is the FMU a nessacity?

128chris · 08-09-2005

i have my 87 notchback with all the usual, rebuild, heads, cam, intake, basically everything new. I have 30lb injectors with a 75mm TB and calibrated mass air. I just had my old sn92 rebuilt and repaired and put it back on. This unit is going to push 6lbs tops even with my smaller pulley. My question is, is the FMU really needed for such a small amount of boost? Only reason im asking is cause i have to fix one of the lines. And can anyone recommend a proper fuel pressure. Thanks for any help.

NoSlow5oh · 08-09-2005

Yes, you definitely need the fmu, unless you have a custom burned chip AND are not making more than around 350hp at the crank with the charger. There is a very big difference between N/A and 6lbs. of boost. Just to give you an idea of how much mine changed with 7lbs. boost, I went from 24lb. injectors and 40psi. of fuel pressure without the blower at WOT to 30lb. injectors and 90psi of fuel pressure at WOT. I'm running out of fuel at the top end too. Thats with a 255lph in tank pump. Now, given, the computer now knows that the injectors were changed, and the injector pulse width has trimmed down a little bit, but not enough to really make a difference. My 7lbs. of boost added almost 200hp, but mainly because of the design of the DOHC motors. As for a starting fuel pressure, I would start at around 35psi at idle with the vacume line attatched. Depending on how good the table values are in your mass air meter, you may need to increase the pressure just a bit. Most aftermarket MAFs run the car a little lean to make a little more hp. Thats not what you should be looking for. Do yourself and your car a favor and get it on a dyno with a wide band before you go romping on the gas and blow it up.

JUSTNLX · 08-10-2005

FMU’s are for people who don’t want to spend the money on there fuel system! Most people don’t take the time to build a whole package. They want their blower, N20, or turbo, but they don’t do everything necessary to run these items. Instead of larger injectors, good fuel regulator and larger fuel pump (and in some cases fuel rails) they stuff a Band-Aid like an FMU on there.

You already have the injectors, now get a good fuel regulator (Kirbin) and a fuel pump. Don’t just jump to a 255l pump you need to calculate your fuel requirements. If you don’t know how to do this find someone who does! If you dump a 255l pump on there and you only need a 190l pump, then your fuel is only making laps around your fuel system. This creates heat in the system and WILL cost you horsepower.

My 91 ran 10lbs turbo boost with 42lbs injectors (a little over kill) a Kirbin regulator (set to 35lbs) and a 190l pump and ran perfectly fine. A/F was around 12.5 and no chip. Although I do think a dyno tune and chip is a good thing to do.

hotwheels · 08-10-2005

i have to agree with both statement's.......any injector under 42#er's, need's an fmu.......fuel pump is a toss of the coin......i put in a 255, but i planned on and do run more then 10#'s of boost.......and yes, the kirban fuel pressure regulator is awesome........

hotwheels of insanemustangs.com

128chris · 08-18-2005

i have a 190lph in tank in it now. And i also have an aeromotive adj fuel pres regulator. My problem seems to be the tuning of it all. Yes, i do need to find someone around my area with a dyno. Only one i know is someone here(not personally) but to my understanding gets 375$ an hour for the tuning on the dyno, just dont have that kinda cash at the moment. Not running a chip either and have no idea if i need one either. And i was under the understanding all my fuel pressure adjustments had to be made without the vacuum plugged in. I am gonna put the fmu back on just for peace of mind anyway. Someone else told me set it at 42 without the vacuum. Seems the way it runs at that, it needs more, for my car anyways. Thanks for the help, and more help would be great too.

hotwheels · 08-19-2005

that is correct.....you will need to adjust your fuel pressure regulator with the vaccuum line disconnect......42psi is fine.......that is about where i have mine set at........Just listen for detonation/pinging, but with the fmu, that will help ensure you always have enough fuel underboost....

hotwheels of insanemustangs.com

12second5.0 · 08-31-2005

An FMU is just a band aid as JUSTNLX stated.