Upgrading fuel pump power feed wires

[spark_38]

Anyone running extensive mods in their normally stock standard wrx or upgrading their fuel pump to the ever popular Walbro 550hp pumps should consider this simple mod

The factory fuel pump feed wires have a voltage drop of 0.2 - 0.3V from the relay to the fuel pump, losses also occur between the two or so connector plugs: one behind the kickpanel and the other under the rear seat

Step 1: locate the relay and the feed line, usually black with either a yellow or red stripe, cut as close as possible to relay plug and attach a 8 gauge or 10 gauge wire. It wouldn't hurt to also obtain a clean power feed straight from battery to relay (fused about 30A or so).

Step 2: cut the factory pump feed wires after the plug so as to minimise the connections and the length of the higher resistance factory wiring. i have used bullet terminals here for ease of connection, butt connectors are just as good - i do not recommend soldering 8 gauge wire, unless you have a blowtorch or similar - dry joints do not conduct as well. Run the ground wire to a clean metal point and keep as short as possible - it is ok for the ground to be slightly thicker than power (but not vice versa).

From this simple mod (about $5 in cables & connectors), the fuel pump gets a clean 13.56V feed. Prior to this i was only getting about 12.83V - sometimes less.

Note how thin the factory wires are in comparison

[5UBZRO]

where exactly is the relay for the fuel pump? im running a fuel cell in the boot with a bosch 044 pump, and hoping to run the wires off the original relay.

[Stoffa]

Would it be better to join the new cable closer to the fuel pump? I.e straight off the plug on the top of the fuel tank in the boot?

I suppose its only saving a few feet of cable and its not going through any extra connections.

[spark_38]

It probably wouldn't make much difference running directly to pump - i didn't see much more benefit as it involved going through the rubber grommet, outside the car in between the tank and back up... too much hassle, and the last bit only runs about 30cm or so

The factory relay is located behind the fusebox in the 1998 Ver 4 upwards, remove the fusebox and you'll spot the 3 relays mounted on a bracket, it is the bottom one with the green plug.

Ver 3 and below it is above the fusebox and a bit trickier to get to - you have to unbolt the fusebox anyway and move a few other bits around to gain access, unless you running the immobilizer to the relay trigger wire at the same time I would recommend cutting it a bit lower as i struggled to get my crimping tool in there

[Stoffa]

sweet, good info thanks.

every tuner i've talked to has mentioned this problem with subie fuel pumps.

another thing on my list of stuff to do

[spark_38]

yip as with most electrical systems the cleaner the feed to the source the better. alternators, starter motors, fuel pumps, ignition system are just the main ones to look for - straight off the battery you'll usually get between 13.8 and 14.4V with the car running - turn on the headlights and A/C and it may drop slightly 0.5V or so, throw in 1500W or so of amps and you get another small drop; probe your multimeter around the various sources and you'll be surprised at the differences

[subieboy]

 Stoffa said:

Would it be better to join the new cable closer to the fuel pump? I.e straight off the plug on the top of the fuel tank in the boot?

I suppose its only saving a few feet of cable and its not going through any extra connections.

My thoughts too in this is similar:

I vaguely remember reading somewhere that joining two wires of differing diameter is a bad idea, the reason being that it creates a blockage or voltage drop accross the join (therefore heat, etc) and can in worst case result in a fire... I do stand corrected if this is not the case tho...

But wouldn't it better to get a new connecter (on both ends) for the new wire and replace it at the plastic bit itself? Just a thought tho

[spark_38]

 subieboy']

[quote name='Stoffa said:

Would it be better to join the new cable closer to the fuel pump? I.e straight off the plug on the top of the fuel tank in the boot?

I suppose its only saving a few feet of cable and its not going through any extra connections.

/quote]

My thoughts too in this is similar:

I vaguely remember reading somewhere that joining two wires of differing diameter is a bad idea, the reason being that it creates a blockage or voltage drop accross the join (therefore heat, etc) and can in worst case result in a fire... I do stand corrected if this is not the case tho...

But wouldn't it better to get a new connecter (on both ends) for the new wire and replace it at the plastic bit itself? Just a thought tho

I found out in the weekend that the factory relay is rated at 22A - just the more reason to use a thicker wire to minimise voltage drops - a proper crimp joint almost has near zero resistance/voltage drop across it, whereas a bad soldier joint does - hence why crimp terminals are preferred

[rusteh]

Mean Idea, and pretty simple!

what sort of flow gains would you estimate from that extra 0.8V?