, but I want to get it right first.

I will be building a new engine that will be setup for nitrous, that's what the larger jets are for.

 

It's not safe to tune your engine based on the stock O2 sensors. They are very inaccurate at any reading other than 14.7:1 air/fuel ratio. Take the car to a dyno with a wideband O2 sensor and tune it properly. You'll also be able to monitor your fuel pressure under load to determine if your pressure is dropping.

 

I found my problem, and it was not fuel pressure.

There was a problem with the fuel jet.