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Discussion Starter #1
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?
 

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Discussion Starter #2
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...
 

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Discussion Starter #5

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Why is your base fuel pressure under vacuum at 40 psi? Is your tune off and you are compensating by adding fuel pressure? It must go to what, 50 psi or so when the vacuum hose is pulled off the regulator and 60 psi under boost??

Unless you are running 50 psig of boost, 100 psig fuel is the wrong way to do things. It's actually a very bad idea, to be blunt about it.

You are trying to make a 30lb/hr injector be a 60 lb/hr injector by more than doubling up on fuel pressure.

Good luck on that!

The normal use of an FMU or boost controlled regulator (same basic thing) is around a 1:1 increase, so the injector has the same differential pressure when working between the rail and manifold pressure. You can "tune" with an FMU, but it is a snake pit at high pressure ratios.
 

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100psig

will be a pump, injector killer

people stop using this method a long time ago

some just need to find out on their own
 

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100psig

will be a pump, injector killer

people stop using this method a long time ago

some just need to find out on their own
I tried to tell him the same thing on another message board Indy, but some people need to learn for themselves.
 

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One of the biggest problems with FMUs is that they break. Usually at inopportune times. I have received several brand new out of the box from Vortech that were no good.

As for your experiment, when they work, they work fine. I wouldn't go raising the fuel pressure to 100 psi with one though. 70-80 psi is probably about all you want to do. You can limit the maximum fuel pressure with a bleed valve in the vacuum line going to the FMU. The ratio of the FMU determines the rate of rise.
 

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I really struggled with an fmu on my vortech 302 setup. Cobra intake edelbrock 1.90 heads, stock cam. Pulled only for 6-7 lbs. I got frustrated with a professional tuner who just didn't make any progress, so I decided to go old school with an fmu and btm. The btm actually worked pretty well. But I tried every disc combination on the fmu and it just never got my wideband to get much under 13.5 AFR at Wot up to about 5200 rpms. Not safe at all. This was with a 255 Walbro and 42 lb injectors, so should have done the trick, but it didn't. I think one random highway pass I got it to go to about 12.25, but there was no real consistency to it and never got me to the 11.6-12.0 I wanted to see. Self tuning with binary editor and a quarter horse now, and it was super easy to ditch the fmu and get exactly the AFR I want every time. Never even tuned a car with software before.
 
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