FMU Ratios and injector sizes

[white91gt]

Can someone explain the ratio on a FMU.

If it is 12:1 and you have 6psi of boost this causes the FMU to raise fuel pressure to 72psi, right ?

Is this 72psi in addition to the factory regulators 38-40psi ? or is this 72 psi total ?

Also, what size injectors would be recommended for a stock cylinder head 302 running 6 psi,GT40 intake, 65mm throttle body, 73mm MAS, 1.7 roller rockers, 15/8 equal length shorties, and dynomax exhaust?

Should I ditch the FMU and go with a tune ? What is everyone's opinion ?
Thanks in advance.

[Ackbar00]

If it is 12:1 and you have 6psi of boost this causes the FMU to raise fuel pressure to 72psi, right ?
Well, It raises 12 PSI for every pound of boost, so it would add 72psi to your static pressure. But this is WAY too much, Try a adjustable FMU, that way, you dont have to screw with disc.


Is this 72psi in addition to the factory regulators 38-40psi ? Yes

Also, what size injectors would be recommended for a stock cylinder head 302 running 6 psi,GT40 intake, 65mm throttle body, 73mm MAS, 1.7 roller rockers, 15/8 equal length shorties, and dynomax exhaust?If you are going to run a FMU, just stick with the 19#'rs. You be fine with at 6#'s

Should I ditch the FMU and go with a tune ? What is everyone's opinion ?
That is always the best approach.. But I am runnig 30# injectors with a BEGI FMU. Should make over 400 RWHP when it is all said and done.

[jonnyk]

I would ditch the FMU and get some 30# injectors and a dyno tune. The FMU will only cause problems tuning...

[white91gt]

Ackbarr00:
Are you sure that the FMU adds to the static fuel pressure.
I have used the analyzer, on this website, and it calculates the fuel pressure required to support 300hp with 19 pound injectors to be 64psi, and that a 12:1 FMU is required.

Additionally the 12:1 FMU was standard in vortech's entry level kit (A-trim), and as this kit does not require any additional fuel mods, I would find it hard to believe that a stock 88lph fuel pump would be able to function if pressure under 6psi of boost was spiked to {12*6 + 39psi }= 111psi

I don't mean to question good advice as it is always appreciated, however I am just trying to figure this thing out.

Thanks.

[Stang_Crazy]

Ackbar is correct. A 12:1 ratio FMU will add 12 psi of fuel for every 1 psi of boost. 12psi (fuel) x 6psi (boost) = 72 psi that will be added to your static pressure to acheive your total fuel pressure under boost. If it were me, I'd toss the FMU and go with bigger injectors (30#)...don't forget about a fuel pump too.

[jonnyk]

How does the FMU add fuel pressure? How does it know how much fuel pressure your rail regulator is holding?
I still think you guys are wrong on claiming that.
9 psi with a 12:1 FMU would put your fuel pressure at like 150. That is totally unbelievable to me.

It makes even less sense when you think about what an FMU really is. It is a boost responsive regulator. But, it runs AFTER the rail reguatlor. So if it's responding to 2 psi of boost at 12:1, it will attempt to hold 24 psi of fuel pressure in the rail. However, that is not restricting any fuel flow since the rail regulator holds closer to 40 and the remainder is just bled off. So the pressure coming out of the rail is 40 psi, the FMU is only attempting to hold 24 psi, therefore it's not a restriction in the picture and is NOT adding any more pressure to the system. Unless there is some sort of voodoo I am missing here....

[white91gt]

Excellent response JohnnyK.
This is exactly what I am trying to point out.

If you have two regulators connected in series, the regulator with the highest setting will determine the pressure in the system.
If you have two regulators in series, both set at 50psi, you do not get 100psi in the system, you would get 50psi.

[Ackbar00]

Got me, it was what I was told

But 12:1 sounds a bit high to me

[Stang_Crazy]

Originally posted by Ackbar00
Got me, it was what I was told

Me too. I don't know how the FMU raises pressure but it certainly does....some how. I'll do some research and find out.

And FYI, 12:1 is the raito that my Novi 1000 FMU came with. 12:1 is a common ratio for stock motor or stock injected cars.

[jonnyk]

I know 12:1 is common. I sold the FMU out of my Paxton kit not long ago.

I'll spare you the research. In return style fuel systems, the way to increase fuel pressure in the lines and rails is to pinch off the return line. This causes the fuel pump to work against the restriction and builds pressure in the lines and rails.

So it simply has a diaphragm that controls the amount of flow going through the FMU, and responds to a positive pressure from a vacuum feed.

[Stang_Crazy]

Originally posted by jonnyk
I'll spare you the research. In return style fuel systems, the way to increase fuel pressure in the lines and rails is to pinch off the return line. This causes the fuel pump to work against the restriction and builds pressure in the lines and rails.

So it simply has a diaphragm that controls the amount of flow going through the FMU, and responds to a positive pressure from a vacuum feed.

If you knew all of this, what was the problem before?? Maybe your researching methods are better than mine?

[jonnyk]

No problem...just curious why everyone thinks FMU's have an additive effect on fuel pressure, and aren't absolute referenced from 0 pressure and build pressure based on the FMU ratio and boost. I thought they knew something I didn't...