I could do a search and find out but I was talking to my friend who has done a bunch of turbo setups on his friend's cars before and said the FMU's were good and could be run beyond 6 psi (I told him they couldn't handle past 6 psi) I knew they sucked but he seemed to know more about it so I ended up looking stupid and he successfully defended the abominations that are FMU's. If somebody could give me the top 5 reasons FMU's suck that would be great.

 

if they malfunction, your motor blows up. what more of a reason do you need

 

ok heres and even better reason. as fuel pressure goes up fuel delivery volume from your pump goes down. IE

at 45psi a pump delivers 125lph
at 55psi same pump now delivers 00lph
at 65 psi same pump is now at 85lph
at 75 psi same pump is now at 40 lph.

not really a good situation. Also these are dead head pressures not accounting for boost. and are not actual nubers. they serve only as an example. but typically for every 5 psi in pressure increase pumnp volume drops 10% so if you increase pressure with and fmu say 15 psi youve lost 30% of you pump volume. its not a perfectly linear thing but it gets progressively worse as pressure goes up. IE at 5psi increae its 10% but at a 10 psi increase its more like 20 % and that is the real reason why fmu's suck.

 

Uhhhh thats why you upgrade your fuel pump buddy.

 

lack of tunabilty.....bottom line

 

Well... I run FMU 10:1 disc with inline fuel pump.
So far.. me happy.
But, here are pros/cons for an FMU:

Pros: you'll easier pass smog test, whilst not in boost car drives like stock - idles like stock and accelerates, etc... no sputterings... usually cheaper than AFC+injectors; it works

Cons: high fuel pressures (you *do* need inline fuel pump), hard to tune (only by changing discs), not good for higher pressures due to rising fuel pressure

Overall... as said, I have FMU with inline pump. According to A/F gauge I'm rich all the way with 10:1, even when boost creeps up at 8psi (it's set to 7 psi).
People who say FMU is not 'adjustable'... it is, by changing discs... either buy 'em, or make yr own. Yes, you actually *can* do that! Imagine that...
This mostly goes to those who were running pig rich by using 12:1.. why not use 10:1, or even 8:1 if you were running *that* much rich?

 

Exactly. Sure, you can change a disk, but how good of a tune is that? I'd like to see how fucked up the A/F is across the RPM range with an FMU.
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boost transition sux

 

What boost transition?!