Battery relocation causing poor performance?

So my st165 was running perfect about 3 weeks ago, pulled hard and had no issues, but the floor on the drivers side was basically falling out so i decided to park her and rebuild it(wich i did). But while i was waiting for parts to come and shopping for a new mig welder so i could finish the job i decided to relocate the battery to the trunk. I used two 4 gauge wire for the starter/efi fuse then a splitter to 0 gauge all the way to the back. finally found a lincoln at a price i liked and finished up the floor. took her out for a drive and while she ran perfect once again acceleration was terrible, felt like she was pulling tons of timing. so i whipped out the multimeter and found i was only getting 10.5 volt at the pump and 13.5 at the battery with the car running wich is not right for either of them. At that point i decided to hook the pump to a relay and a switch wich gave me 12.5 at the pump and at the battery wich isnt quite right either but the car still ran just fine, decided top take her out and while power was a ''little'' better is was still awfull.

So a few months ago i decided to install my aem 340 and give the tank a refurb. got that done took her out and power was awfull then too, was only getting around 9 volts at the pump, so i did a bit of inspecting and noticed the inside of the connector was rotted and holding by barely a thread. hard wired it directly and voila! 13.8 volts at the pump and finally the car pulled like a train again and i was happy. now remember this is when the battery was still in the front of the car.

So now what im thinking is the car is not enjoying the battery relocation, it usually had around 14.5 14.7 volts at the battery and 13.8 13.9 volts at the fuel pump with the battery up front, but now with it the trunk im gettin 12.5 volts at the battery and the pump. tomorrow i think ill be taking all that out and just bringing the battery back up to the front and hooking up the fuel pump like stock again(no more relay/switch either) the car pulled/ran perfect that way but before i do i wanted to get some input from you guys. now i know some of you are gonna say check your grounds, dont worry i did and they all tested fine. i also hopped on youtube i found a clip of a dude who works at airtex explaining that even a 1-2 volt drop at the pump can cause HUGE performance issues, hard starting, poor acceleration etc...so 13.8 minus 12.5 means my pump is missing 1.3 volts. This pretty much falls right in line with what the dude on youtube was saying.

So what do you guys think?


edit: i did a bit more research and learned that low voltage at the pump also affects fuel pressure. My car is 205 swapped so fuel pressure with the car running is 36psi. A fuel pump needs at least 13.5 volts to operate the car properly. with teh battery up front i was getting 13.8v at the pump so i was definitely getting my full 36 psi. But now with 12.5v at the pump a bit of math tells me im only getting about 32.6psi. So i imagine a loss of 4 psi is ok for normal driving but not sufficient for heavy acceleration.
 

BlackCat13

New member
Was gonna say maybe your wire to the back was too thin, you can check your voltage at the bat, and then at your splitter to see if your resistance is too high
 
Its 0 gauge and not the cheap stuff, oem bmw with the 250amp fuse that goes with it, so i doubt its too small or faulty but ill check that for sure anyways. im gonna test the alternator too and also check if my belt is tight enough, ive been hearing it squeel a bit on startup lately. i get the feeling that when my battery cables were only about 10 inches long there was still a small issue but it was so small it couldnt be felt but now that ive added about 13 feet of cable it exacerbates the ''problem''.
 

Domspun

Member
The battery relocation is not a factor here. Been around race cars for 20 years and they all had relocated batteries and never saw a problem like this. Most probably is your connections or alternator.
 

underscore

Well-known member
How did you ground the battery in the back? It might test okay but not be up to flowing all the current the far is needing. I'm also not totally clear on how you hooked up the front or what you did with the stock terminal connections.

Honestly though I'd just skip relocating the battery on a street car. I've seen far too many badly wired, not vented/not a sealed battery, not secured properly, etc. It just doesn't seem worth the hassle.
 
The stock battery cables are long gone and havent been there for over a decade. like i said in the first post i used two 4 gauge cable, one from the fuse box and one for the starter leading to a splitter then 0 gauge all the way back. Anyways i took the alternator off and sanded all the contact points and the chassis ground and added dielectric grease. Battery voltage went up tp 14.2 and fuel pump is at 13.3 now. Much better but still not what id like to see, id prefer 13.8 at the fuel pump at minimum and right now im getting a .9v voltage drop. ill test wich wire is loosing current tomorrow since its raining here but im betting its the ground seeing as i installed a brand new positive wire yesterday while installing the relay/switch.
 
k so after scratching my head all week thinking about what could be the actual problem i decided to reinstall the stock fuel pump. I hooked it up to the relay/switch i installed and she finally pulls like a train again, i got full volatage at the pump,13.9v-14.0v so it wasnt the wiring or grounds. It does leave me wondering whats wrong with the aem 340 though, why was it fine before i redid the floor? Why did it ''die'' within 250 miles? And also why was the positive wire getting burning hot? Did i receive a lemon? Is it a knockoff? I dont know but i do know that the next one im buying will be a genuine Denso for a Supra TT.
 
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