FMU's, Are they needed? Options

[WALKER]

Alright, so this will probably sound like a very dumb question to those of you that are experienced turbo people, but basically what is an FMU?

I am new to the world of turbos and am slowly building up my knowledge so that I can build a turbo on my 5S. My research has brought me to the point of FMU's (fuel management unit, I am assuming). People everywhere on other sites are talking about them and FMU ratios of 4:1, 6:1, 8:1, 10:1, 12:1, etc. Well this is the first I have heard of FMU's and by conducting a search on 6gc it looks like there are some comments about them from a ways back, mostly just asking about what ratios to use. Basically what I have been able to figure out is it was used in the past by some 6GC's to push the stock injectors and fuel system...is this correct?

From what I have read recently about the 5SFTE there is no mention of FMU's, so are they needed? Basically all I have read about requirements of the fuel system for a turbo 5S is to get the 460cc injectors and a Walbro 255 fuel pump (or similar) and use either a SAFC2 or eManage.

I am hoping some of you with experience can clarify this for me. Also if you could explain exactly what an FMU is and how it works and what do the ratios mean? Does a standard fuel system have an FMU ratio?

Thanks in advance for your help!

[Glitch001]

to put it simply, its your a/f ratio and the ability to hold and maintain boost. Horsepower is created by air+fuel+ spark, more or less airflow. If you run to lean boom, if you run to rich your loosing hp. All fuel systems have their own sensors (o2, MAF, AFM ect ect) which measure the air coming in and compensate by adding fuel to the mixture. the safc / emanage / full standalone are 3 different levels of tuning. Its just basic understanding of whats an approperatie fuel to air mixture to run. Lean is dangerous, rather run a little rich.

[WannabeGT4]

An FMU (Fuel Management Unit) is pretty much just an additional fuel pressure regulator... It is generally installed after the stock FPR in order to raise the fuel pressure beyond the stock setting.

The numbers 8:1, 10:1, 12:1, etc are just ratios of the amount of fuel pressure increase per pouud of boost. For example with a 12:1 FMU your fuel pressure will raise 12psi for every one psi of boost.

Using an FMU is the simple and basic way of increasing fuel output through the stock injector for forced induction applications. The more accurate way would be to use oversized injectors and either a piggyback fuel computer or a standalone ECU. This will result in a more precise and reliable tune.

Also worth noting is the relationship of fuel pressure to flow rate when it applies to fuel pumps. As the pressure rises the amount of fuel that can flow over a given amount of time will decrease. That's just one more variable that you have to worry about when you're working with an FMU as opposed to a set fuel pressure and proper tuning.

This post has been edited by WannabeGT4: Jun 22, 2006 - 3:19 PM

[WALKER]

Thanks alot WannabeGT4. So basically what an FMU is a bad way to bypass using proper injectors, tuning, and fuel pump. That would be why it was looked at in the past before knowledge of what larger injectors and setup could be used.

Basically an FMU seems like a terrible idea then for any more than a few psi of boost since the stock fuel pumps could not handle it. It would even be challenging for a walbro to handle it if I understand correctly. If stock fuel pressure is 43psi, then with a 12:1 FMU @1psi need 55psi, @2psi need 67psi, @3psi need 79psi?

One thing I am still wondering about is if you just use larger injectors, what type of ratio is applied when boost. Is it a 1psi increase to the fuel system per 1psi of boost? I am just trying to plot out a graph to determine the fuel pressure requirements. I am assuming that the stock fuel pressure would be around 43psi, correct?

What type of impact does an SAFC have, I know you can add and cut fuel, but does that have any impact on the pressure?

[WannabeGT4]

Thanks alot WannabeGT4. So basically what an FMU is a bad way to bypass using proper injectors, tuning, and fuel pump. That would be why it was looked at in the past before knowledge of what larger injectors and setup could be used.

Basically an FMU seems like a terrible idea then for any more than a few psi of boost since the stock fuel pumps could not handle it. It would even be challenging for a walbro to handle it if I understand correctly. If stock fuel pressure is 43psi, then with a 12:1 FMU @1psi need 55psi, @2psi need 67psi, @3psi need 79psi?

One thing I am still wondering about is if you just use larger injectors, what type of ratio is applied when boost. Is it a 1psi increase to the fuel system per 1psi of boost? I am just trying to plot out a graph to determine the fuel pressure requirements. I am assuming that the stock fuel pressure would be around 43psi, correct?

What type of impact does an SAFC have, I know you can add and cut fuel, but does that have any impact on the pressure?

Well when you use upgraded injectors you swap the stock MAP sensor to a Turbo Pressure Sensor from an All-trac or MR2... That alone changes the way fuel is delivered at a given PIM output voltage to the ECU. You're in essence tricking the N/A ECU into fueling your engine like it was turboed. Obviously with all this trickery going on you need the SAFC to fine tune things and get them as close to normal as possible. The E-manage does quite a better job at this than the SAFC since it's a much more in depth tuning device.
Pressure2 should chime in any second here and go into more detail. He could probably explain the tuning side of things a lot better since he's been working with his for over a year now and studying the E-manage Ultimate for several months.

[presure2]

wannabegt4 did a great job of explaining the fmu.
you have the right idea.
its IMO a very dangerous way of increasing the available fuel you have.
with the 2 bar map sensor, and larger injectors, it works like this:
the stock map sensor only reads vacume.
0 volts = 100% vac (these are not exact #, this is just to give you the idea of how it works)
5 volts = 0% vac (WOT)
now a 2 bar map sensor works the same way, only 0volts is 100% vac, and 5 volts is +14.7psi
the ecu uses the map sensor voltage, along with the other sensors in the engine (TPS, IAT, Coolant temp ect) and determines the amount of fuel to inject.
so, becuase the stock injectors are ~220cc, if we use injectors around 440cc or so, we end up with double the fuel.
the SAFCII intercepts the signal going to the ecu, and tricks it into thinking there is more or less air going into the engine(depending on weather you add or remove fuel) which in turn affects the amount of fuel the ECU injects.
the drawback of the SAFCII is that by tricking the ecu about the amount of airflow, that affects how the ecu applies timing to the engine, so, i personally dont like to make big adjustments with it. i think of it as a fine tuning thing.
the emanage ultimate is a better option, because with it, you can adjust fuel flow independantly of timing.
it can adjust fuel in the same manner as the SAFC, or you can adjust it by controlling the injector opening time.
you can also adjust timing, which IMO will really open the door so to speak once we get an experienced tuner willing to take the time and tune it properly.

to be honest, i really still think the BEST setup on the 5sfe is still out there, waiting to be made.
with the stock ct-26, im thinking the 460's are a little big.
if we had more peope willing to try some stuff, id really like to see someone try a set of injectors like maybe 390cc or so.
that would give you enough fuel for ~235whp, and wouldnt be as rich as most of the guys are running now.
IMO there are 3 main keys in building a sucsessfull 5SFTE:
#1 start with a healthy motor.
#2 eliminate weak points in the setup (IE: oil and waterlines, IC piping ect..)
#3 (and prolly the most important, aside from starting with a good motor) TUNING.

[WALKER]

Manny, Thanks again for your enlightening answers. I learn so much from reading your words. Initially I am hoping to just build a reliable 5SFTE that puts out 2/3's the power of yours (of course I would like to equal you! ). Initially I am more interested in understanding the 6GC engine and learn how to do some minor tuning (on a dyno of course!). Perhaps down the road, once I have the knowledge you have built up I can look into finer tuning and a better setup, but for now you are just going to have to put up with me asking you questions and copying you for the most part!

One question I do have for you about the emanage. Is it as easy to do minor tuning (ie. fuel only, such as the SAFC) or is it very advanced and in depth? I would look into getting it, as long as it isn't rocket science just to adjust fuel flow. So are you able to adjust the fuel flow with it and still have the timing remain the same, or do you need to adjust the timing to compensate for the fuel adjustments in emanage? I know that you are just running the SAFC right now and have removed your MSD, if I were to go that route would you suggest just the SAFC or would you suggest I get the MSD as well?

Another question for anyone to answer is still relating to the pressure vs boost. Anybody know what relationship of fuel system pressure to the boost pressure? Is it a 1:1 setup or is it quite a bit more? Also what is the stock fuel pressure in a 5S?

[phattyduck]

Another question for anyone to answer is still relating to the pressure vs boost. Anybody know what relationship of fuel system pressure to the boost pressure? Is it a 1:1 setup or is it quite a bit more? Also what is the stock fuel pressure in a 5S?

Generally, stock setups have a 1:1 FPR. This allow the pressure differential betweent the fuel and the intake manifold (where the injector lives) is the same at all times. For most cars, this is somewhere near 40psi. The idea behind this is to keep the amount of fuel that flows out of the injector for a given time period the same under all operating conditions.

This sometimes breaks down for stock N/A cars after they are turboed if the stock FPR cannot keep up (either with the fuel flow of a high flow pump, or the boost pressure not causing a rise in fuel pressure). This is a possible concern on aftermarket turbo kits, but it doesn't seem to be a problem on the 5s (somebody correct me here if I'm wrong! I've only worked on the 3s-fe and stock turbo cars).

-Charlie