Fuel Pump Voltage Drop vs Pressure 1999
 

Did this test today, results below. I measured the voltage drop on each wire. Looks like the wires are too skimpy for my 255HP.

https://www.miataturbo.net/attachmen...ine=1426489987

Hmm, a current measurement would be useful. I was already planning on installing a circuit that regulates the FP voltage at the pump. A 1V drop across a wire is significant for DC.

Just ask Edison!

I'd be really curious to know how much of that 1v drop on the supply side is occurring across the fuel pump relay itself. As Ted75zcar noted, 1v drop on a 12v supply is massive, especially for something as nominally trivial as a fuel pump motor.

Do you have access to a DC ammeter capable of reading several amps?

I suppose, since you made the measurements, it is likely that you are planning to jack the pressure up. If not:

The stock FPR is set at 60 psi, so that is where the operating point of the pump is. Flow goes either into engine, or back into tank. Very nearly a steady state proposition from the pump and wiring point of view.

I have a meter that will got to 10A.

Looks like relay is under the dash.... Not sure I want to go digging for it to measure voltage drop across it.

Just checked walbro specs though

60 PSI = 10A current
90 PSI = 12A current.

I think the fuel/pump has 16 gauge wire going to it.

This calculator: American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies and wire breaking strength

Shows, with 12 ft of wire, at 12V

10A =1 V drop
12A = 1.2V drop

So not that far off.

Well, I guess the numbers add up then.

Just more incentive to build a high-current input spanning switch mode converter. The non-linearity that is tolerated in automotive electrical systems just baffles me.

I imagine you are probably going to install a higher gauge feed. I think if I was going that route I would probably drive a secondary relay at the pump using the existing control signal. That way you can wire directly from the battery with the larger conductor, but you don't have to run it back and forth.

I suspect this is discussed at length somewhere around here.

Dig the SC build by the way, I am in the process of piecing together my own compound ... even though everybody tells me not to do it :)

Many years ago, there was a movement within the industry to transition to a 42v electrical system for this exact reason. Interest in the concept died out when the automakers started assuming that within 10-20 years we'd all be driving EVs with much higher-voltage electrical systems and the point would be moot.


Here we are in the 21st century, and our few EVs still have 12v electrical systems to drive all their conventional accessories.

Yes sir, that's the plan. Just going to add a relay near the battery area, and have it trigger off the old F/P wire. And thanks! I tested the oil pump flow rate on my engine last night, so getting close! (unintentional test btw)

Sorry for the necro revive but what ever came of this? I feel like my car has some serious voltage drop during certain situations. I can hear the Walbro 255 change pitch with my turn indicaters. It's even more pronounced when the headlights and/or fan is running.

I ran a 40A fuse to the battery, 10 gauge wire, 50A relay, and 10 gauge wires into the tank. 10 Gauge ground to chassis just outside of tank on the shelf. Voltage drop of 0.02-0.03 with twin walbro 255's. I now have a Walbro 450 and a Walbro 255, didn't measure it again but it's negligible with the new wiring.

How did you get 10 gauge into the tank? I need to do this soon...

I'm using the racetronix adapter thingy. Got it on sale over black friday.

Racetronix - Universal Bulkhead Wiring System, 4-Way

Not big enough for me, walbro 450 pulls 20A or more at high pressure and wants ~16A at 5 bar.

https://www.miataturbo.net/attachmen...ine=1454541332

Well, I guess I could double up. I only need one fuel pump...

How is that not big enough.

2 14 gauge wires is equivalent to 1 11 gauge wire. Should work fine.

If I double down it should be big enough is what I meant to say. I don't want to ground inside the tank, so 4 wires is plenty fo me.

Cheaper and easier than what I was planning on doing.

Thats what i was assuming you were going to do. 2 power 2 ground.

Pat where did you ground outside the tank?

You could even use the 2 stock wires too if you wanted all of the wires.

Make sure you get the racetronix e85 plug/install kit for the walbro450

A 10mm bolt on the shelf back there close to the tank.

I was about to say "40A? Jesus, why not just measure the actual current and then size accordingly, rather than build a bomb right behind the driver's seat?"

Then I saw the chart which deezums posted.

Holy fuck. 21A at 13.5V is 280 watts, which is a pretty substantial heating element. Granted, not a lot of us are running 110 PSI on the fuel rail at all times, but that's still amazing. Put three 100w light bulbs next to each other on your desk and turn them on, then see how long you can hold your hand over them.

Then put that in your fuel tank.

Then add 60% (for the other pump).

We've known since 1999 that the stock wiring is inadequate for the stock fuel pump -- How is this news in 2016?

For a 20A pump driven by the battery via relay, you'll want a minimum of 12ga wiring. If you were gangster, you'd use a spare PWM output to duty cycle that big ass fuel pump at low loads. Easy breezy with a solid state relay.

Which is why you PWM and/or stage your big fuel pump.