I know, I know, but hear me out…

I, like I’m sure many of you thought I knew how these things work. That is until I took one apart last night an took a good look at it.

I assumed: big piston on one side (the replaceable “disk”), and little piston on the other side (the machined plunger that fits in the back side and seals the fuel passage). I measured them on the known 12:1 FMU and got a 19:1 ratio of surface areas. HUH?

Took a better look at the thing and had the brainstorm “the area inside the outlet orifice on the small piston side does not act on the piston until it’s open, so until it’s open it doesn’t count.” Well, that’s moving things in the wrong way, to about 27:1.

???

So what’s the deal with these things, how do they work? My only thought is that there is a small spring on the fuel side that does counteract the diaphragm that senses the boost pressure, so maybe the force of the spring slightly counteracts the force of the boost on the diaphragm, but that would result in something that does not have a linear rate. It would only have a 12:1 ratio in a small range. It would start at much lower (till the force of the boost of the diaphragm counteracts the force of the spring), and as boost increases the force on the diaphragm would get greater…

 

Maybe this will help a little:

(man do I need to start using a real cad program)

 

sorry don't have a answer for you,

I've just found out that

FMU PSI= 1/2(stock rail) + (FMU ratio)(Boost)

 

I can tell you for sure that that's not right.

which ever is the biggest restriction in the line (causes the highest pressure) is the one that determines the current pressure.

I'm curious, where did you find that formula? I've seen 4 or 5 different ones on the corral...

 

I aggree, but thats comming from a vortech tech..

 

Really? That suggests that I either understand this less then I thought I did or they don't have a clue... huh...

 

Na, I believe them

My buddy used to have a 12:1 FMU on his 5lbs vorteched mustang.. he said he used to get like Xpsi at 5lbs and we did the math.. it came out right

 

sorry, one more thing..

I've got a 4:1 FMU on mine right now and if you didn't add half the pressure of the stock rail that would mean the FMU wouldn't work over the stock system at all.. and it obvioiusly does

 

OK, you said that you're running something like 9-10psi boost right? If what I measured is roughly correct and the FPR has no effect on on the fuel pressure after the fmu takes over, I'd expect you to be seeing about 70psi at that boost level. If that formula is correct you should be seeing about 61psi. If it's a straigh 4:1 ratio (or whatever the advertized ratio is) like I've always assumed then you should be seeing about 53psi.

So what are your actual numbers?

 

i've never had a gauge on mine. When i fabricated the fuel rails I decided against putting a schrader valve back in b/c of time issues

The math we did was for my friends mustang

I'll tell you this, I did burn a chip a few days ago and used the simple formula Flowrate2= Sqrt(new fuelpressure/oldfuelpressure)*old flowrate to run with out the 4:1 FMU..

I was just seeing if the FMU was causing a heavy AE bog I was seeing.

With the assumed 61.35 Psi and the above formula I modified my PE tables and it was the exact same AFR I was shooting for.. SO the FMU formula must be pretty close.