420A FMU question

[rstare]

Okay, so is a 12:1 fmu disk really better? I looked at a write up on 2gnt.com and they suggested a 8:1 disk would be better. Because the walbro 255lph hp only supplies up to 120psi, with a 12:1 disk you @8psi you would be well over this 120psi mark. I realize you can rewire it and get 13.5v to the pump but it still only delives up to 140psi and that will be at 2 gallons per hour which isnt nearly enough. So im asking if you guys think a 8:1 disk is a better way to go. I really would like to order the fmu and the pump tonight so i need to know which one to get. I cant wait to go turbo anymore so im getting everything ready. sorry its so long
Thanks for your help
Ryan

 

[VelocitàPaola]

I do not agree that a 8:1 is better because you're deviating too much from an acceptable fuel curve in the lower boost range. Sure, a 255lph can only supply so much fuel, but if you look at a fuel pressure log with a 12:1 FMU you'll see it begin to level off and plateau up top: this isn't as big a crisis as no fuel down below. Of course, most of us see the FMU as a sub-par temporary solution anyway.

 

[rstare]

I think i understand what you are saying. Correct me if im wrong im fairly new to all this so im still learning, are you saying that the fmu stops raising the pressure as much the higher the pressure gets? I definatley understand about having less fuel in the lower boost range. I guess i forgot to think that im not going to be at 8psi everytime i drive, .
thanks for your help
Ryan

 

[chris712vt]

Paul is saying only using a rising rate of 8:1 won't provide enough fuel. The 12:1 should give you the fuel you need, but when the boost rises to 8psi the FMU is still trying to raise your fuel pressure, but the problem then becomes your Walbro 255 not being able to provide that super high pressure. Possibly not having enough fuel just at peak boost is less problematic that not having enough fuel pumped in all the time. Did this help in any way, or just restate what you already understand? haha sorry if I am no extra help. I do have a 12:1 FMU I can sell you though.

 

[dr1665]

Do it up right. If you're not wanting to mod on the cutting edge and feel there is merit in doing things the we used to do them (circa 2005), there is no shame in doing so. It will be considerably cheaper, but the fuel system of a turbocharged car is not the place to skimp! At a minimum, pick up the SFMU.

Even so, despite the daunting task of learning how to install and tune with MSnS, with everyone talking about it these days, you really owe it to yourself to give some more consideration to such a modern set up before resigning yourself to a world of little plastic discs.

Food for thought.

 

[RSTeclipse97]

I would suggest the sfmu as well. Im currently running a fmu with larger injectors and in runs good but it could be a hell of a lot better with something you can actually tune a bit

 

[rstare]

Thanks guys, the sfmu doesnt it only go up to a ratio of about 8:1, i believe i read that somewhere I could be wrong though. What are the benfits of the sfmu over the fmu, as far as i know all it does is change idle fuel pressure and you can change the ratio.

 

[VelocitàPaola]

]...before resigning yourself to a world of little plastic discs.

They're metal disks...


OP, the S-FMU works via lower ratios, but that's okay because it affords you enough control over the fuel curve to use larger injectors. There is simply no control at all with an FMU.

 

[rstare]

Okay, I see. Thanks a lot guys, I think im going to go with bigger injectors and the sfmu, then.