F.I. fuel pressure theories

[Dave Noll]

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

[e vassar]

Originally Posted by Stan Weiss

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

Stan

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?

[Terry Drinkwater]

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

[Signman]

Originally Posted by e vassar

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?

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!

[Pierce Jones]

Originally Posted by Stan Weiss

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

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.

[Stan Weiss]

Originally Posted by Pierce Jones

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.

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

[e vassar]

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!