If you are not comfortable with wiring and soldering, do not try this.

 

Sweet. I'll have to do this sometime this year. Hopefully I won't see the pressure drop as much at high RPM.

Thanks for the writeup!

 

I quick drawing.

 

I can't enlarge it!!!!

 

is this for 3rd gens ?
i could not enlarge attachment so its pretty blurry .
i was unable to save the word document as well. computer is wacky this weekend

 

Word, help us out here Cew, great post, I was talking with Cam at Pettit on Friday about this very item! Can you enlarge the diagram for us please?
Art

 

Good deal! I pioneered some fuel pump rewire mods on the TII way back in the day, and found on the TII that even with a VERY healthy charging system the fuel pump got pretty lousy voltage. With all the accessories on (stereo, headlights, AC, etc.) I saw as little as 11v to the pump at full throttle.

I ran a fused 12 gauge power wire from the positive terminal straight back to the fuel pump and used the stock fuel pump wiring to switch a relay on and off. At that time it was unknown if the fuel pump resistor relay was really needed, so I brought that circuit to the back of the car as well and fed the output of the first relay to the resistor circuit. Once done, I had full battery voltage at load every time.

This could likely be done even keeping the stock resistor relay if you wished with your mod. This will definitely give more headroom on the stock fuel system, even on a near-stock car - it's a good idea.

Thanks for the good writeup!

Dale

 

Was there a measured increase in fuel line pressure after the modification? That's the only change that would allow you to reduce injector duty cycle.

 

Drawing is blury, looks like we are bypassing the stock 20 amp fuse or the stock ignition switch. But where does the stock 12 VDC side of the ignition switch go to? Where exactly does the new wire tie into? And the 20amp fuse is used on the same side as the new wiring correct? I'm sure its really simple but I'm missing something here. Can't think, I haven't had my Starbucks today.

 

I just completed the mod yesterday and it has been raining all day today.

It will help at high rpms and high boost at this would be noticed by richer AFRs than before. It will make a difference as anyone who ever rewired their FP will tell you. My buddy Kyle did it and it help his high end fuel supply problem, but he did a total rewire and his battery was in the trunk. My mod was designed to be simpler and cheaper.

Just look at Mazda's FP wiring diagram in their manual and then read my notes and then you will understand. This mod is not for novices!

 

damm that's hilarious i was just thinking about this mod because all the supra guys do it. So i decided to do a search and sure enough you guys have a how to about it. Thanks cewrx7r1

Terrence

 

Well this thread intrigued me so I did some testing today. I hooked up my fuel pressure gauge and made a temporary jumper from the circuit opening relay and battery voltage. I first tested with the bleeder valve on my gauge closed (static pressure ) and had 75lb stock wiring and 80lb with the jumper. Then I set the bleeder valve for 30lb ( dynamic pressure ) stock wiring and the pressure went to 32lb with the jumper. Good enough for me. So I did the mod, but instead of using a aftermarket fuse holder, I used one of the empty fuse holders in the box.

Anyone have any speculation as to why Mazda ran such a high amp circuit thru the ignition switch? Maybe a redundant switch incase the relay failed on?

 

i think it is to prolong the life of the fp because running it at a higher voltage will make the fp work harder which will cause the fp to wear down quicker. there's a lot of info about this on the supra forums because their pumps work a lot better with this mod. my 2cents

 

can someone explain how the resistor works? ign on power goes through the circuit opening relay then splits to fp relay and fp resistor. how does the resistor lower the current if there's another path with less resistance, through the fp relay, for power to go through?

 

It's controlled by a relay. When the relay is open, the only path is through the resistor & the pump runs at low speed. When the ECU closes the relay, the resistor is bypassed, the path is direct and you get higher voltage and a higher speed out of the pump.

 

i understand that the relay controls it but in the wiring diagram above the relay is controlled by the ignition switch only not the ecu

 

no the ground side is ecu controlled.

 

Chuck I really think I can perform this mod and will help me alot. Do you think you can give a larger/clearer version of the thumbnail you posted?

Jeremy

 

Mad 7tist is right. The ecu grounds the circuit, closing the relay. The wiring digagram posted above does not show the ecu. The full page diagram does.

 

Just bend the contact on the relay so it is always closed.

 

http://www.roadraceengineering.com/f...pflowrates.htm

this site shows how providing full volts to a fuel pump can increase flow, at various pressures.

nice fix by cew ... most cars I checked out had fp power direct from battery, relayed by ign switch. surprised to see fd feeds current to pump through the ign switch.

 

Exactly what I'd expect, but it doesn't show that increasing voltage automatically increases fuel pressure, and increasing fuel pressure is the only way to reduce injector duty cycle and inject the same amount of fuel, short of going to larger injectors.

Adding an aftermarket rising-rate fuel pressure regulator would allow you to decrease injector duty cycle by raising line pressure. Increasing voltage at the pump just ensures that you'll have maximum volume available at the fuel rails at x pressure. Great mod, certainly, but the claimed benefit was a little off.

 

The attachements were deleted. Here they are again.


RED line is new wiring. BLUE lines are parallel wiring.

 

You know, for those that have re-located their battery to the trunk... You could just add your own relay that switches the fuel pump on (acting the same as an ignition source, not accessory) and a dedicated wire straight from the rar bin area to the fuel pump. I would think that this would be the most beneficial way to wire the pump.

Now for a rising rate regulator. How would that benefit a stock FD?

 

But then the fuel pump will be running full voltage all the time which will shorten its life. OK for a race car or weekend warrior, but not for a dayly driver.

One of my friends lost his engine in his weekend warrior when his new pump died with such a setup. Was the pump just bad or the continous higher voltage the cause?