Fuel Pressure question

I am helping a friend with his 2012 GT500 that its running on E85. The car is not running right. It has several mods with new cams, ID1000 injectors and a small pulley upgrade. I want to put it back on 93 octane gas and make sure everything is performing correctly. I have tuned many standalone ECM but no factory ECU's. I have a stupid question about the fuel pressure. I looked at his tune and his initial fuel pressure is set at 39.5 psi plus his flow characteristics for the injectors are wrong according to the manufacturer. I spoke with Injector Dynamics and they said the 2011-12 GT500 fuel pressure for returnless systems should be 55psi and their flow characteristics they gave me were based on that pressure.

My question is how do you adjust the pressure? :) is it done in the ECM and it raises the pressure as the load increases? I have always set up the initial pressure with an external regulators that increases 1 to 1 as boost increases? Thanks for your help and be nice...I am 60 years old!

Ernie,

I did some poking around online in an attempt to help you and found conflicting info on that vehicle, on info from sources I felt I could trust to be known good.

Generally you want to apply injector characterization data based on the differential fuel pressure you'll be operating the injectors at. That concept is covered in HPA course material and on this forum if you aren't already familiar.

If as some people state online, and it seems you've found in the ECU, base fuel pressure is about 39 psi, and it goes up and down from there based on intake manifold pressure, then it seems the differential fuel pressure will be about 39 psi and that's the fuel pressure I'd expect to use injector data from. Again, the info I found online is by no means conclusive in my mind and may not be correct. The folks at ID are great, so if they're certain the differential fuel pressure your vehicle operates at is 55 psi, then you would use characterization data from that pressure instead.

I'm sorry I wasn't able to find conclusive data on your vehicle in my search.

My general suggestion for anyone trying to determine what their differential fuel pressure is, is to datalog fuel pressure via a real sensor (not inferred data) on the vehicle over the range of intake manifold operating pressures by doing idle and decel, cruising around moderate and full throttle. This does require being able to operate the vehicle safely that way.

Then you can histogram the fuel pressure data vs. engine speed and intake manifold pressure and see what the fuel pressure does at varied intake manifold pressures. You can then either do the math yourself, or use a math channel to calculate what differential fuel pressure is over those various conditions.

Logged Fuel Pressure - (MAP - Baro) = Differential Fuel Pressure

I hope that at least gives you an option to confirm what differential pressure is, so you can then enter it into the ID spreadsheet and get appropriate characterization data for your system.