Sorry for all the stupid questions, but I want to understand this stuff better before I install my blower.

1. What is the normal fuel pressure range in the newer C5s?

2. When should you get concerned? What is an acceptable psi?

3. When an injector is rated at 36#/hr it is based on a specific fuel pressure. Is it always based on 3-bar 43.5psi?

5. What is considered to be an acceptable duty cycle for your injectors at full throttle?

6. What is the big concern about upgrading the fuel pump in FI setups? Is the concern about not having enough psi, or is it that the duty cycle for the pump is too high and the probability for failure is increased.

Thanks in advance for your responses.

 

TTT.... Come on guys, just throw out what you know.

 

Off the hip thoughts...
85% D.C., no more.
56-60psi normal, dips below 50psi could be an issue.

I believe that's accurate...

 

As far as pressure goes, on my 98, after the SC was put on, here is what happened on the dyno. Before upgraded fuel pump and boost-a-pump My fuel pressure would go from about 60 PSI to 80 PSI, but then start to fall off at 5200 RPM. After fuel upgrade I had steady pressure up to redline.
JB

 

Injector Rating: pounds of fuel it can flow in one hour at a fuel pressure of 300kPa or ~43.5 psi is what I read, though a tuner once told me 45psi, but I think he rounded that off. I believe Bosch rates at the ~43.5 psi. FYI: I did hear of one place selling 50# injectors but they were really rated as 50# at 60psi, so in actuality at a standsrd 43.5psi they were more like 36-38#.

Duty Cycle: 45-75% duty cycle would give you some leway. I would go for the lower end with forced induction, since you need to overcome the intake manifold pressure at full boost. I would go with a larger injector that can flow more fuel at a lower fuel pressure than to perhaps overburden a fuel pump by trying to up the pressure to compensate for the extra fuel.

Rule of Thumb: general rule of thumb is that it takes about .5 pounds of fuel to make 1 HP for a naturally aspirated motor and .6 pounds for a forced induction motor. Ex: say you are looking for 600 hp with your supercharger, then 600/8(cylinders) x 0.6 = 45 (#injectors). But this is at 100% duty cycle, and rated at 43.5-45psi, so I'd still go larger even running about 60 psi on a fuel pump, around 50-52#.

Acceptable Max Duty Cycle at WOT: I would feel 75-80% to keep some leway.

Fuel System Upgrade: I think you are correct on both counts, adequate injectors but also strong fuel pump. Again, I would go with larger injector size to meet demand than just count on raising the fuel pressure. I think it pays to do both. I did both.

Here is a link I found to an injector size calculator I found useful instead of having to calculate everything out. Actually I did it both ways, and came out nearly identical for my needs based on my fuel pump, hp, etc:

http://www.rceng.com/technical.htm


Disclaimer: I am no tech/mechanic, this is just information I was told or read. I know, you can't beleive everthing you read. Maybe some of the more knowledgeable folks will add their thoughts.

ZZ

 

LT1 injectors are rated at 3.0 bar
LS1 injectors are rated at 3.5 bar...?
Bosch/Ford are rated at 2.7 bar

1.0 bar = 14.5 lbs......NOT 14.7 lbs. (atmospheric)

Helpful?

 

This is great info guys. Keep it coming. The above site with the calculator is great.

A few follow up questions.
2.1. Is it possible to size the injectors too big. Would it be difficult to tune if you went with 50# injectors but only ran them at say 50% duty cycle. Wondering if the PCM injector scaling can handle that or if there would be issues with a non linear flow curve when only going to 50% duty cycle. Someone had a link to site that showed some injectors were pretty linear, so my first thought is that this would not be a problem.

I’m starting out my D1-SC at 5-psi, but I would like to go ahead and size the injectors for 7-8psi.

2.2. As far as the fuel pump goes, I’m assuming that you have to have a steady pressure or your air/fuel ratio will suffer. Is there anything built into the PCM to allow for changes in pressure at the fuel rail during WOT operation. Don’t remember ever seeing anything like that, but just thought I would make sure.

2.3. I also want to make sure that fuel pressure is a completely valid indicator in this system. Is it possible to have good pressure, but not enough volume. In other words, is there any other bottlenecks in the system besides the fuel pump. (say the fuel rail diameter…).

 

I am interested in seeing these questions answered also. Thanks :D:D

 

Good questions. Here are a few of my thoughts, FWIW:

I would think that if the injector is too large, then at idle it will open and close so quickly that idle will be poor and the engine will be hard to tune. I would think 50# running 45-75 duty cycle should not be a problem.

I think the fine tuning is a challenge, and likely one of the reasons for the problems with Superchips. I know my tuner is having a time getting mine just right, the tuning is especially challenging on modified engines.

I asked my tuner about the fuel rails also, I've seen some aftermarket ones, but he felt they were not needed.

ZZ