some background info. Had my car tuned a few months ago and I was told that my injector duty cycle maxed at 5500 RPM causing a lean condition past 5500. So, I set my rev limit low on the UTEC with plans to upgrade injectors from 440cc to 650cc or 750cc and install a return fuel system.

I have since noticed that when I am aroud 6-8 PSI of boost around 5k RPMs that my fuel pressure drops from 52 PSI to around 35 PSI.

So, right now I am trying to resolve my issue without throwing unncessary parts at it. I really want to try and determine the root cause of the issue before I start changing things.

My tuner says the injectors can't keep up with the amount of fuel needed and that I should add 750cc injectors. If I go to 650 or higher I will require a return fuel system. Now I understand to be on the safe side a RFS is needed and I don't mind adding one, if it's necessary. Also, For some reason I'm not convinced that throwing larger injectors is going to completely resolve the issue.

When I added my walbro 255 I did not do any modifications to the regulator or siphen jet. I just dropped it in and had an idle pressure of 52 PSI, so I figured I was good to go. Usually people experience 70-80 PSI when they install a walbro without modifications.

So, right now I am trying to decide the following route to take:

1. Replace walbro (maybe I have a subpar unit)
2. Install RFS (use stock rails)
3. Install stage 1 RFS and 650/750 injectors

Anybody with more knowledge of fuel systems (Phunk) have any ideas to help me find the root cause of this issue...thanks

 

Walbro and 440's require the vortech fuel system..The FMU/ Aux fuel pump to maintain adequate fuel. I ran like this for 2 years prior to recently adding a RFS, UTEC and canning the vortech fuel..My fuel pressure was 80 psi at idle and went up under boost to around 100 psi with the FMU/Aux..I did not drill the Walbro as is suggested.

You have UTEC and I would suggest you need a RFS, stage 1 (using stock fuel rails) and a tune..Larger inj's for insurance but not critical..

 

So you think all I need is the RFS to keep the pressure up? FYI I have the 28T sc cog on a rev-up so I'm running more PSI than stock.

 

I have a non rev up w/ 3.12 serp pulley, w/ 34T cog reaching 11.6 psi. Fuel press at 52 psi and rising with boost about 1 psi/ lb of boost..A RFS should be all you need.

 

+1 Start with a return kit and a 255. Then see if you need to upgrade your injectors.

 

You have 2 problems it sounds like, maxed out injectors, and low fuel pressure. One is causing the other, or at least making it worse.

Your injectors wouldnt be so maxed out if your pressure could keep up, they might still max out but it wouldnt be until a higher point if you had better pressure supply. Even tho you dont have a return fuel system, the walbro 255 should be able to at least keep the pressure up for your HP level.

First you need to isolate the fuel pressure problem, then you can see where you need to go more clearly from there.

Either you have a big pressure leak, your pump is bad, or there is a big restriction somewhere.

If you cannot find an external pressure leak, you need to rig up a leakdown tester and apply 60-70psi at your fuel pump assembly into its output nipple and see if there is an internal leak. If there is an internal leak, it would either be the pump o-ring grommet thing that seals the pump output nipple to the assembly, or it would be a regulator or regulator seal problem, or it would be a crack/leak in the plastic housing itself.

If there is no leak, try replacing the pump with a new one. While you are in there, perform the regulator drilling mod and the siphon jet mod to give yourself a proper pressure curve.

Lastly, if that doesnt solve it, there must be a restriction somewhere, possibly in the non-replaceable filter that is built into the fuel pump assembly. You can only replace it by replacing the entire assembly.

After the pressure issue is resolved, you should be able to get a little more out of those injectors, but not much. At that point its up to you if you want to move to a RFS and/or larger injectors. Its easier to tune larger injectors with a RFS since at idle the vacuum will cause the pressure to go lower and less MAF voltage correction is needed, and then under boost the pressure will raise and you can get the most out of your injectors. You could basically just about assume that 650s without a RFS will get you about as far as 550s with a RFS, and tuning the 550s and RFS will typically result in a smoother tune as there is less work to do to get it to idle and cruise right.

 

I would agree with Phunk in particular regards to the pump mods that were neglected. This could be the main restricting point in the entire set-up, but there is probably more to it than that. However, it would be the cheapest place to start. Of course by removing the pump from the tank you are only looking at $12 bux to replace the pump O-ring gasket. Not too bad.

Well now that you are spending money, I guess you want to go in with a plan. Be prepared to drill out the 2 points stated above and see what happens. Your pressure will probably stay higher, longer on boost.
Good Luck.

 

i was seeing 100% IDC with 600cc injectors on my Vortech kit. With my stage 2 RFS and 600cc injectors i dont even see 90%.

FWIW, I saw 72 psi fuel pressure when i added my 255l, but i didnt drill the FPR. I did drill the swirl jet. only 50ish PSI is pretty low to start with.

I now run 42 psi with my RFS and its perfect.

 

the drilling mods wont help the pressure maintain on the top end, both are specifically to reduce the excessive pressure at idle that happens since the walbro overwhelms the internal passages of the built in return plumbing

 

I installed the basic RFS 3 weeks ago. I was still loosing pressure up top, but it shifted to about 6k RPMs and dropped to about 40 PSI instead of 30...

besides that everything was going fine until thursday driving home from work the fuel pump decides to take a crap well I assume it's the pump. I haven't pulled out the assembly yet. A few miles before it died at low RPMs the pressure was all over the place jumping around like crazy.

I hear the pump come on, but before I start the car it will only pressurize to 20 PSI.

No leaks. Maybe I had a bad walbro to begin with.

 

phunk - what drill bit size should i use for the siphon jet?

 

http://cj-motorsports.com/342install.htm

 

thanks!!!

 

ok funny story...I fell like a total a$$. So I pull out the pump assembly and look at the tank and it hardly has any fuel. I guess the level arm got caught on something when I put the return system in and was stuck on 1/2 a tank. Anyway thats an easy fix. So I continue to take apart the assembly to verify everything is good as well as drill out the siphon jet. I take the pump out to inspect it and realize it's the STOCK pump???

I must have been so high on fuel fumes when I did the original install (injectors then pump) that I re-installed the stock pump instead of the walbro. Needless to say I found the walbro and installed it this time.

So now i know why I had 52 PSI without assembly modifications...DUH. Anyway now I am not seeing any drop in pressure and I really need a new tune.

Lesson learned. Step away from the fumes for a little while and make sure you are doing it right!!!

BTW we now know that the stock pump with 400cc injectors is good for 375 whp. LOL

 

^ haha, that is awesome! I could actually see this happening to me - I do this kind of crap all of the time. I wanted to slide some shrink wrap over wires that I just extended, but it wouldn't fit over the connections on each end, so I cut the wire to slide it on - and soldered it back up right away w/o sliding the shrink wrap on...idiot!

 

/\/\/\/\That is hysterical. Good catch! Glad you figured it out.