Injectors Vs Fuel Pressure

[brian617]

In my opinion high pressure small injector vs low pressure big injector you still end up a similar flow rate. The hardest part is knowing how much injector/pressure is needed for your particular build. I too had problems with one of those "18psi" springs coil binding at 17psi causing fluctuations. I went with a 94-95 BBC spring thats good for 30psi (thanks Dave) and 305 injectors @22psi to get the flow rate I needed. The key I believe is after finding that magic flow rate you need is to find a spring that can achieve that desired pressure easily and consistently, then you can finally tune from there. I spent a lot of time tuning around that coil bound spring chasing my tale trying to sort out a VE table and other issues. With my current setup I had to start over completely with a fresh bin because I had so many things changed chasing my fuel problem.

[Six_Shooter]

You could go to an external regulator to have better control of pressure.

[BLG355]

I was hoping I could get the same flow rating with small injectors and more pressure, but i didn't know if there was any bad side effects to doing that. I used a injector flow calculator to determine i need about 22-23psi from 61lb injectors to get the same fuel as I was getting from the 7.4 injectors at 14.7 psi. I have been through so much on this project so far, i almost forget what i did and didn't do. I've been using carburetors forever, this is my first time messing with EFI, so i am feeling like a major rookie again.

My regulator is actually working quite well now. I can easily get more pressure if needed or less. I tacked a lock nut onto the bottom of the housing to help lock the allen bolt I used in place. The head of the bolts fits perfectly into the hole in the throttle body base and i could adjust it if i really wanted to on the truck.

[buddrow]

BLG, one minor issue you can run into by running higher pressure on smaller injectors, or any injector really is injector latency. More pressure on the pintle can require more time to open. Just a thought.

As long as your injector DC% is under 80-85 at WOT max load then your good. So, if your blm's are in line and your injector DC is inline then you should be golden.

I ran into issues with using the 7.4L injectors on an engine that didn't really need that big of an injector. Sure, I could have tuned it out but it seems a bit like running an 850cfm carb on a 231 6cyl, low speed driveability suffered but once you got rolling it was fine. switched to the smaller 5.7 injector, refined the tune a bit and ran like a top throughout the operating range.

Buddrow

[RobertISaar]

More pressure on the pintle can require more time to open. Just a thought.

this is what i was going to bring up....

i've seen injectors that are intended to operate at 43.5PSI get run up to 50PSI and have a complete failure to open. whether it was due to age or just injector design, i don't know.

[BLG355]

BLG, one minor issue you can run into by running higher pressure on smaller injectors, or any injector really is injector latency. More pressure on the pintle can require more time to open. Just a thought.

As long as your injector DC% is under 80-85 at WOT max load then your good. So, if your blm's are in line and your injector DC is inline then you should be golden.

That is what I wasn't sure about, I didn't know what kinds of potential side effects that could be of doing it. I'm going close to double the 9-12psi normal service range. I've read some guys have no issues and others say to run bigger injectors but don't say why. I personally like the feel of the smaller injectors at more pressure, the engine feels smoother, especially below 2800 rpm. I've seen some guys running similar pressure so I assumed it was OK, but I'm far from an expert at this stuff and I trust all of your advice and information, I read on this forum a lot and I am very impressed with how nice and knowledgeable everyone is.

Is there a way form the data log to determine injector duty cycle? I found calculators but I couldn't make sense out of it. Mark told me when I had the big injectors in that we were good, but i didn't know what it is. So i just figured I'd get the BLM to go close to where is was on those logs and that would be a way to have a decent idea i am in the ball park, I hope. I figured the numbers were high with 19psi, so I'd bump it up to 20 and try it again and so forth. I think it worked to get me flowing close to where i was.

Barry

[brian617]

80pph injectors @ 14.7psi is 88.5pph to achieve that with 61pph injectors is 25.5psi equating to 88.9pph which with a BSFC of .50 and 85% duty cycle is good for 300hp

[Fast355]

80pph injectors @ 14.7psi is 88.5pph to achieve that with 61pph injectors is 25.5psi equating to 88.9pph which with a BSFC of .50 and 85% duty cycle is good for 300hp

GM actually made a set of "high pressure" 61# injectors that operated at 28-30 psi......Flowed around 104pph IIRC.

[Hog]

GM actually made a set of "high pressure" 61# injectors that operated at 28-30 psi......Flowed around 104pph IIRC.

Did you ever find out any info about those super huge flowing TBI injectors from Brazil? Went they something like 160 lb/hr or something. Designed for all teh Ethanol that Brazil uses.

peace
Hog

[BLG355]

80pph injectors @ 14.7psi is 88.5pph to achieve that with 61pph injectors is 25.5psi equating to 88.9pph which with a BSFC of .50 and 85% duty cycle is good for 300hp

This has been the hardest part for me, determining how much fuel. I used Desktop dyno to estimate the power, which depending how you enter the cam, it's either 275hp or 325hp. As far as the injectors I had, they were replacement from a BBC and i never replaced them, they might actually be 74lb? They are napa injectors and they don't know what they flow. I think i used 74lb when i figured on what i would need with the smaller injectors because the math comes out of then. I used them originally because I knew the engine needed more fuel, and i already had them, so i put them in and truck ran decent on stock chip, better than it ever did on any of the six TBI chips.com ones.

Brain617, I see you build specs are pretty similar to mind, do you know roughly how much HP yours makes? I did little things to get the extra HP i do with all engines, tight quench, balance it, minor head work, port matching, just to try to gain a tad more than if i didn't do anything at all ya know. My cam is an isky roller, 204/209 dur @.050 and makes .480/.496 lift with my 1.6 roller rockers, so it's marginally larger the the ram jet cam.

BLG

[brian617]

Brain617, I see you build specs are pretty similar to mind, do you know roughly how much HP yours makes? I did little things to get the extra HP i do with all engines, tight quench, balance it, minor head work, port matching, just to try to gain a tad more than if i didn't do anything at all ya know. My cam is an isky roller, 204/209 dur @.050 and makes .480/.496 lift with my 1.6 roller rockers, so it's marginally larger the the ram jet cam.

BLG

Wish I knew! I had hoped for 300hp but I think my stock intake and throttle body bore are limiting me. Practically I think it's around 250-275 and with 54pph 305 injectors at 22psi that puts me at 73.1pph. So far on my data logs with PE disabled i've only gotten to 60% duty cycle at around 4500 rpm. Need to do a WOT PE run to about 5k and see where i'm at. Have considered getting some 1.6 rocker also to give me a little more cam.