12v Fuel Pump Mod

[r32_nate]

Ive been searching and searching, but cant seem to find a straight answer. Im about to go to the dyno again on Friday, and debating doing the hardwire mod for my fuel pump to give direct battery/alt voltage. My understanding is this is to eliminate the 9v-12v resisted ground when idel/low rpm, and to give more fuel if needed (along with a a/m FPR). My only issue with this is, unless it absolutely required, wouldn't u just be giving your fuel pump 14.4v alt voltage and burning ur pump out faster, since it more thna the 12v its designed to run?

Is their another benefit I dont see?

[bobbo]

PM CobrAA (Alex). I'm pretty sure he has it done.
I'm not sure what power number you're shooting for but at 427whp I haven't touched the fuel system up to the injectors.

[Skym]

Depends on the fuel requirements of engine.

Can buy fuel pump hardwire kits that come with instructions -

http://www.deatschwerks.com/products/accessories/fuel-pump-hardwire-installation-kit-detail

Most aftermarket fuel pumps are rated at 13.5 volts to get the rated lph. Can actually run up to I think was around 18 volts max (usually most run to 17 volts) to get more lph from the fuel pump. Fuel pumps wear out due to the brush design.

The reason for the drop in voltage at idle is not to overload the alternator / draw too much power when the aircon, headlight's, etc are on at the same time. Also the lower voltage at idle doesn't send too much fuel to engine which gets returned to fuel tank and heats up the fuel (on N1 GTR racecar they do this hard wiring mod, but have a inline fuel cooler installed and aircon, etc removed). They might change the stock alternator as well, as Nissan have up to I think was 120A alternators.

Also the vacuum of engine sucks fuel out of injectors at idle, so higher fuel pressure is not needed. Under boost the boost / pressure stops the fuel from exiting injectors, so need a equal or higher fuel pressure to overcome the boost pressure (hence why engine has a FPR that opens under vacuum and closes under boost and voltage is increased to fuel pump).

A way to have the same input voltage, but have higher output voltage to fuel pump when needed (on boost) is via this method (solenoid on engine that triggers it at a set boost level) -

http://www.kennebell.net/KBWebsite/Accessories_pg/Boost%20A%20Pump/layouts/boostapump.htm

It has it's own tuning problems associated with it.

The hard wiring method is said to be the most reliable way.

[r32_nate]

Depends on the fuel requirements of engine.

Can buy fuel pump hardwire kits that come with instructions -

http://www.deatschwerks.com/products/accessories/fuel-pump-hardwire-installation-kit-detail

Most aftermarket fuel pumps are rated at 13.5 volts to get the rated lph. Can actually run up to I think was around 18 volts max (usually most run to 17 volts) to get more lph from the fuel pump. Fuel pumps wear out due to the brush design.

The reason for the drop in voltage at idle is not to overload the alternator / draw too much power when the aircon, headlight's, etc are on at the same time. Also the lower voltage at idle doesn't send too much fuel to engine which gets returned to fuel tank and heats up the fuel (on N1 GTR racecar they do this hard wiring mod, but have a inline fuel cooler installed and aircon, etc removed). They might change the stock alternator as well, as Nissan have up to I think was 120A alternators.

Also the vacuum of engine sucks fuel out of injectors at idle, so higher fuel pressure is not needed. Under boost the boost / pressure stops the fuel from exiting injectors, so need a equal or higher fuel pressure to overcome the boost pressure (hence why engine has a FPR that opens under vaccuum and closes under boost and voltage is increased to fuel pump).

A way to have the same input voltage, but have higher output voltage to fuelpump when needed (on boost) is via this method (solenoid on engine that triggers it at a set boost level) -

http://www.kennebell.net/KBWebsite/Accessories_pg/Boost%20A%20Pump/layouts/boostapump.htm

It has it's own tuning problems associated with it.

The hard wiring method is said to be the most reliable way.

So my Walbro 255 is designed to run off alt power and not battery to run at full 255lph? and if I understand right, to see the true benefits of this mod, you would need to have an aftermarket FPR to be adjusted based on needed fuel?

[bobbo]

I was told by a very credible source that a healthy oem GTR pump will be more consistant than the Warlbro 255. Especially if you rewire the oem pump.

[Skym]

So my Walbro 255 is designed to run off alt power and not battery to run at full 255lph?

I think fuel pump would need 13.5 volts to flow 255lph. Factory is roughly 10-12 volts. Some see 10 volts at higher rpm (engine runs lean or leaner than it should on dyno).

From what I understand, runs off battery (look at fuel pump hard wiring diagrams on the internet for GTR) that gets recharged by the alternator, which keeps the voltage steady (without alternator the battery would drain, engine stalls). If have aircon, headlights, etc on the voltage drops (can test this at the battery or ECU itself) hence can see a lower voltage to the fuel pump which drops fuel pressure, thus engine runs leaner.

Higher amp alternator supposedly helps to overcome this problem, but most people get away with using a stock alternator. That's why the module is a good mod, as it still provides a steady output voltage, even if the voltage into the module drops.

There's many ways to solve the same problem, each have their own set of problems and some are more reliable than others.

and if I understand right, to see the true benefits of this mod, you would need to have an aftermarket FPR to be adjusted based on needed fuel?

If up the lph via the fuel pump hardwire mod, the stock FPR orifice is too small thus fuel pressure increases and engine runs richer at mid to high rpm where the injectors are open more. The engine can run into the 10's with A/F ratio, even if boost level is raised via fitting breathing mods.

With adjustable FPR you can drop the fuel pressure to stock fuel pressure if using stock injectors. But it's recommended to reduce amount of fuel via ECU tune (fuel map) and to never drop the fuel pressure due to the boost preventing the fuel exiting injectors (might be running higher boost level than the factory 7-8psi), as will make engine run lean if drop the fuel pressure with a higher boost level.

If look at injector specs, they have a recommended fuel pressure you run (bigger injectors might have a higher psi, hence look at what the adjustable FPR can handle psi wise). But injectors supposedly can handle higher fuel pressure without failing.

Stock FPR is known to fail with age, so replacing the stock FPR with a rebuildable FPR from Tomei or Sard, etc is worth it (most people do).

Also adjustable FPR is a mod that can help to increase the limit of stock injectors cc wise via upping the fuel pressure. It's a band aid fix instead of upgrading injectors which is the proper way to increase cc of injectors.

The stock FPR provides 1psi of fuel to 1psi of boost or seen on most FPR spec sheets as "1:1" (with no vacuum the FPR raises fuel pressure by 7.1psi over the stock 35.5psi at idle, which is designed to match the 7-8psi most stock engines ran at factory around that time). More fuel is added via the ECU tune that makes engine run into the 10's with A/F ratio.

Can pull vacuum hose off stock FPR and block vacuum hose nipple on intake plenum and engine runs richer at idle due to the higher fuel pressure.

A tuner could explain it better than I could, but that's how I understand how it works.