fmu question

[diablizzard]

I'm just wondering if I could just get by with an FMU and not have to pay for a whole stand alone unit. Could I also get a few recomandations?

[DRIFTEADOR]

you can get by up to a certain point, but it won't be optimal for power, even at low psi. what kind of power are you looking for?

[Structure240sx]

for up to 9 psi and no more than an 8:1 ratio fmu, i ran this for months

understand the rate an fmu increases fuel pressure. an 8:1 fmu increases fuel pressure 8psi for every 1 psi of boost. so at 9psi that is an extra 72psi of fuel pressure on top of the base of 35-40psi. 100psi give or take is about where you want to limit fuel pressure using an fmu. too much pressure (along with 100% duty cycle) can cause premature injector failure

[DRIFTEADOR]

i'm almost certain the numbers don't add. from my understanding a fmu installs inline after the stock fpr and lets any fuel that has gone past the fpr go by with no restriction at anything under 0psi (non-adjustable at least). this is where your fpr controls fuel pressure. after so much boost pressure, depending on the diaphram/disk used, the diaphram inside the fmu won't allow fuel through until pressure surpasses that pressure set by the diaphrams ratio. in case of a 10:1 fmu, it would take about 5psi of boost (50psi fp) , before the fmu even begins to raise fp over stock. I could be wrong, for example if even non adjustables have a universal static pressure, but I really doubt they do. If they do, who knows what that number is to begin adding from on the different brand fmu's?

[diablizzard]

I would also have to factor in the cost because I definately can't afford a stand alone for probably a long time

[chandler]

im in the same boat, whats a good FMU?

[DRIFTEADOR]

good - cartech, but that probably out of your price range too. vortech, blox, drag, will all work. if possible, try to get an adjustable one

[:: orion ::]

DRIFTEADOR...you're incorrect.

All a basic FMU does is raise the fuel pressure a certain ratio ON TOP of the base fuel pressure. The bad-*** new CARTECH's can add pressure BEFORE getting into boost, but I don't know of one that will hold the rise off for XXpsi.

So Structure's math is right on.

...

And one point t note...despite what people have run in the past...the common acceptable or safe TOTAL fuel pressure is ~100psi. That's only 7psi from a 8:1 FMU to break that barrier.

So if you must use an FMU, use it for 7psi and LESS.

Set the base timign back a couple degrees, and you'll have a nice, cheap setup that should get you decent HP.

- Brian

[nissanfanatic]

I ran my 8:1 at 6psi for seven months along with a MSD BTM. did 12-12.5:1 AF ratios.

[DRIFTEADOR]

well, then it raises pressure on top of its own diaphram's base pressure which i guess is assumed to be 40psi . It doesn't take the stock fpr's pressure and build from there. cartech's can adjust the pressure before boost and build from there; a non-adjustable is the same except it's base pressure can't be changed.

nissanfan, did you really get 12-12.5 throughout the entire band or just in one area? i'd like to see a dyno with afr. if its true, thats what i'll be doing to hold me off for now.

[nissanfanatic]

Yup. From spool(approx 4k) to redline, I would pull a 12-12.5AF. I also have my walbro 255lph fuel pump hotwired.

[Craving4Boost]

isnt 11.5afr what you want?

[nissanfanatic]

For higher boost street applications, yes.

But for lower boost, 12.5 is safe. 12.5 is actually ideal for maximum power. Most just tune for 11.5's to keep it safe.

http://www.stealth316.com has an article on it somewhere.

[:: orion ::]

well, then it raises pressure on top of its own diaphram's base pressure which i guess is assumed to be 40psi. It doesn't take the stock fpr's pressure and build from there.

cartech's can adjust the pressure before boost and build from there; a non-adjustable is the same except it's base pressure can't be changed...

Again...and I'm not trying to be an ***, just put up the correct info...but that's not exactly right.

Most of the basic FMU's (like a Vortech 8:1, non-adjustable) don;t do anything until they see boost.

They are installed in the fuel return line, and do NOTHING under vacuum...they let all the fuel pass through, unrestricted.

As it sees boost, it restricts the fuel return line, and therefore raises the fuel pressure.

So your base pressure could be 20psi, or 65psi...either way, the FMU lets it pass right through until it sees positive pressure...then it modifies the pressure in the rail.

- Brian

[:: orion ::]

Oh...and the Cartech, as I understand it...can increase pressure BEFORE it sees boost (to eliminate the lean spot as the boost comes on)...but can't hold the increase off until a desired PSI....but that may be wrong, check their website to be sure.

- Brian

[DRIFTEADOR]

like you, i don't want to come off as an ***, this is just the way i understand things. if i'm wrong i'd like to know why.

I'm having a hard time understating how a fmu can read what the stock fp is -what psi to build from- when it has to relation to the fpr. it's just inline after it to restrict any fuel that has already gone by the fpr from going by it too, thereby increasing the fuel pressure. picture the stock fpr not being there, what would happen? since theres no restriction during vacumm, like you said, fuel perssure wouldn't build up, right? then at 1psi of boost what would happen? using a 10:1 fmu ratio for convesations sake, would fuel press. shoot up to 50psi or 10psi? I would think 10psi, then 10psi more for every lb of boost there after. the only difference when the fpr is there, is that fuel during vacumm is not zero. other than that, the fmu doesn't adjust to the incoming pressure. is there something i'm missing?

[Mustangs_Suck]

For higher boost street applications, yes.

But for lower boost, 12.5 is safe. 12.5 is actually ideal for maximum power. Most just tune for 11.5's to keep it safe.

http://www.stealth316.com has an article on it somewhere.

dead on

I'm running 7psi and my 12.5:1 is perfect.

Before I got it tuned i was at 14.5:1 - that was DANGEROUSLY high, I just thank my lucky stars it made the 45 min. drive up north to get it tuned properly without blowing anything

[DRIFTEADOR]

i guess i'll try a fmu first. I found one on ebay that comes with 3 different disks so i can change it if i need to.