Fuel Injectors, Fuel Pump, Sequential Injection, and O2 Sensor Locations


Famous Member
Guy's, I am getting ready to order some parts and I am muddling my way through this. A fault of mine is I like to understand the numbers (engineer brain). So see if I am looking at this correctly.

All along I have wanted this build to be in the 400 HP, 400 torque range. However I want the ability to grow if I so desire. Using various calculators I am coming up a bit short on my injectors at 60 lb/hr getting to the 500 hp range.

I have been looking at these injectors:
Static Flow Rate @ 43.5 PSI ( 300kPa ) w/Gas: 60 lb/hr
Static Flow Rate @ 87.0 PSI ( 600kPa ) w/Gas: 85.7 lb/hr

  • Looking at 500 HP and a .75 BSFC, that leaves me at 62.5 lbs/hour at 100%. At 80% that is 78 lbs/hr. Am I being too conservative with .75 BSFC for a supercharged engine?

  • Next question is about the injectors themselves. It appears if I increase fuel pressure the injectors output increases. Does that affect the duty cycle?

  • Based on 500 HP, 43.5 psi Base Fuel pressure, and 10psi boost, the calculator says I need 210LPH @ 53.5 psi. Does this mean the regulator should be set to 53.5 psi, or does this mean the pump needs to be capable of 53.5 psi, and regulator is set to 43.5 psi?

  • Do I need 80 lb/hr injectors? Or would you let that fall into that 80-100 percent range since you will only be using the full rating at the higher HP and a conservative BSFC?

In regards to the pump, the walbro GSL392 is rated at 255LPH @ 50 PSI (max). That means at 500HP and 10 psi, the Base pressure would have to be 40 PSI, to keep from exceeding the max pressure rating. This would limit the 60LB/hr injector to 57.5 LB/hr with this pump. At .75 BSFC this limits the HP to 368.

So it seems I need to go to bigger injectors, or a higher rated pump to push the 60lb injectors harder? Am I correct? Or do I need to back off of the BSFC and recalculate?

Next question is in regards to Sequential injection and O2 location. I was under the assumption that in order to tune for sequential I needed an O2 sensor in exhaust port (6 in total). However talking to a local with a dyno, he says I do not. He said best bet would be to collect both collectors into a y pipe and install my O2 sensor there for a dyno run to tune. Is this correct? Can we setup sequential injection this way?


Famous Member
Since the BSFC of the engine is poor to begin with, .75 seems safe to base the fuel system off of on gas.

First bullet: flow doesn't go up with less duty cycle.

When increasing differential fuel pressure across an injector, the flow goes up, but not linearly. The dead time of the injector also goes up and some lesser injectors quit operating above 70psi differential. So, it kinda affects the duty cycle, but not really the correct way to look at it I guess.

The Walbro 255 is at its limit around 550hp on gas, but that's also pressure dependant. As pressure across it goes up, flow drops. There is an upper limit where the internal relief opens and it drops rapidly, but there is a high pressure version that can operate up to 90 or so psi. At that point, I'd recommend a Bosch 044, as the 255's flow isn't much at high pressure.

Base pressure at 43, pump needs to handle 210LPH@53psi at 10psi manifold pressure and a 1:1 fuel pressure regulator.

I'd recommend a bigger injector than 60lbs/hr. using your calcs, unless the injector can handle higher than 70psi differential and use a Bosch 044.

Sequential doesn't require one o2 per cyl. It can be tuned the same as batch fire and any individual cyl trims can be made in the tune after looking at plugs or best guessing. Individual cyl EGT can be used as well, either real time or after the fact in the tune. Both fuel and timing can be trimmed depending on what you want, or none at all. One wideband o2 in a common pipe has been used for years and still works for global fueling. One benefit of sequential fuel injection is also requiring smaller injectors than batch fire, since they're firing at least half as often and the dead time doesn't add up.


Famous Member
P.S. You can also get a global tune with a single o2 sensor in the exhaust after the merge, then go back and put the o2 in each primary runner one at a time to log the differences and make changes from there. Histograms probably make the best sense there.


Famous Member
Your first post you said "if" the injector can handle 70 psi...the specs say it can handle 87psi at 85.7 lbs per hour. Or am I missing something?

Second post is where I was headed. I don't want to repaint headers. So if I want to individually tune I need to install o2 bungs now. So it isn't necessary, but would further improve performance. Wonder if having that bung welded in affects exhaust flow?


Famous Member
Unless the bung is seriously sunk in the runner, I doubt it affects anything measurably. I question more when the o2 itself is placed far in, but haven't seen any info on it.

The Deka 4 injectors are good, but I wouldn't try to run them at the limit of their pressure rating. Injectors like to lock up and quit when at the limit. A base pressure a bit below should be fine. Maybe 80psi max? You're still taxing the whole system a lot more than a 43psi base - pump, lines, regulator, injectors, connections, etc.