Gonna be swapping a 450 hp 6.0 LQ4 into my K5 but how do I determine the fuel pressure needed to run it? I'm gonna be using an Aeromotive A-1000 pump so that's not an issue lol. I've searched a bit but I couldn't find much 
CK5
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Determining Fuel Pressure Needed
- Thread starter Coontail
- Start date
Depends on size of the injectors. I was told to start at stock pressure and monitor the duty cycle of the injectors, if you are getting close to maxing them, up the pressure.
I'm sure someone here probably has a formula to figure it out without testing though.
I'm sure someone here probably has a formula to figure it out without testing though.
What injectors will you run? Typically they work best within a certain range of pressure. Going from 325 - 450HP is 38%, so you would go from a 25lb/hr injector to something like 35lb/hr instead of cranking the pressure to 83psi. So pressure is a tweak near the end, as stated above, not the main path to your fuel delivery needs.
I'm buying the engine from my brother and he hasn't given me the full run down on what this thing has got so I really don't know what size injectors it's got lol
This is all makes sense though, what should the duty cycle be for the injectors?? Or is that entirely dependant on the size of the injector?
yes, dependent on injector and pressure
Is this a return or returnless fuel system? Might just run the Vette filter/regulator. It's fixed at 58psi and seems to be what all the LS swaps use when it's a returnless fuel rail. Mine works good but I'm only at 300/300. My 400sb is only 330/420 IIRC, only runs 42psi.
Is this a return or returnless fuel system? Might just run the Vette filter/regulator. It's fixed at 58psi and seems to be what all the LS swaps use when it's a returnless fuel rail. Mine works good but I'm only at 300/300. My 400sb is only 330/420 IIRC, only runs 42psi.
80% on a warm day is what I was told. If tuning at sea level on a cold day with really dense air I could see 90-95%, but that won't leave much room for growth.
That's the duty cycle at your HP peak at WOT and it's kind of an upper end. At idle duty will be pretty small. Having it too small at idle can also cause problems, but generally that's not an issue on newer engines unless you go to 100lb injectors or something. If the injectors are only at 70% max, that's probably OK. As long as you get everything in the right range, it all dials in with the tune.
Will you be tuning it or taking it to somebody?
Will you be tuning it or taking it to somebody?
I'm planning on doing a Diablew Tune
Do they do actual tuning, or just provide canned file programmers? To get 450hp, the engine has a different cam and probably some other stuff done to it that makes a factory engine tune wrong. At minimum you need a custom mail-in tune where you tell them everything on the truck and they adjust cals for it. Better yet is where you drive around with a datalogger and mail the data to them. Best is an in-vehicle tune on a dyno, which can hit all operating modes and be done in a day. Mailing files back and forth iteratively can come close to that, though.
From what I've read the Diablo Tuner itself is a canned file tuner but there's a guy named Lew who writes custom tunes for you. He essentially does what you said for a mail in tune. The hand held tuner itself is only used to load the tune. I could be wrong though. I haven't set my mind on that though, I've just heard very good things about what he does
So I just got out of work and took a picture of the injectors. It appears to be a Delphi 8.1L injector but there are different varients I guess. Any clues?
EDIT: they're 38# injectors
EDIT: they're 38# injectors
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It appears to be a return less
There is a mathematical equation at this link for what you are looking for https://www.dynamicefi.com/TBI_Fueling.php
So wait a second, all I have to do is supply a constant fuel pressure. It shouldn't matter what the pressure is in the fuel rail as long as it's not too low and is consistent. Am I over thinking this? LOL
That's kinda what I was thinking. As long as it's enough, and I would assume a Vette regulator at a fixed 58psi, would be enough.
The tune will adjust how long the injectors need to open for, according to the load and injector size.
My FAST system has an adjustable fuel regulator and requires me to input the fuel pressure. I can move the pressure up or down depending on the duty cycle of the injectors. If I'm close to what Chris said, I would up the pressure so the injectors are maxed out.
You start getting into the higher HP numbers is when volume comes into play. Pump might not be able to keep the pressure correct. At that point we need more pictures
The tune will adjust how long the injectors need to open for, according to the load and injector size.
My FAST system has an adjustable fuel regulator and requires me to input the fuel pressure. I can move the pressure up or down depending on the duty cycle of the injectors. If I'm close to what Chris said, I would up the pressure so the injectors are maxed out.
You start getting into the higher HP numbers is when volume comes into play. Pump might not be able to keep the pressure correct. At that point we need more pictures
For more pictures follow my build threadThat's kinda what I was thinking. As long as it's enough, and I would assume a Vette regulator at a fixed 58psi, would be enough.
The tune will adjust how long the injectors need to open for, according to the load and injector size.
My FAST system has an adjustable fuel regulator and requires me to input the fuel pressure. I can move the pressure up or down depending on the duty cycle of the injectors. If I'm close to what Chris said, I would up the pressure so the injectors are maxed out.
You start getting into the higher HP numbers is when volume comes into play. Pump might not be able to keep the pressure correct. At that point we need more pictures
Kind of. As long as that injector size at that pressure can deliver enough fuel, it all gets fixed in the tune. But the injector works best in a certain range, for correct "atomization". In many systems the pressure regulator is referenced to manifold pressure, so the pressure you read in the rail goes up and down as you drive, the but pressure from injector inlet to outlet is close to constant. It's supposed to give more dynamic range from the injector and make the tuning more linear.
Just set it up in the stock configuration with the right injector and fuel pump and get it tuned.
