Carl Fausett

A good question.

First, understand that even if you did re-chip the car, you would never be able to increase the fuel pressure. The chip, and the computer it gives instructions to, do not have the ability to increase fuel pressure even if they wanted to.

So, thats why a) you need something else to do it, and b) we leave the chip alone.

We use a Fuel Management Unit (FMU) in our kits to increase the fuel pressure only when the system is under boost. It is installed in the return line to the tank, and has a signal line (vacuum) that connects to the manifold to sense vacuum/pressure. At idle or bottom end (generally under 2500 rpm) the supercharger is not making boost, the intake is in vacuum as nortmal, and the FMU does nothing. There is no reason to change the fuel map at all.

When the FMU senses the manifold swing into boost, it begins raising the fuel pressure at the injector rail by a ratio of roughly 2 psi fuel pressure increase per 1 psi of manifold pressure. This keeps the injectors in their proper pressure differential range, and atomization is optimal. It also adds the extra fuel we need.

There are 2 adjustments on our FMU - one allows you to set when the FMU begins to cut in, and the other adjusts the rising rate. This allows owners to easily tweak their own tune (as it is always a little different from car to car).

The FMU is all we need on the Stage 1 kits. Stock injectors work fine.
On the Stage 2 kits, we include larger injectors to help even more.

Mark944na86

Just out of curiosity, does this approach work as well for (low boost) turbocharged applications?

Is there a level of boost after which this approach starts to not work very well?

Damian in NJ

You know Carl, maybe you should answer questions like this through a pm-do you really want to provide info for people trying to copy your setups?

Fox944

who else is in the process of developing a SC kit that would copy Carl's set-up?

Jfrahm

The FMU works pretty well in boosted Miatas as well as 928 applications. I think the Flyin' MIata turbo kit uses an FMU. I know some Miata supercharger kits use them.

Given the fuel curves from the dyno of the supercharged 968 it is hard to fault the FMU. It is also nice that you can take the SC off and go get smogged or whatever and there is no need to swap chips. In all except boosted conditions everything runs as stock.

-Joel.

Carl Fausett

No, not so much that a FMU will no longer work, but there is a level where even more changes need to be made. For example on my 928 Race car... first larger injectors were good enough, then we raised the boost and found that was no longer enough, had to install a bigger fuel pump too. Then we installed a larger supercharger, and found that we had to increase the size of the lines from the gas tank to the engine... and so it goes. When all that is done, you install a different Engine Management System where you can control the phasing of the injectors (BTDT). (Speed costs money) and its very addictive!

On the 968, our Stage 1 kit is the maximum I can get out of the stock injectors and a FMU. Thats why on Stage 2 we add the larger injectors.

I am sure there will be those that start with our kit and take them much further. They will no doubt upgrade the 968 fuel pump next, and go on from there.

Carl Fausett

I know. I used to try to keep the barn doors closed much more. Now I have learned that all the time I spent trying to keep 1 horse from getting away, I lost others out the other end of the barn.

So I try to not worry about it so much and just concentrate on better products with better customer service.

RajDatta

+1 Carl. As we all know, you have already been burnt on your IP already. Not trying to rehash what some people did to you, but you should seriously consider not sharing so much of your IP publicly.
I hate to see vendors get burnt by other vendors, by copying their ideas and designs.
Good luck.
Raj

edz968s

And one here in the UK
I`ve got mine installed, but cannot use it. Fueling problems, don`t know what it is, but high pressure all the time. I removed and replaced the fuel press reg with a stock new item but fouls the plugs, hunts then dies in a cloud of thick black smoke.
I fitted the air smoother filter to no avail.
Waiting on a fuel rail guage to see what pressure I have and an air/fuel ratio gizmo to hook up.
Carl stage 2 belt slippage, not being able to confirm it, this would no doubt affect mine?
With belt tension and fitting the a/c alternator bracket, to non a/c cars, there is a hunt on for a suitable turn-buckle to allow the alternator to swing out, thus gaining tension.
Hopefuly give you some on road feedback and track time soon.

Carl Fausett

When you get a fuel rail pressure gauge we will be able to find out why your fuel pressure is so high (we think).

Your installation is gorgeous!

Jfrahm

You might try running without the FMU to see if it will idle and run a bit, but DO NOT BOOST!

Might the FMU be installed backwards? It's on the return line so flow is from the rail to the FMU and then RETURN to the tank.

Cheers,
-Joel.

Rogue_Ant

How an FMU works is not IP, nor is its typical application and useage.

I do agree that blatant copying of someone's hard work is wrong, but don't blanket everything under IP.

Mark944na86

One of things I find interesting about the FMU approach is how it works given the non-linearity of fuel rate of delivery with respect to fuel pressure.

My understanding is that the rate of fuel delivery varies as the square root of the increase in fuel pressure, so doubling the fuel rail pressure from 3 bar to (say) 6 bar results in a fuel delivery increase of about 1.4 times.

But from what I'm reading above the FMU control pressure in a linear fashion with respect to manifold pressure. If the rate of air delivery is roughly proportional to manifold pressure, then at some stage of boost level, the air is going to be delivered disproportionately to the fuel to the point where lean conditions are inevitable, I would think. From the example above, you would be getting twice the air delivery but only 1.4 times the fuel.

Now obviously the duty cycles of the injectors can be adjusted up by the ECU to compensate within a range, but past a certain point, I would think air would inevitably outstrip fuel with the FPU approach, which is why I was wondering about the limits to boost using such an approach.

Bigger injectors would help, of course, but really just moves the boost limit higher, delaying the inevtable so to speak.

Maybe in practice none of this is a real issue. Or perhaps I'm misunderstanding something basic. Just curious, really! Interested to understand how this all works, since it's a simple approach that seems to work (surprisingly?) well.

Rogue_Ant

Mark, you are understanding it perfectly.
FMU's are typically used only in low boost applications (<15psi).
Trying to account for the increased airflow with just an FMU becomes problematic:
Assuming air flow increases according to boost (without VE loss), then at 10psi of boost it would take an ~8:1 ratio FMU to supply the exact same increase in fueling (3Bar base FP). The problem with FMUs then becomes obvious - if you are increasing fuel pressure at a rate 8:1, then at 10psi that is an 80psi increase in fuel pressure! Add that to the base of ~43.5psi, and you have an unreasonably high fuel pressure.

Now with Carl's kit, it is only a 2:1 ratio FMU. This is not increasing fuel at the same rate as airflow (thats ok, it isn't meant to in this situation). The 968 DME understands and is measuring the increased air flow (MAF), and increasing IDC accordingly. Thus the FMU is not trying to account for the extra airflow, but only used to help the DME to enrich the A/F mixture for the added boost.
A simple, proven, and effective way of doing it, IMO.


-Rogue

RajDatta

My statement was not in particular to FMU, but some of the information that Carl shared here and elsewhere, that was then used to develop a competing product. I guess you are not aware of some of stuff that went on.
In either case, my statement was a more general statement.
Regards.
Raj