Fuel calculation Questions

So by searching and coming up with the fuel formulas for Injector Sizing and fuel pump sizing I have a few questions and concerns. I took my car to the dyno to do some tuning a couple weeks ago. And I seem to be running out of fuel at anything over 4800rpm. During WOT runs my FP would spike to about 50psi under boost but when I would get up into the higher rpm I could see it drop to about 40psi. No matter what changes I made in my PE table my AFR was always pretty much the same.

So now I think I am running out of fuel. I when through and checked all my fittings, injectors and could not find any leaks. So my fuel pump was a couple years old so I replaced it. (255l/hr). Found the main pump hose was swolen and kinked, I thought cool, it was probably killing my volume and my pressure was droping. NOT! Still the same result but a little bit better. I also changed my fuel filter.

So here I am searching for some anwsers.


My fuel system consist of:
42lb/hr racetronix injectors
255l/hr pump
New fuel filter
Stock FP regulator
I have a FP guage in the car

My car only made 430rwhp running up to 6000rpm pulling about 9lb of boost through a P1SC. AFR would cosisantly drop points above 4800rpm, from 12.2 up to 13.2@6000.

While sitting here doing all these math problems, If I have 42lb/hr injectors, @100% DC they could only use 194.5 litre/hr of fuel @43.5 psi. That would still give me plenty of overhead for the fuel going back to the tank. I just don't see the pump being the problem.

So if I was losing say 110hp being an A4, that would be 540fwhp. 540 x .6 = 324 / 8 = 40.5 lb/hr/cyl

40.5/ .85= 47.65lb/hr injectors

Could my injectors be to small? But what is with the FP drop?

I'm just kind of stuck right now.

Did you upgrade things like the fuel lines and filter to accomidate the increased flow?
As for you injector sizing do you have a datalog from the dyno runs to see what duty cylce they were operating at?

Did you upgrade things like the fuel lines and filter to accomidate the increased flow?
As for you injector sizing do you have a datalog from the dyno runs to see what duty cylce they were operating at?


No I did not, Stock fuel lines should be fine for the power I'm making. The filter should not be that much of a restriction, it is brand new. ;)

What I see on my one datalog was 100% duty cycle but I had my PE tables super high and isn't the DC calculated. Anyway at 5400 it was 101% That just doesn't seem right.

Stock diameter lines are fine for 600 flywheel HP. I run 800HP through a -6AN supply line, which is essntially the same diameter as the stock feed line.

If the fuel pressure is falling off, its only going to fall off it the system can't supply the required volume of fuel to the injectors. A faulty fuel pressure regulator could also cause the problem. The injectors seem to be flowing whatever fuel they get, they just can't get enough.

Are you using an FMU at all? Or are you just relying on the vacuum line to the stock FPR to raise the pressure based on manifold boost?

Duty cycle is calculated from injector pulse widht and RPM. What data logging program are you using? DataMaster seems to overstate duty cycle.

Stock diameter lines are fine for 600 flywheel HP. I run 800HP through a -6AN supply line, which is essntially the same diameter as the stock feed line.

If the fuel pressure is falling off, its only going to fall off it the system can't supply the required volume of fuel to the injectors. A faulty fuel pressure regulator could also cause the problem. The injectors seem to be flowing whatever fuel they get, they just can't get enough.

Are you using an FMU at all? Or are you just relying on the vacuum line to the stock FPR to raise the pressure based on manifold boost?

Duty cycle is calculated from injector pulse widht and RPM. What data logging program are you using? DataMaster seems to overstate duty cycle.


No FMU here Injuneer. I do use datamaster. I am relying on the vacuum line to the stock FPR. My PW at 5400 is 22.25ms. I have used datamaster on one of my previous motors and always used the inj DC as a ballpark.

I'm still looking over things but I can't really find anything that is abnormal. No leaks, Gas smell, pin holes, nada....

Sucks! :mad:

Is your FP dropping? If so, it's a supply problem. What is your injector duty cycle? If it's close to 100% and it is still leaning out, you need more injector. Real world data > Internet formulas every time.

Rich