how does an FMU work?

[Loochy88]

does it only deliver more fuel under boost? if so, how does it read boost, throug the vaccum line?

could the same thing the fmu does be achieved through just using maft?

[blackFbird_98]

the only time the FMU kicks in is under boost by using a vacume line, as far as a maft I have no idea if it will work or not

[grayman]

Positive pressure (boost) pushes on diaphrams, (etc.) inside the FMU. the fmu then multiplies this pressure by a preset or adjustable factor such as 6/1. when the FMU is 'working' it reduces the amount of unused fuel that would normaly return to the fuel tank via the 5/16" fuel return line on the fuel rail. This creates back pressure in the fuel system which in turn raises the fuel pressure in the fuel FEED lines on the fuel rail.

So...a fuel pressure sender is mounted in the FUEL FEED line and an FMU is mounted in the Fuel Retrun line.

The fmu only delivers more fuel under boost. it is not 'electronic' rather it is mechanical. If my engine is only seeing 1/2 psi of boost the fuel pressure increases about 3 psi. (i have a 6/1 ratio fmu)

[Loochy88]

thanks grayman. thats exactly what i was wondering [img]smile.gif[/img]

i was wondering why it only hooked into the return fuel line. its a lot more simple than i thought.

so, it seems to be that the wastegate, bov, and fmu are all t'd into the same vaccum line. (same line i t'd my pressure regulator for the nitrous system into)

[ September 25, 2005, 04:57 PM: Message edited by: Loochy88 ]

[1DangerousV6]

^yup..i actually used the vacuum line that comes off of the TB for my powerdynes..but since positive boost is seen by both that line and the one that comes off of the MAP sensor..i assume you can use both..but they can all attach to the same one

[Loochy88]

thats going to be a lot of T's. Im going to have a bit of trouble making it look like a clean install i guess. Has anyone with a front mount turbo managed to not make their engine bay look like a huge plate of spaghetti?

[asdf1234]

Mine looks pretty.


..Not


It's not bad though. You can loom up the vac lines if you want. I'm going to dress it up this winter and get it looking nice.

[Teufel Hunden]

Originally posted by Loochy88:

could the same thing the fmu does be achieved through just using maft?

No, a MAFT manipulates the MAF signal. You'd use one if you made a mild change in injector size, and didn't get the MAF recalibrated, or didn't want to tune for it

[ravenp]

I'm moving my vacuum lines under the overhang close to the firewall. Should be clean enough when i'm done. It is kinda annoying, but worth it [img]smile.gif[/img]

[grayman]

oh yea...as regards the FMU and the injectors:
I believe the different fuel pressure in the system due to the fmu 'turning on' forces more fuel per cycle through an injector as opposed to the amount of fuel an injector delivers at a lower psi.

injectors have limits but the threshold of maximum flow can be manipulated by increasing/decreasing fluid pressure on the valve like metering system of an injector. the electronic control of the injector is not changed by the use of an FMU but the dynamic relationship of fluid delivery capacity under pressure is manipulated.

[Rob Bell]

Is the general consensus that a 6:1 FMU plus an inline or intank hi-flow pump sufficient for boost levels of around 6-10 PSI?

[grayman]

the stock pump can handle 6 psi at 85 psi fuel pressure. It can do more as well in terms of power of the stock pump but it is supposedly not as durable as aftermarket pumps. a stock pump might even be able to handle 9 psi of boost on stock injectors as well. but that can put the fuel rail at about 110 psi or more with a 6/1 fmu. some people don't want to put that much pressure on the fuel rail. (part of the fuel rail is rubber hoses and connections, etc.)

As far as my comment about durability of the stock pump: once the fuel rail starts pushing higher and higher psi that is the same as back pressure on the pump. the pump now has to work that much harder to move fuel to the engine. Like going up hill. supposedly the stock pump is more prone to fail prematurely under such conditions. on the other hand every once in a while somebody mentions that they have been running a stock pump on their boosted car a very long time.

Some others believe that an aftermarket pump is good insurance to avoid pump failure while under high boost and then developing a lean condition in the motor at high rpm.

but to answer your question more directly:
stock pump seems fine (for sure, well documented) at 6 psi and less. it is at higher boost levels like 7+ where at least in the supercharger realm the use of the stock pump (or any pump) and an fmu is far less documented.

I don't know what turbo dudes do about the fuel system other than swap to bigger injectors and tune the fuel curves because all that data seems to be locked up in fort knox.

[Red]

instead of t's, why not use a small block manifold with a couple sideports? If it is all going to the same vaccuum line you could also use 1 small t andhook in a remote manifold with several ports to connect using a length of hose to be virtually unnoticable.