Thread: Walbro 255 upgrade

Saw this in another thread and made me think, why would the fuel pressure go up if the stock FPR is set to 3bar? Just because of a new fuel pump shouldnt mean the fuel pressure jumps up to almost 4.5bar.

well it does jump up, thats a fact. you can always find out the hard way though.

Well why though? Because I have a 255 pump and the stock FPR I am infact running at 4.5 bar? Would explain my shitty gas mileage....

the extra pressure from the pump raises the fuel pressure. Get a nismo apfr or equivalent. With the hose off reading should be 43psi

hobbs, that is a very good question with a not-so-simple answer.

When I installed my Walbro 255 HP (High Pressure) fuel pump I didn't have a fuel pressure gauge so I couldn't see how much the base fuel pressure went up. But just like you, I assumed it wouldn't go up any at all, because the FPR should do it's job in keeping pressure regulated at 3 bar above absolute. I did however have a stand-alone ECU in, and I noticed that around idle and cruising I had to make a few minor changes to the fuel, but under moderate to heavy loads nothing changed and no adjustments were necessary. Around idle and low load I had to take away small amounts of fuel. Anywhere from 5-10%. A bit of adjustment, but much less than you'd expect from a jump in idle fuel pressure from 36 psi to 55 psi which is a 52% increase in idle fuel pressure (different from base fuel pressure).

It seems to me from my finding the pressure might have gone up a slight amount (5-10%) at low loads.


The reason for this I assume is because of the extra flow of the fuel pump (not the extra pressure). The way the FRP works is by letting a certain amount of fuel to return to the tank to keep the pressure at the desired 3 bar above absolute. So with the increased flow of the Walbro, the return line gets overwhelmed and pressure builds up in the fuel rail because the FRP can't return enough fuel fast enough. Once you get above low loads however the FPR has no problems keeping 3 bar because a lot of that fuel is being injected and the return line is now adequate to return enough fuel to keep the pressure down.


I would bet if you put a larger return fuel line on the system you could eliminate this minor problem entirely.

Ive fitted a brand new stock FPR, with a Walbro 255 pump and have noticed no ill effects at all through the running of the car, its running better than it ever has...

But what would a AFPR do if it works in the same manner as a stock FPR? If I get a nismo AFPR and set it to the same 3bar I will end up with the same problem of the flow overwhelming the fuel return line. So pretty much it is useless to run a walbro 255 at 3 bar with stock fuel lines correct? Also how does the FPR return fuel to the tank if it is only a in/out type "switch" it has to flow through the injectors regardless.

fuel comes from the filter into the rail then to the FPR where it holds the pressure, before allowing it to return to the tank...

I ran my stock FPR on my setup with no ill effects with my old car. Car ran great on 4bar with a walbro and no AFPR. Even got 230-250 per 11 gallons (which is decent the way I drive).

Based on my theory you are correct. An adjustable FPR would not be able to fix the problem. If it is not a return line flow issue, then maybe an adjustable FPR could solve the problem.

TheSam explained that one.