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Fuel Pressure Troubleshooting??

vetteboy86

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I currently have a fuel pressure issue. I have set my holly AFPR for 45 to 47 psi with the vacuum line disconnected. I find that as my RPM's increase the fuel pressure will actually drop instead of approach the psi I set it for.

I have replaced the fuel pump with a WALBRO. I replaced the fuel filter last spring. I have 24# injectors. This is the second AFPR the first was a crane, and now this holly.

I am going to drive the car again monitoring the fuel pressure only this time I am going to unplug the vacuum line to the AFPR to see if the pressure still drops. I am going to try to scan the car and determine the duty cycle of the injectors. My thinking is that the injectors are undersized, and they are full on causing a decrease in fuel pressure. The only thing I cannot get over is that I think the new fuel pump should supply more fuel then the injectors can provide.

Any ideas let me know...
 

bossvette

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24 lbs is probably too small for a 383 when I built mine the techs at Holley recommended 30 lbs injectors for my set-up.

is the cam computer-compatible?
 
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vetteboy86

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Well I have ran numbers from a few equations and I am believing that the injectors are too small as well. However that doens't get me past the issue of loosing fuel pressure, I would think that the pressure would remain, unless the duty cycle is greater than 85%. I had some tuning done, although I think I am not exactly there yet.
 

bossvette

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I would think its simple plumbing, you can't build pressure without a restriction; if your injectors are open all the time to try to supply enough fuel for the engine you won't have any restriction to build pressure, but what do I know I'm just a back yard mechanik ;)
 
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vetteboy86

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I understand your thought process and it very well may be correct. I guess I dont understand exactly how much fuel the injectors supply. In my head I see that they dispense a spray of sorts. Best analogy I can think of is a nozzle on the end of a hose. With no nozzle the pressure is low, but the volume is pretty good. When you put the nozzle on the spray setting you may bet more pressure but not necessarily more volume. Does that make any sense?

I dont want to necessarily buy injectors, but I think they are undersized anyways. How radical is your 383? I will probably just buy them and then address any issues that arrise from there. Also do you have any knowledge of a different fuel rail, or will the stock supply all the fuel I would need??
 

bossvette

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Not radical at all, computer friendly Isky Hyd roller cam .485 lift 52 MM throttle body Brodix street heads flat top pistons. SLP runners ported baseplate and plenum pulls good to 5500. I am using a Holley Commander 950 ECU replacement for the stock TPI it tunes with a laptop.
I will eventually be putting it in my 68 after a rebuild. When I built it in 99 I contacted both Holley and Accel Holley was the first to respond with their recommendations and was always quick to answer questions. I'll probably swap out the hard parts for their stealth ram when I do the swap.
a 383 is almost 10% bigger then the 350 so the 30 lb injectors don't have to work as hard as the 24lb do. One of the things that the Holley manual cautioned against was to go "static" with the tuning which is when the injectors are open all the time. You may have that condition.
 
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vetteboy86

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Hey it works for me. It makes sense that the 30's wont have to work as hard or be oversized since fuel pressure can be lower and the computer can cut back the duty cycle. What fuel pressure are you running? I think I might just go ahead and purchase some 30's. Do you have any suggestions? I have heard alot about the Ford pink tops!
 

bossvette

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I don't remember the exact fuel pressure but I believe I cut it back to 40 its been a while since I have done anything with the 76 (C5's will do that to you)
I have Accels but like you I hear that the fords are the best I have a set of ford yellow tops for the 305 thats going in the 76 for Craig jr to drive.
to many projects to little time or money ;LOL
 

bossvette

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I had the same type of problem with Thunder.
I do not understand how but I get better MPG with 30# injectors than I did with the 27# ones. :crazy

:L but its probably like i said the 30lb injectors don't have to work as hard, you know the old saying "nothing beats cubic inches" ;LOL
the Holley manual did say something about better off being a little bigger then too small, I'll have to try to find the manual it may even be on their web site
 
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vetteboy86

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Any other suggestions or opinions??
 

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