Question about fuel injection from a newbie

Kdw1403 · 05-05-2017, 06:06 PM

I just made a street outlaw pass down the hi way in front of my house with data logger monitoring fuel pressure. I set it at 60lbs, left the line easy and got it in high gear real quick then nailed it. Data show instantly fuel pressure dropped to 25 and stayed there till I let off at 6700rpm. Pressure went right back to 60. Pretty sure its the fuel pump or filter. Since I am new to fuel injection is there anything else that can cause this? I don't think so but again I never thought thought a President would say that its ok for men to go into the womens bathroom. Thanks

Ed Wright · 05-05-2017, 07:16 PM

Listen to Cooter. Your injectors are big enough. You need to worry about your fuel pump, and filter. Your fuel pressure should not drop off like that. I use 36lb injectors in my Super Stock LT1, at 60 psi.

Painter · 05-05-2017, 08:03 PM

I am really thinking that we might have a plumbing issue.

Buick6 · 05-05-2017, 09:38 PM

Low fuel pressure at WOT is usually caused by a voltage drop at the pump, a failing pump or a pump too small to keep up with needed flow. A lot of people run a secondary hot wire to the pump that ensures proper voltage but can cause early electric pump failures. Definitely start by checking the fuel lines and filter. Also make sure you have enough fuel in the tank to cover the pickup.

pbp1 · 05-06-2017, 10:51 AM

Now that you are logging Fuel Pressure, and you see that the pressure is dropping drastically, you have narrowed the root cause of the issue down to the fuel delivery system.
I personally think that 30# injectors running at their standard set pressure are borderline, but lets get your fuel delivery problem fixed first, and then, re-evaluate the injectors. Once you get the fuel delivery problem fixed, you will need to lower the pulsewidth numbers in your fuel table because right now, they are inflated from trying to compensate for the lack of fuel flow. Once the fuel pressure is constant, you will need to lower the pw numbers, which will bring your duty cycle down.
Think of it like this, at 25 psi, you injectors no longer flow 30# per hour. At 25 psi, they are more like 20#s per hour, so that's what you have at WOT.
Here's some simple math that applies:

The Fuel Consumption for this type of engine should be around .42 - .45#s per hr. If your engine makes 500hp, then it will need between 210 - 225#s per hr.

225 divided by 8 injectors = 28.125#s needed per injector.

If your injectors can flow a maximum of 30#s, and you need 28.125#s from them, then 28.125# divided by 30 = .9375 or 93.75% duty cycle.

If you raise the fuel pressure (and the fuel system is able to maintain that pressure at WOT), that raises the potential flow of the injector.
If your injectors are rated to flow 30#s at 45 psi, then they will flow around 35#s at 60 psi.

Now, you have 35# injectors, so the fuel you need - 28.125# divided by 35# = .80 or 80% duty cycle.

If I were you, I would fix the fuel delivery issue first. Then, I would upgrade to 36# injectors just to give you a little more margin for error.

Ed Wright · 05-06-2017, 04:05 PM

36 lb injectors flow 42 lbs at 60 psi. Smaller injectors at higher fuel pressure atomize the fuel better. Finer mist is easier to ignite. There is a reason the later (LS family) GM engines went to 60 psi.

GUMP · 05-09-2017, 04:36 PM

Quote:

Originally Posted by Kdw1403

Data show instantly fuel pressure dropped to 25 and stayed there till I let off at 6700rpm.

You need to fix this first. There should not be the voltage drop that you have reported. You will burn something if you continue to run things with this problem.

I am assuming that you have your regulator plumbed after the fuel rails and that you are running a return line?

Ed Wright · 05-09-2017, 08:34 PM

I didn't think about him maybe having the regulator in the wrong place. HAS to be AFTER the fuel rails, controlling return only.

Surely his battery or batteries is/are in the trunk. I let my FAST ECU control my fuel pump relay, just like a factory vehicle. My relay is back there too. All 10 gauge wire from my batteries to the relay, then to the pump, and a ten gauge ground from the pump back to the battery's ground terminals. My batteries are grounded directly to the engine, with a long, large gauge ground cable. I have a ground cable to the body too. My battery cables have all terminals soldered on. I've logged my fuel pressure. Never changes. Solid at 60 psi.

05-05-2017, 06:06 PM	#1
Kdw1403 Member Join Date: Nov 2016 Posts: 205 Likes: 49 Liked 68 Times in 22 Posts	Re: Question about fuel injection from a newbie I just made a street outlaw pass down the hi way in front of my house with data logger monitoring fuel pressure. I set it at 60lbs, left the line easy and got it in high gear real quick then nailed it. Data show instantly fuel pressure dropped to 25 and stayed there till I let off at 6700rpm. Pressure went right back to 60. Pretty sure its the fuel pump or filter. Since I am new to fuel injection is there anything else that can cause this? I don't think so but again I never thought thought a President would say that its ok for men to go into the womens bathroom. Thanks

05-05-2017, 07:16 PM	#2
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 390 Times in 170 Posts	Re: Question about fuel injection from a newbie Listen to Cooter. Your injectors are big enough. You need to worry about your fuel pump, and filter. Your fuel pressure should not drop off like that. I use 36lb injectors in my Super Stock LT1, at 60 psi. __________________ Ed Wright 4156 SS/JA

05-05-2017, 08:03 PM	#3
Painter Member Join Date: Dec 2012 Posts: 227 Likes: 4 Liked 162 Times in 45 Posts	Re: Question about fuel injection from a newbie I am really thinking that we might have a plumbing issue.

05-05-2017, 09:38 PM	#4
Buick6 Junior Member Join Date: Feb 2015 Location: Florence, SC Posts: 84 Likes: 16 Liked 16 Times in 7 Posts	Re: Question about fuel injection from a newbie Low fuel pressure at WOT is usually caused by a voltage drop at the pump, a failing pump or a pump too small to keep up with needed flow. A lot of people run a secondary hot wire to the pump that ensures proper voltage but can cause early electric pump failures. Definitely start by checking the fuel lines and filter. Also make sure you have enough fuel in the tank to cover the pickup.

05-06-2017, 10:51 AM	#5
pbp1 Junior Member Join Date: Mar 2013 Posts: 80 Likes: 12 Liked 27 Times in 6 Posts	Re: Question about fuel injection from a newbie Now that you are logging Fuel Pressure, and you see that the pressure is dropping drastically, you have narrowed the root cause of the issue down to the fuel delivery system. I personally think that 30# injectors running at their standard set pressure are borderline, but lets get your fuel delivery problem fixed first, and then, re-evaluate the injectors. Once you get the fuel delivery problem fixed, you will need to lower the pulsewidth numbers in your fuel table because right now, they are inflated from trying to compensate for the lack of fuel flow. Once the fuel pressure is constant, you will need to lower the pw numbers, which will bring your duty cycle down. Think of it like this, at 25 psi, you injectors no longer flow 30# per hour. At 25 psi, they are more like 20#s per hour, so that's what you have at WOT. Here's some simple math that applies: The Fuel Consumption for this type of engine should be around .42 - .45#s per hr. If your engine makes 500hp, then it will need between 210 - 225#s per hr. 225 divided by 8 injectors = 28.125#s needed per injector. If your injectors can flow a maximum of 30#s, and you need 28.125#s from them, then 28.125# divided by 30 = .9375 or 93.75% duty cycle. If you raise the fuel pressure (and the fuel system is able to maintain that pressure at WOT), that raises the potential flow of the injector. If your injectors are rated to flow 30#s at 45 psi, then they will flow around 35#s at 60 psi. Now, you have 35# injectors, so the fuel you need - 28.125# divided by 35# = .80 or 80% duty cycle. If I were you, I would fix the fuel delivery issue first. Then, I would upgrade to 36# injectors just to give you a little more margin for error.

05-06-2017, 04:05 PM	#6
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 390 Times in 170 Posts	Re: Question about fuel injection from a newbie 36 lb injectors flow 42 lbs at 60 psi. Smaller injectors at higher fuel pressure atomize the fuel better. Finer mist is easier to ignite. There is a reason the later (LS family) GM engines went to 60 psi. __________________ Ed Wright 4156 SS/JA

05-09-2017, 08:34 PM	#8
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 390 Times in 170 Posts	Re: Question about fuel injection from a newbie I didn't think about him maybe having the regulator in the wrong place. HAS to be AFTER the fuel rails, controlling return only. Surely his battery or batteries is/are in the trunk. I let my FAST ECU control my fuel pump relay, just like a factory vehicle. My relay is back there too. All 10 gauge wire from my batteries to the relay, then to the pump, and a ten gauge ground from the pump back to the battery's ground terminals. My batteries are grounded directly to the engine, with a long, large gauge ground cable. I have a ground cable to the body too. My battery cables have all terminals soldered on. I've logged my fuel pressure. Never changes. Solid at 60 psi. __________________ Ed Wright 4156 SS/JA