Injectors Vs Fuel Pressure

[BLG355]

Hey Guys. I have been working with Mark for a few months, he burned me a starter chip, and I have to say he has done so much for me with helping me get my truck dialed in, he should really charge more because he offers a service that is priceless. He really is awesome to deal with and I'm so glad I came here after getting screwed by TBI Chips.com. Before Mark went in for his first surgery I talked with him about some issues I was having and I wanted to try different injectors and run some logs, and I was going to send them over to him for so he can check them out and see how it's going. I was hoping maybe you guys could give me some feedback on what you see or think.


I am not burning my own chips, I was going to at first but after talking with Mark I decided to let him to it for me. I am on a starter chip right now, and the truck runs and drives good. I am trying to get a good setup that doesn't have the smaller injectors maxed out. I am very engine savvy and have been building engines for years, but I am not so super with this TBI stuff. My question I have is, what is better, larger injectors with less pressure, or smaller injectors with more pressure. So, on Mark's starter chip, I was running 80lb injectors with 14.7psi fuel pressure. The truck seemed pretty good but just never felt quite right. I talked with Mark about running 61lb injectors with more pressure and run some logs to see what it looks like. So I started by making an adjustable regulator which has been quite interesting, and I ran some logs with varying fuel pressure.

My 80lb injectors are new Napa injectors which I think they may be junk. The engine never seemed real smooth. So I wanted to try my GM 5.7 injectors with more pressure. I cold never get the pressure over 18-19 psi without it being very finicky. I learned the spring was pretty much coil bound when trying to adjust it higher than that, so I took found that a spring from inside a wheel cylinder feels about half as strong as the stock pressure spring and fits inside it without and issues. So I doubled the springs up and can easily get my pressure deep into the 20 psi range.

Run 1 was too much pressure, before I double the springs and I was coil binding the stock spring. It was 26 PSI
Run 2 was at 21 PSI
Run 3 was at 22 PSI
Run 9 was one of the last logs I ran with the big injectors on 14.7 PSI


I noticed the engine feels smoother and seems to run a lot better over all with the smaller injectors, and again, this may be because I have junk 80lb injectors. I was hoping to find a set to have sent out and cleaned and flowed but I haven't found a set yet, and I am hoping the smaller injectors with more pressure will be fine. Is there a way to tell how hard the injectors are working? I was basing it where the BLM was going, it seems close to where it was with the big injectors.

Thanks in advance to anyone who responds, your feed back is really appreciated.

I guess you may want to know the specs:
95 K1500 5.7
355ci, zero decked block (.039 quench)
flat top 2v pistons (9.6:1 compression)
72cc dart 165cc heads
isky roller cam 211252/257 w/1.6 roller rockers
Edelbrock 3704 intake
stock manifolds, dynomax cat back

Thanks,

Barry

[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