Just Curious, How Does An Fmu Work

[sen2two]

I've never actually seen one of these or heard of them outside of the Foxbody world. And so they seek to be popular for supercharged Foxs. I did some searching and found minimal info. I'm really only asking because I keep seeing them mentioned. I'm not interested in using one really, but I would like to be informed how they work. Not if they are ideal, or if they are or not a bandaid. Or how they compare to a real tune. I'm just curios how they function...

From what I read they work similar to a rising rate Fuel Pressure Regulator. Which are normally a 1:1 ratio. So with each pound of pressure added, it raises the fuel pressure 1psi.

I seen 6:1 FMU's and other ratios. So it's basically a higher ratio regulator? For example, if my base fuel pressure at idle is at 40psi with the vacuum line connected. And I add 10lbs of pressure (boost) then my fuel pressure at 10psi will be 100psi with a 6:1 FMU?

But do you still need a fuel pressure regulator, or does it takes its place? Are the ratios adjustable, or do they sell as standard ratios that are known to work? Does it do anything else?

If allspice it does it multiply the fuel ratio, it's just a fuel pressure regulator with a higher ratio. Why does it have its own fancy name?

 

[sen2two]

Did a bit more research. I think I'm actually going to try this out. I have an AEM that I will be tuning with down the road. But I want to experiment with this first...

My plans and reasons:

30lb injectors with matching calibrated MAF
6:1 FMU
Fuel Pressure at idle set at 40 with vacuum line connected

I will be running 10psi with a Paxton Novi 1000

I estimate this set up to make around 450-500 at the crank (360-400 at the wheels)


To meet the fuel requirements to handle this power. I would need to have 30lb injectors at 100psi of fuel pressure if the injectors were operating at 80% duty cycle. So says the fuel injector calculators anyways.

I believe this will work well and run like factory while under cruise conditions. Which is ideal for me since this is my daily driver. I will not be extracting every last HP, but that's not my goal here. Having some fun and trying something new is.


If your experienced in using and FMU with supercharged set ups and you see a flaw in my plans, please let me know...

 

[mikestang63]

You better rethink that grasshopper You dont want to go over 60-65% duty cycle on injectors with any power adder. I would run at least a 42# injector and get rid of the FMU and get a tune. The FMU is nothing more than a band aid to increase the fuel pressure to compensate for smaller injectors. With a T5 expect around 10-15% drivetrain loss of HP so if you are 450 crank then you will be around 375-400 rwhp. With an AOD expect around 20% loss.

Your car.. good luck with your decision.

 

[sen2two]

80% duty cycle is the norm.

I have an AEM EMS fully programmable computer. And I have experience tuning them. But I want to try this out. What makes cars fun for me is experimenting and learning.

 

[TOOLOW91]

I get you want to try it but with an aem,42s and a 340 pump you would be golden. The car should drive stock . The injectors really are not built to run that much fuel pressure over the top of them . You run the risk of a injector sticking open etc .

Seeing 80% duty may be the norm but the injector isn't living there very long .

 

[mikestang63]

IMO FMU's are the worst thing you can put on a supercharged car, as they pinch off the return line to build more pressure and make the injector think it is bigger than it really is. With a 6:1 disc at 10PSI you are adding 60 PSI on top of the base 40 psi fuel pressure, so you are running around 100PSI through 30# injectors.

Let us know how things work out.