F.I. fuel pressure theories

[Stan Weiss]

Originally Posted by KRatcliff

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


YMMV.

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

Stan

[Bobby Fazio]

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.

[Pierce Jones]

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.

[Stan Weiss]

Originally Posted by Pierce Jones

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.

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

[Dave Noll]

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

[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!

[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?