a few FMU qustions

[luckythirteen13]

ive been looking at superchargers and see that some kits have them and some dont. usually its the lower boost kits that dont. at what boost level do you need one? how exactly does it work? i know it increases fuel pressure with boost but how does that tie into the stock fuel pressure regulator and how would you plumb it?

 

[Daggar]

The FMU is plumbed through the fuels system's return line. In a nutshell, it works opposite of what the fuel regulator does on the supply side. As the fuel regulator open up to increase fuel flow the FMU closes off the return line so that the fuel has nowhere to go bu through the fuel injectors. It's an oversimplified explanation but that's the gist of it.

If you're using 19 lb injectors then you need an FMU even if boost is just 3psi. 19 lbers on a stock motor and FMU are good (generally) for 6-7 psi of boost. If you've got better flowing heads or other go fast goodies then that figure drops. If you've got 24lb injectors, you can usually get away with not running an FMU on a stock motor up to 5-6 psi. but you will still need a PMS or tune in many cases because the stock computer is not expecting to have to provide a proper air fuel ratio for a boosted application even though the injectors might be able to proved the necessary fuel for the application.

 

[luckythirteen13]

ok, just checking. i have 24# injectors but want to run 8-9lbs of boost so i guess to be safe ill buy the FMU. sounds like cheap insurance and will probably allow me to get a better tune on it.

 

[Daggar]

For 8-9 lbs of boost it's not insurance... it's a requirement. Without ramping up the fuel, those 24s will not do the job.

What fuel pump do you have installed?

 

[Daggar]

You need to determine what ratio FMU you need as well. A typical 10:1 ratio FMU is going to be too much for 24s... You may need an 8:1 or even a 6:1.

 

[mustangmarks]

If you've got 24lb injectors, you can usually get away with not running an FMU on a stock motor up to 5-6 psi. but you will still need a PMS or tune in many cases because the stock computer is not expecting to have to provide a proper air fuel ratio for a boosted application even though the injectors might be able to proved the necessary fuel for the application.[/QUOTE]


I got 36lb injectors and i'm gonna run 8-10lb of boost. Is my car gonna run like crap with out a tune?

 

[luckythirteen13]

i have a 255lph fuel pump so that wont be a problem. ill talk to the factory and see what FMU they recomend for the boost i want to run.

 

[Daggar]

I got 36lb injectors and i'm gonna run 8-10lb of boost. Is my car gonna run like crap with out a tune?

Not necessarily but you'd be wise to get the car onto a dyno and see what the AFR is across your power band at WOT (the majority of boost occurs at WOT particularly when you have a bypass or blow off of some sort). Remember that WOT, the computer ignores inputs from sensors but fuel and spark tables are partly constructed of the readings your computer takes during part power and idle. Without a tune, you run the risk of being too lean (or too rich but that's not generally a threat to the motor). I've noticed that in most cases, folks who are running 36s or larger injectors on the facotry EEC and tune tend to run a bit on the rich side anyway. It's still a great idea to get a good reading and find out exactly where your AFR is for at least when you're under boost instead of leaving it to chance. If you do discover that you're too rich (12.5 or lower depending on amount of boost) you may also be actually loosing power when under boost (although your engine will be safe(r)). I'd rather be a hair rich than a hair lean when we're talking forced induction. It pays to actually "KNOW" though. Not everyone is lucky enough to have it work out on the rich (safer) side.

 

[Daggar]

i have a 255lph fuel pump so that wont be a problem. ill talk to the factory and see what FMU they recomend for the boost i want to run.

Just keep in mind that the higher you ramp up fuel pressure the less fuel volume you will end up getting out of that pump. With 24 lb or larger injectors, this "should" not be a factor but you'll need to be able to look at your Air Fuel Ratio in order to be sure. At 12V and 80 psi for example, your fuel volume will be cut in just about half.