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FMU vs. SFMU -> aint FMU dangerous?

Old 10-14-2003, 04:32 PM
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Default FMU vs. SFMU -> aint FMU dangerous?

Ok, so here's the deal...
FMU - raises fuel pressure... more-less same as SFMU.
But...

How FMU works: it raises fuel pressure depending on air pressure. Let's assume we use 10:1 disc. If statis fuel pressure is at fixed 35psi, that would mean that FMU would begin rising pressure at 3.5psi of air pressure, and at 5psi air pressure, fuel pressure would be 50psi.

How SFMU works: you can set also static fuel pressure, etc.. BUT.. SFMU starts raising fuel pressure as soon as there is boost! So, if we have SFMU with 5:1 ratio, that would mean that at 5psi we'd get 35+5*5psi = 60psi -> what is yet greater at 5psi air pressure than with standard FMU!

But, here's thing that troubles me.... what happens if air pressure is for example at 2psi (driving with 2/3 throttle for example) - wouldnt that mean with standard FMU you'd be driving lean? -> as 2+10=20 -> thus this is yet lower than static fuel pressure at 35psi!

What I'm saying... isnt FMU somewhat dangerous when driving at part throttle due to lean conditions (if air pressure is under 3psi.. you still have stock fuel pressure!!)?

.. I'm though planning on buying SFMU purely because of that reasons.. and also - it's possible to tune it somewhat.
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Old 10-14-2003, 04:52 PM
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Default Re:FMU vs. SFMU -> aint FMU dangerous?

What you wrote just confused me.

An FMU with a 10:1 disc raises the fuel pressure 10 psi per 1 psi of boost. Example if your normal fuel pressure is 35 psi and your boosting 5 psi your fuel pressure would be 85 psi at full boost. I guess with an SFMU you can change a few things or something I dunno.

I wouldn't even bother with an FMU unless your boosting really low and just need something cheap to get by until you can get real fuel management. Here read this and see if you can make sense of it...

http://www.vortechsuperchargers.com/.../fmu_sfmu.html
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Old 10-14-2003, 07:32 PM
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Default Re:FMU vs. SFMU -> aint FMU dangerous?

Originally Posted by Nick7
If statis fuel pressure is at fixed 35psi, that would mean that FMU would begin rising pressure at 3.5psi of air pressure, and at 5psi air pressure, fuel pressure would be 50psi.
Yeah, I'm a bit confused also. I think you have something screwed up. I don't know where you got that information, but it's weird. With a 10:1 FMU your fuel pressure will raise 10 pounds at 1 pound of boost and 20 pounds at 2 pounds of boost. It will raise it above the normal set fuel pressure of let's say 35 psi so your fuel pressure at 1 psi of boost would be 45 psi and your fuel pressure at 2 pounds of boost would be 55 psi. I don't know what the hell you were talking about with that 3.5 ----. I hope this helped.
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Old 10-15-2003, 02:06 AM
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Default Re:FMU vs. SFMU -> aint FMU dangerous?

OK, maybe you'll understand better if you hop to this page:
http://www.capa.com.au/vortech_fuel.htm

c/p for Vortech FMU:
---------------------------------------------------
In Summary, the FMU backs up line pressure at the given rate for this example 10:1, therefore if the original regulator still in place holds a rail pressure of 40psi, it will take 4psi of boost for the FMU to raise pressure to 40psi. At this point there still will be no effect at the rail pressure simply because most factory regulators also raise fuel pressure at 1lbs of fuel pressure per 1lbs of boost. So you would have 44 lbs in the fuel rail and 40 psi between the FMU and the original Reg. But at 5lbs of boost you would then have 50psi held by the FMU thus raising the rail pressure by 5psi. Then at 6lbs of boost you would have 60lbs at the FMU and 46lbs held by the original reg, thus raising the fuel pressure by 14psi. This proccess will increase as boost increases.
---------------------------------------------------

This is why they also have 'spring loaded FMU' which increases pressure even at 0-2psi... or an SFMU.

Guess many people didnt know that... that's also reason why people tend to use 12:1 FMU's... so they do get some fuel pressure increase at lower pressure (5psi for example).
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Old 11-05-2003, 05:54 PM
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Default Re:FMU vs. SFMU -> aint FMU dangerous?

totally agree with Nick7. what he said is what i know by reading around quite alot.
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Old 11-05-2003, 09:04 PM
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Default Re:FMU vs. SFMU -> aint FMU dangerous?

Why are we arguing about FMU's? FMU's are ghey end of story. That website where you got that info is ghey too.
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Old 11-06-2003, 04:52 PM
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Default Re:FMU vs. SFMU -> aint FMU dangerous?

well the FMU really sprang from the misunderstanding of how to tune the factory EFI systems. the earliest FMUs were actually designed to keep the pressure differential between the pump and the craberator at the same level for blow thru carberator applications. the best thing to due yet again is get your ECM chipped. or get a standalone or one of the many products out there. i cant say this enough. also an FMU wont manage your spark. and running those really low boost levels 6-8 psi doesnt provide the substianial gains in HP that i feel really even warrant a installing turbo in the first place. if you dont want to invest in good engine managent strageys then i highly advise going another route with your engine build up.
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