I came up with a pretty cool invention to make the signal supply to the FMU adjustable in different pressure zones which removes the original design limitations.
Tell us more about this invention.
Back in the day, I "tuned"
my FMU with a small brass shutoff valve for fish tanks, installed in the vac line to the FMU.
I've been meaning to post about this. I was going to put it in my build thread but figured it'd get buried and lost there. It's been a while since I posted in this thread. Essentially I was looking to lower my fuel pressure and get better reading on my AFR guage by trying some "tuning" on the FMU.
I did install some flow matched Bosch 24lb injectors and a ProM 80 MAF calibrated for them. Then I started playing around with the FMU. Here is what I found.
If anyone has ever messed around with a FMU and viewed your changes with a wideband, we've probably all come to the same conclusion. They get extremely rich at full boost.
Most probably know this, but for those that don't it's best to understand how these work as boost pressure is applied to them. The pictue below shows a few discs with the blue diaphram and FMU top cover with vacuum / boost port.
As boost pressure enters the boost port it pushes on the blue diaphragm. That diaphragm in turn pushes the round disc (looks like a washer). The washer sots in a machimed aluminum seat to keep it centered and pushes a plunger which squishes two plates together in the bottom of the unit that fuel is flowing through. The harder those plates push together the more of a restriction of the return line and the higher the fuel pressure gets. It's like some sort of crazy Rube Goldberg contraption.
The pic below shows the washer which sits under the blue duaphram seal. That's what boost pressure is pushing on. This one happens to be an 8:1 ratio.
Below the washer is the plunger in the FMU body. In the pic below its the round silver button in the center. That's what connects to the "wet" area on the bottom of the FMU where fuel flows. The harder that plunger is pushed the higher fuel pressure gets. It doesn't move much at all. Maybe 1/16". It's more about applying pressure to it.
There are a few ways to adjust how the FMU responds to boost pressure. The first is with different size discs or calibration kits as Vortech calls them. To test this out I bought all the sizes they offer. I also had several sets made by a local machinist in sizes between what Vortech offers. Below are some of the various sizes I tested.
The idea of the different sizes is strictly to increase or decrease the surface area that boost pressure has to push against. The larger the washer the easier it is for boost pressure to push the plunger harder. Resulting in higher fuel pressure. A smaller disc has less surface area so the same amount of boost pressure pushing it doesn't push the plunger as hard resulting in less fuel pressure.
In my testing I found it works pretty good up until about half of the boost you run. For example is you run 10psi of boost and use an appropriate size FMU disk to richen up your AFR at 0-5 psi, it will get extemely rich above 5psi because pressure keeps increasing on the disc as boost builds.. Conversely if you run a smaller disc to get acceptable AFR values at maximum boost it will be too lean at lower boost levels. It's always a catch 22.
Even the custom size discs I had made exhibited this same behavior. It essentially just shifts the problem around.
Another option Vortech makes is the bleeder. Picture of the bleeder below.
This replaces the nipple on top of the FMU woth a bleeder valve. The idea of this is to bleed off pressure to fine tune the mechanical advantage of the disc. This is poorly executed because it's a constant bleed, meaning at idle or cruise it's a vacuum leak. Not ideal. In testing I found it behaved exactly like the "in between" custom discs I had made. It still exhibited the typical FMU behavior of getting excessively rich at maximum boost. So rather than offer much adjustment of a disc set, in practice it makes a disc set operate like a slightly smaller disc set. Same behavior.
If you allow enough of a leak it will reduce the pressure acting on the disc all the time. So a larger disc acts like a slightly small disc all the time because it leaks all the time. Therefore it isn't any different than using a smaller dics set. Then you end up with the same dilemma. Either rich up top and good down low, or lean down low and decent up top.
After some tinkering I came up with a pretty good improvement. The problem with the FMU is that it works in a linear non adjustable fashion. As boost goes up so does fuel pressure until you are essentially dead heading the pump. There needs to be a way to introduce a curve. Ideally you could apply all the boost pressure to the FMU calibration disc at low boost levels but then vent some at higher boost levels so it doesn't get so rich up top. With the Vortech offerings there is no way to do that.
After a few different trys I came up with the idea to use a mini presicion adjustable air pressure regulator. Below is a picture of it. Model number on box.
It's made by Air Logic in the USA. This particular one is adjustable from 0-10 psi. It installs between the vacuum tree and the FMU and is either surface mounted with the bracket mounted to it, or panel mounted by removing the knob and using the threaded body.
To prepare for this I decided to re route all my S/C accessories together off the vacuum tree. The bypass valve, BTM, and boost guage share the same supply. I ran a dedicated line to the Air Logic by adding a vacuum port. I used a UPR billet aluminum firewall mounted tree and re did all my lines with some thick wall silicone high pressure tubing.
This allows to set the point where boost pressure to the FMU is vented. Rather than just being a metered all the time leak (and a vacuum leak like the Vortech bleeder) it allows the FMU to operate with a curve. It also does allow any vacuum leaks at idle or cruise.
I've found it best to look at it as two curves. Assuming we make 10 psi then a low boost curve 0-5 psi set with the FMU disc size, the the high boost 5-10 psi set with the Air Logic.
So i start with a FMU calibration disc large enough to get rich enough in the first half of boost. Normally that would be end up excessively rich at maximum boost, but with the Air Logic it can be vented above whatever PSI it's set to. In my case I used a custom made 7.5:1 FMU disc set to get the AFR i wanted in that low boost zone.
Then I recorded the AFR guage during a pull and note when it's getting too rich. Also noted the boost level at that point. A cell phone recording works great. That's a good starting point. For me I saw the AFR nudge below 11 and into the high 10's at 5.5 psi. Then after that it went very rich amd kept getting richer reading of AFR 8:1 were typical (way too rich) at max boost.
With that info I set the Air Logic up out of the car. I applied shop air to the input of the valve. On the output side I attached a hand held pressure pump guage to monitor the pressure allowed through it. I turned the knob until my guage read 6psi. This means it will begin to bleed pressure at 6psi and stay there. Thats a good baseline so I took it for another ride and it was way, way better. After sime knob twisting I was able to keep the AFR in the 11's the entire pull.
I don't claim this to be as accurate as a digital tune or stand alone setup by any means. But it's way, way better than how it was before.
An added benefit of regulating the air pressure to the FMU is the reduction of fuel pressure. I'm liking what I see on the AFR guage and maxing out at 60 psi of fuel pressure. For an FMU that's pretty good.
I was also able to disconnect the T-Rex auxiliary fuel pump.
If anyone gives this a try just be aware you need to take it easy the first few times up to max boost to make sure you have enough fuel pressure. It won't continue to build like it did before. So be cautious until you're sure. Start on the rich side of course.
I'm happy enough with it for now. I'm sure I'll upgrade one of these years to a stand alone setup. But for now it's good enough. I'm suprised Vortech doesn't have something like this. It's so simple.
I still say you're fine with a high 11 afr under boost.
