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Discussion Starter #1
Can somebody tell me what the ratio of the FMU means? Is the ratio the total fuel pressure under boost or the amount of extra fuel pressure added to the base pressure under boost?

My setup is a 306 with TFS high port heads, GT40 intake, B303 cam, MSD 6A, Vortech Supercharger making 5 or 6 psi boost, Paxton 14:1 FMU, T-Rex inline pump, 155 lph in tank pump, long tube headers, off road pipe, dynomax mufflers, total exhaust is 2.5" diameter, 4.10 gears, 5 speed trans, 65 mm throttle body and some other things that I can't remember right now. Injectors are 24 lb/hr.

When I start getting into boost, it looks like my fuel pressure gauge is pegging at 100+ psi. I am in the process of remounting my gauge where i can see it better to be sure of what I am seeing, but I want to make sure nothing gets damaged while getting this system to work right. Makes me wonder if the FMU is working properly or not.

Looking for some help. My base fuel pressure is set to 35 psi. Any thoughts or suggestions are welcome.

Charlie
 

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Discussion Starter #3
Thanks for the reply. If I understand you correctly, if I am making 5 lbs of boost with a 14:1 FMU and base pressure of 35 psi, then under boost my fuel pressure is (14x5)+35, or 105 psi.

Am I doing this right?
 

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Since you asked for opinions on FMU's in general - here's mine. Ditch the FMU, upgrade your injectors and of course match your MAF to it. You definately have ample fuel delivery via your 2 pumps. The FMU is just too risky IMO.
 

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Actually it won't raise the fuel pressure 14lbs for every 1lb of boost. Alot of people including myself, basically just split the rate of gain in half. So say for a 6:1 fmu, I would use 3lbs for every 1lb of boost. There are different factors that limit the FMU giving full rate of gain. I don't like the stock FMUs, but I really like my Cartech FMU. It had adjustable rate of gain and onset gain. I've had no problems with it. In a few months I will be go with 55lb injectors and no FMU, but not because I think the FMU sucks.

Troy
 

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Redsnk95 said:
Actually it won't raise the fuel pressure 14lbs for every 1lb of boost. Alot of people including myself, basically just split the rate of gain in half. So say for a 6:1 fmu, I would use 3lbs for every 1lb of boost. Troy
Say what ? Why do they rate them the way they do then?
 

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B/c in a perfect world (excuse the expression) the FMU would raise the fuel pressure that much based on boost. But there are some factors (some of which I can't explain) that limits the FMU's ability, plus the FMU has to overcome the amount of boost. I remember way back when in 5.0 Magazine, Rick Anderson explaining all the factors of why FMU work this way. But I've always split the rate of gain in half. And it was actually spot on with my math.

Troy
 

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That's why I hate stock FMUs. I love my Cartech FMU, adjustable rate of gain and onset gain. The Cartech FMU excels in the vacuum to boost adjustment, that's where stock FMUs fall behind like your describing with your FMU. An FMU lacking in the 1-3lb area can lead to broken parts.

Troy
 
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