Part I: Stepping Up Input Voltage (underhood)
Normally, the stock fuel pump relay gets its
power(pin #5) from the output of the MFI relay (pin #1).
The first step is to disconnect pin #5 of the fuel
pump relay from the MFI relay. This is where you
cut the wire loop at the stock harness.
Now, the fuel pump relay power input (pin #5) needs
to be connected to some power source so that the
fuel pump can have power. Part of the point
of this modification is to boost the voltage
at the fuel pump, which is why the we change this
input connection (pin #5) of the fuel pump relay
from the MFI relay output (12.5V) to the battery's
positive terminal (13.5V).
Connecting the fuel pump relay input directly to
the battery would result in the fuel pump always
being on, which is not good :-) Thus, we need
a relay to ensure that the fuel pump relay
input is only "on" when the MFI relay is on. That's
where the extra relay comes in.
The coil input of the extra relay (pin #85) is connected
to the output of the MFI relay (same as pin #3 of the
fuel pump relay harness). That way, the new relay
is only "on" when the MFI relay is on.
As mentioned before, the power input of the new relay
is connected to the positive terminal of the battery.
This connection must be fused to be safe.
The output of the new relay is connected to pin #5 of
the fuel pump relay, which is the stock relay's input.
Using this circuit configuration, everything downstream
of the stock fuel pump relay remains stock. This ensures
that the ECU can still reduce the voltage at the
fuel pump when it needs to do so (by switching the stock
fuel pump relay so that the resistor is not bypasse).
Part II: Reducing Voltage Losses [optional]
At this point, the largest loss of voltage due to
resistance in the circuit is the long (B)lack wire
with a (L)ight blue stripe that runs from the output
of the stock fuel pump relay (pin #2) to the input
of the fuel pump (pin #3) in the trunk. This is a 14ga
wire that runs the length of the car and, at maximum
[stock] fuel pump current, results in about a 1.0V loss
between the engine bay and the fuel pump.
To reduce this voltage loss, you can either replace the
stock wire with a thicker one or splice a new wire in
parallel with the stock wire so that both wires are used
to carry current to the fuel pump. I found it easier to
just run an additional wire in parallel with the stock wire.
As far as exactly where to connect this additional wire,
the optimal solution would be connecting the new wire
to pin #2 of the fuel pump relay and pin #3 of the fuel
pump harness. Another, IMHO more convenient, way is to
connect the new wire at the point shown in the diagram