F.I. fuel pressure theories
 

36lb injectors at 50 psi or 42lb injectors at 40 psi . If I am able to achieve an optimal A/F either way...which will produce more power and greater consistency? Either? My theory is the higher pressure going through the smaller orifice will produce a finer mist..or better atomization which is always better? Or not?

Re: F.I. fuel pressure theories
 

Has anyone looked at say running the 42 lb injector at say 55 PSI which would have a reduced pulse width / duty cycle verses the 36 lb injector at 50 PSI?

Stan

Re: F.I. fuel pressure theories
 

I run my 24 lbs injectors at 70 psi. Theory is the injector acts larger and high pressure atomizes fuel better. I believe Ed Wright posted on this topic in the past.

Re: F.I. fuel pressure theories
 

So turn the pressure back up to 50 and shorten the duration cycle?

Re: F.I. fuel pressure theories
 

Yes, as you increase the fuel pressure the injector flow rate increases. But if fuel needed has not changed then you need to short / decrease the amount of time that fuel is flowing.

Stan

Re: F.I. fuel pressure theories
 

Yes, that is definitely the right direction based on our experience. Larger injectors, higher FP (50 psi minimum), and reduced pulse width.


YMMV.

Re: F.I. fuel pressure theories
 

Does your ECU let you control an individual injector's flow position in the 720 degree cycle?

Stan

Re: F.I. fuel pressure theories
 

I've been wondering this also. An oversized injector to get down to a low pulse width duty cycle? This is beneficial if you can phase the injector to spray at optimal time. Or a smaller injector with higher pressure for higher mist and atomization but it stays open longer and could be spraying at a closed valve.

Re: F.I. fuel pressure theories
 

Looking at a data log, my injectors (24 lbs/hr) are open 7.7msec (88% duty cycle) at 7000RPM (42 deg/msec). The intake has 308 deg dur @ .020”. So injectors are open 323 degrees of rotation and in my case spraying at a closed intake valve.

Not sure if any would be gained by a 36 lbs/hr etc.

Re: F.I. fuel pressure theories
 

One crank rotation @ 7000 RPM is ► 1 / (7000 / 60) = 0.008571 of a second

My understanding of Duty Cycle is it is the percent of open time against 2 crank rotations (720 degrees).

1) @ 7000 RPM @ 7.7 Mill seconds pulse width I get 44.9% Duty Cycle

2) @ 7000 RPM @ 88% Duty Cycle I get 15.0857 Mill seconds pulse width

I don't know what your engine BSFC or HP is. But a 24lb injector running at its rated PSI, if I guess .45 BSFC and 375 HP, that would 2# above.

Stan

Re: F.I. fuel pressure theories
 

Spraying onto a closed intake valve is losing the advantage of Sequential injection. Makes it similar to having a carb on it.

Re: F.I. fuel pressure theories
 

I think so its a big stuff 3 i had it programmed for my car (99 Camaro) and had it installed First time at the track black smoke was rolling out at idle and was trying to load up. I haven't learned how to tune it yet so i turned fuel pressure down to about 42psi to clean it up. So my thought was to change to 36lb injectors so I could turn it back up....but if I can cure the problem by shortening the pulse width.....whats the difference?

Re: F.I. fuel pressure theories
 

For my own education, what is triggering the initial pulse cycle? Crank position sensor?

Re: F.I. fuel pressure theories
 

When you change fuel pressure the whole map is leaned out or enriched.
Chances are your idle is fat but the rest of the map is close. adjusting the cells used art that RPM and Load will take care of that easily.
Raising fuel pressure makes the injector think it's larger there is a limitation on how high you can go based its construction. When an injector is tested and sized it is based on a specific pressure one thing to consider asking your supplier how high can fuel pressure be raised until the injector does not operate efficiently.

Changing injectors to enrich or lean won't be accurate or recommended. The ECU has info on the specific injector characteristics changing them out without telling the ECU will make fueling inaccurate. Your concept: injector size is scalable may be true when using the same model line if available. The idea of replacing injectors in a performance application is to size it to the fuel demand choosing a model that atomizes well is fast acting reliable and fits the manifold.


Your thinking is in the right direction but you must do some reading and ask questions methods you're considering are a real hatchet job!

Don't know your case but if known good tune was flashed to your ECU that is just a starting point and could be the cause of your idle issue since that tune may work perfectly on the engine is was developed on.



Have you run the car on a chassis dyno with a good tuner? If no the time and money will be well spent and you will learn a TON! When the tune is optimized on the dyno with a good tuner you will be well in the ball park with only minor tuning necessary at the track if you so desire. Load at the track is different from any dyno engine or chassis.



Many are intimidated by EFI tuning but all it requires is an understanding of how it works some time building confidence a calculator pad and pen.


The most valuable advise anyone can give a new tuner is to be sure to save every map changed with a unique name to assure when you get off into the weeds which we all do it's as simple as installing an older tune and back to where you started that rabbit hole.



Goof Luck!

Re: F.I. fuel pressure theories
 

I'm new to FI, only self taught and going on 3 seasons. I just got back from the chassis dyno. Engine is an LT1 with Holley HP. Running 24 lbs/hr @ 70 psi, log shows 7.99 msec, 90% duty @ 7000RPM. Chassis dyno showed 415hp ish ;)

Stan - Maybe Holley calculates duty cycle on 360degrees?

Unfortunately I didn't have a larger set of injectors to throw in and try.

My biggest issues continue to be hot start and idle quality, decided to try and sort that out at home instead of on the dyno.

I started playing with the Injector End Angle (IEA) Table for the first time. Very slick in V5, essentlially plug in cam specs and it calculates IEA based on fuel table. My set-up uses the crank tigger only.

Re: F.I. fuel pressure theories
 

Pierce,
I have never used the Holley system and so do not know how it works.


Maybe Rob from Holley will see this thread and explain it.


Based on my understanding of what you have, and running some numbers I believe the 90% is correct. I believe the 7.99 msec (what should be the pulse width) is not correct.


Stan

Re: F.I. fuel pressure theories
 

Thanks for the great advice...although I understand little of it.....I think I will not change those injectors for now. I think my ECM is close, just needs a little tweaking. I am willing to pay someone down here in D4 to throw a laptop on it!