fuel pressure question

[rb_mike]

i have a 240sx with an rb20det in it. i have a walbro 255 pump installed in it and im only gettin 35 pounds at the fuel filter. that is kinda low isnt it? i have 11.3 volts at the pump with the key on and 14.4 with the car runnin so im not gettin low volts at the pump. the pump sounds the way it did when i installed it, the noise has nevered changed. the car is runnin good but can always get more out of it im sure. the other thing im wondering is why is the fuel pressure regulator after the rail?

[OutToWinPAHC]

Seems a bit low to me. I dont run an RB but I would suspect more. I run a 255 at 65-70lbs

[compactfean]

The regulator is after the rail because fuel is forced through the fuel filter into the rail and the regulator bleeds off pressure to the return line to regulate pressure. Feal fortunate that your fuel pressure is at base with a walboro. Most of the time when a walboro is installed with a stock fuel pressure regulator your fuel pressure is at like 50psi. Fuel pressure should be at 34 running with vacuum. Line 43 without. If you have cams with less vacuum fuel pressure might be a bit higher, not much. If you want to get some more umph out of that bad boy do the fuel pump wiring mod, get a wideband, crank the boost way up and if it starts to lean buy a adjustable fpr and give a couple extra lbs. I ran mine like that for 2 years no problems.

[compactfean]

Yeah maybe on e-85! That's crazy. Your fuel pressure is fine bro.

Seems a bit low to me. I dont run an RB but I would suspect more. I run a 255 at 65-70lbs

[rb_mike]

thanx for the info. the car runs fine but a coworker said that a 255 should push more. i was hopin i didnt have to buy a new pump. i have the pressure set at 38 idle and 43 wide open.

[compactfean]

Actually it should be more than that at wot. The stock regulator is a rising rate so for every lb of boost you get an extra lb of fuel pressure. That's why people switch to a HP walboro because if your boosting 20psi, fuel pressure will be at 63 which stock fuel pumps cracking fuel pressure cannot handle. Of coarse more volume as well. IT IS ABSOLUTELY NECESSARY TO DO THE WIRING MOD FOR HIGH BOOST! Reason being is because the more fuel pressure you have the pump is under more load. What happens. When something electrical is under more load? The fuel pump fuse blows, you loose fuel pressure and before your car dies you lean out ....bad. detonate and motor gone.trust me.

[rb_mike]

what is the wiring mod.i just bought an rb25 turbo and goin to buy a new exhaust manifold for better flow. plan on runnin 12 pounds or so if the turbo can handle it. not sure what kinda boost they can push and not sure y they r so much better than the rb20 turbo.

[compactfean]

I don't see the point in getting an rb25 Turbo if your only boosting 12psi...you can do that on your stock Turbo. You can get 18-19 lbs out of the rb25 Turbo higher in the revs. The wiring mod, you wire the fuel pump with larger gauge wire and run a 30amp relay and have it wired to the battery. That way at high rpm's /high boost the. Fuel pump has what it needs to push what its supposed to. There are a few write ups here.

[rb_mike]

im havin a hard time finding a good write up. was lookin for pictures or somethin to go off of. what wire do i need to run, is it from the engine bay?

[compactfean]

http://zilvia.net/f/tech-talk/151881-ne ... -wire.html sorry guys. couldnt find it on here.

[rb_mike]

thanx alot for the help, gonna atempt it this weekend mayb.

[rb_mike]

well i gave it a shot today and for some reason i have constant 12 volts at the pump, it is not shuttin off. i went off the diagram but the only problem i had is that the wire colors in the diagram r not in my harness. the car harness has black/yellow red/black green/blue and 2 blacks. the pump harness has yellow/blue white/purple black yellow blue/red. i used the black/yellow on the car harness and the white/purple on the pump harness. that is the power wire for the pump. i made the jumper for the relay also.did i do somethin wrong? my battery is in the trunk so i dont have long wires to the relay or the tank.

[compactfean]

You rather jumped the wrong pins at the relay under the hood or you have the relay set up in the back to still flow through it and not relay the power source. If you look at the. Relay it will give you the diagram with numbers. If you look inside the relay you can see the pins that are numbered.

[sabretooth91]

Actually it should be more than that at wot. The stock regulator is a rising rate so for every lb of boost you get an extra lb of fuel pressure. That's why people switch to a HP walboro because if your boosting 20psi, fuel pressure will be at 63 which stock fuel pumps cracking fuel pressure cannot handle. Of coarse more volume as well. IT IS ABSOLUTELY NECESSARY TO DO THE WIRING MOD FOR HIGH BOOST! Reason being is because the more fuel pressure you have the pump is under more load. What happens. When something electrical is under more load? The fuel pump fuse blows, you loose fuel pressure and before your car dies you lean out ....bad. detonate and motor gone.trust me.

Could you please explain what is going on here a bit more? I'd like to know what mods impact other parts of the car. I was thinking of pushing 15-20 lbs, and this is making me step back for a second. Thanks.

[compactfean]

To. Test this theory I will explain. If you pinch off the fuel line after the regulator it will increase your fuel pressure. Our systems are set up with a 16 gauge wire and 10amp fuse for the fuel pump. A walboro pumps double the fuel and needs more electricity so doing the wiring mod will allow more electricity. If you run the pump (walboro) on stock wiring. At higher pressure (higher boost will cause higher pressure to overcome atmospheric pressure) the pump will try and draw more electricity than the tiny wire and fuse can produce. Than just like if you pinched the fuel line the fuse will blow for the pump. If you've replaced a full filter and pulled the fuel pump fuse when its idling to get rid of pressure so you don't get sprayed in the face...you will notice that it continues running for a bit. That's because there is still enough fuel pressure for it to run but if you looked at a wideband when doing this it Leans out. Although I've seen people just use a bigger fuse and get away with it, that's like putting a bigger breaker in your house because it keeps popping, knowing that you have a 16 gauge wire running your washer and drier!lol as for the "cracking pressure" of the pump itself. Stock fuel pumps sometimes can't handle higher pressure so will bleed back to the tank as a failsafe. Notice that walboro has a low pressure and high pressure pump 255lph. Even though we don't run high base fuel pressure. A high pressure pump is needed because when boosting, the vacuum line. To the regulator gets pressurized causing fp to go up 1 lbs for every 1 lb of boost.

[sabretooth91]

Thanks that makes sense. I guess I wasn't understanding how higher boost from the turbo results in higher pressure from the fuel pump. I have a background in electronics and computers, so I definitely get all that stuff. I guess fluid dynamics is where I'm lacking experience.