Trying to understand LT1 HPFP to LT4 HPFP settings

[Sneakyws6]

So I am running into a problem with fuel pressure on my car. Here is the current setup:

2017 Camaro SS 1LE
ARH Catless headers
Procharger D1SC with 4.25" pulley
LT4 HPFP
LT4 injectors
JMS BAP

The car already had the LT4 LPFP since it is a 1LE car. Now here is the issue. When I updated the fuel system with the tuner (who has been amazing) we had hoped to be able to get around E60 with E85 and be able to make around 700rwhp. For some reason the HPFP pressure is dropping off. Low side stays solid and in the 67psi or higher range, but sometime after the car crosses the 5100rpm point the HPFP side drops from the 2800psi down to 2250psi at the 6500rpm shift point. Because of the HPFP dropping off it is limiting me to E40 and leaving some power on the table.

I thought maybe it was a defective HPFP but after speaking to the GM tech at the dealership I know he said they rarely fail and it is more than likely the ECM opening the high pressure bleed off solenoid on the HPFP allowing it to bleed off pressure.

What are we missing here? I have went thru the entire fuel system, the low side is solid and I even replaced the stock LT1 fuel pipe that has the restricted orifice in it for the non-restricted LT4 fuel pipe with no change.

Why am I losing HPFP pressure once I cross over 5000rpm?

Todd

[sevinn]

Have you logged your desired pressures? What about soi/eoi? You may just be out of pump. I really wouldn't expect it to keep up with that much ethanol to be honest.

[Sneakyws6]

Not yet, we think part of the issue was a scale on the stock N/A tune for the LT1 being scaled wrong from the factory or it is just the way GM scales the N/A LT1 engines.

I fixed the scale on the table to match the ZL1 scale and I also made a tune for the Z06 table. We also found the tune was dropping the low side pressure off from 560 to 500 once the fuel flow rate got over 223lb/hr.

Next step (if my MPVI2 shows up today) is to flash the tune and if that doesnt work check the install height on the LT4 HPFP.

Again what is odd, another car with the same exact mods and fuel system is able to run near 2900 psi while running E68 and I cant get there for some reason.

[sevinn]

Yeah the scale is funny, it's like they moved a decimal point or something. And you are right the factory scaling will allow the low side to get pretty low at high flow rates.

I think it would be helpful to just log it as it sits with desired pressures added so that you can see what you are actually chasing. If you don't know what desired rail pressure and lowside is it's hard to know what to look for.

[Sneakyws6]

I replaced the LT4 HPFP last weekend, seems GM has made 2 revisions to it along with changing the inlet feed fuel line from the HPFP to the fuel rail distribution block. The new fuel line removed a 90* bend in the fuel line and replaced it with 2 - 45* bends on the revised part. So far the fuel pressure is holding better and I have been able to move from E40 to E50 and not lose any real pressure. The injector times have came down as well to the low to mid 5.x range as well now.

[sevinn]

Great to hear. Do you think it was the pump or the pipe that made the difference? Did the pipe you removed have the checkvalve in it by chance?

[Sneakyws6]

The pipe with the orifice you are talking about is the one that provides the low pressure into the high pressure pump. The new pipe that GM revised is the one that goes from the HPFP to the distribution block that feeds both fuel rails. I already had the non-orifice pipe in the system from when I upgraded all of the LT1 engine side fuel system to the LT4 stuff.

So far not fully sure if it was one part or a combo of all of the parts. I am gonna see how it does on E50 for now and slowly take it up some and log the pressure and see how it does along the way.

[Twin16ss]

Got the part number for that revised feed pipe

[sevinn]

The pipe with the orifice you are talking about is the one that provides the low pressure into the high pressure pump. The new pipe that GM revised is the one that goes from the HPFP to the distribution block that feeds both fuel rails. I already had the non-orifice pipe in the system from when I upgraded all of the LT1 engine side fuel system to the LT4 stuff.

So far not fully sure if it was one part or a combo of all of the parts. I am gonna see how it does on E50 for now and slowly take it up some and log the pressure and see how it does along the way.

What was the part number of the new pipe?

[Frost]

The new pipe with 45's is GM and ACD part number 12677005

[sevinn]

The new pipe with 45's is GM and ACD part number 12677005

Thanks!