If you look at the fuel pressure regulator on the driver side fuel rail there is a little vacuum line attached to it that comes from a port on the top of the intake manifold. That is the manifold pressure reference. Our fuel systems raise fuel pressure as manifold vacuum decreases or gets closer to outside pressure(0psi on your boost guage).

So at idle or cruise(low load) the vacuum is high in the manifold which causes the regulator to open the fuel return up which causes the pressure to be low. When you get on the gas the manifold goes from having a vacuum to having the same pressure as outside. If you were naturally aspirated this would show up on a boost guage as 0psi. Now when the pressure in the manifold is high(0psi) it causes the fuel pressure regulator to close the return down some which raises the fuel pressure.

The fuel pressure regulator on our trucks is very similiar in function to a FMU but, a FMU raises pressure at much more than a 1:1 rate like our regulators.

When I had my mechanical fuel pressure guage hooked up and the truck idling the pressure was 50psi. Then I just pulled that little vacuum line off the regulator for a couple seconds which causes the regulator to see regular outside pressure(0psi on a boost guage) instead of the manifold vacuum. When the reg. sees outside pressure the fuel pressure rises to 59psi.

Your fuel pressure shouldn't be staying at 50psi. Could be a bad regulator or clogged vacuum line??

 

Not to hijack the thread but what other fuel system updgrades are you running to feed those 42# injectors?

 

No other upgrades right this minute. The motor only takes a certain amount of fuel no matter what. These injectors just don't work as hard as stock injectors to get the fuel there.

They should be capable of supporting 600rwhp though and that will require something to be done with the fuel pump, still undecided what I want to do about that.

 

Thanks for answering. I figured an upgrade from the stock 24.8# to 42# meant that you needed all the extra capacity already.

I asked because I am undecided if I am going to go with a boost a pump, an inline pump, or nothing when I switch to the 43# delphi 8.1 marine injectors. My whipple ran lean with the auxiliary injectors, which i want to remove, my 5.7 spins higher than my 5.3 did so I want to make sure that I have adequate fuel supply.

I know Kenne Bell kits come with their boost a pump, I read that Lingenfelter uses a boost a pump on their FI trucks, but the radix comes with an auxiliary pump so I am trying to figure out what is most used and most proven.

 

Oops , I wasn't sure what you meant so that's why I answered like that. No, I'm just starting to get my fuel system ready for whatever happens next.

I think parish8 used an inline pump with good results. I've heard good about boost a pumps too but, have no first hand knowledge on them.

BTW, the 42# SVO injectors are rated at 43.5psi so they act like ~50# injectors at our fuel pressure. The only problem with them is that they are not direct plug ins. I think the injectors your going with are the biggest that are.

 

Man I am glad you told me about the rising rate. I really didn't even notice, because it only rose a couple of psi anyway w/o the fmu. I ordere done form my dealer for like 61 bucks
Is it supposed to rise more thana couple psi under load/vacuum/ etc. ?
Thanks.
If so, I will be messing with tuning a bunch again
Dammit I need that Innovative motors wideband soon! Sam

 

Hell, you should've told me you were going to rush out and buy a new regulator. I would've sent you my old one for free that works fine now that I found the $92 o-ring hiding in the old fuel rail.

Did you check your current regulator first to see if it was raising pressure at idle with the reference line pulled off? If it does while the trucks just sitting there in park then it may not be the problem. Could be that the fuel pump is having trouble keeping up or a just a clogged filter or something. KEEP your reciept and check that stuff too. :p

 

xlr8-FYI I have a KB boost a pump that I bought back in 97 and had on my Stang from then til Oct '01. When I put the 42's in my truck I noticed that the pressure was going away at high boost/high rpms. So I took the BAP off the shelf and put it on the truck and I run it full time with the **** turned about 1/2 and so far the pressure has held rock steady. Definitely easier than a fuel pump swap.

 

Yea I tried it, with the vacuum line unhooked. No change. The line has strong vacuum pull thru it too, when I take it off the regulator nipple.
Is it only supposed to raise a couple psi under rpm/vacuum/etc.? (with a good regulator?)
BTW-thanks for the offer

 

That's good to know about the BAP. I may try to find a used one for sale if I start having pump trouble.

Sam - The pressure will vary between ~50-59psi depending on the MAP(manifold absolute pressure) reading. When you get into boost I think it should rise 1psi for every pound of boost. I've never had a blower with a fuel system like we have though so I can't say for sure about the boost part.