Supercharger Fuel Management Unit

Supercharger, Part III: Fuel Management Unit
Paradise Garage

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With a supercharger installed, it's more important than ever for the engine to get fuel. This becomes more difficult with a supercharger, because as boost increases, so does fuel demand -- far beyond what the stock system is designed to deliver. Since we were installing a Vortech S-Trim supercharger on the visiting 1995 Mustang GT, we made certain to include their fuel management unit as well.

In a nutshell, the FMU adjusts fuel pressure when the boost comes on, regulating it at normal levels during normal operation and dramatically increasing it on demand. It is purely a mechanical device, using different sized disks inside the unit to alter the pressure at which the FMU takes effect. This makes it relatively easy to install and adjust.

The FMU is included with the Vortech S-Trim kit and requires no additional parts and only one special tool: a 1/2" fuel line coupling tool. The kit comes with a 12:1 disk for Mustang GTs and a 10:1 disk for Mustang Cobras; we also got an 8:1 disk separately (#6Z170-020) just in case we needed it. The diagrams included in the supercharger manual are actually really good for this step, which is a pleasant surprise compared to many other steps. Refer to it.

Here is the FMU, ready to be installed.
Start by disconnecting the fuel return line at both ends: one end is behind the passenger-side shock tower, and the other end attaches to the fuel rail (this end may already be disconnected if you changed your fuel injectors). The 1/2" fuel coupling tool will undo the connectors.

One of the stainless-steel-braided fuel lines that Vortech provides is 14" long. Attach this fuel line to the center fitting on the FMU. It takes a 9/16" wrench.

Mount the FMU to the passenger-side inner fender between the shock tower and the firewall. Drill a pair of holes using an 11/64" drill bit and use the provided sheet metal screws.

Finish attaching the bottom fuel line to the stock return line (the part headed to the tank). Make sure to put a coil in the line, so that it makes a loop on its way to the stock line.

Attach the second steel-braided fuel line from the side of the FMU to the fuel rail. Use the stock fuel line mounting points to restrain the line and prevent it from chafing any other objects.

The FMU installed. Hey cameraman, sober up!
Attach the vacuum line (provided in the kit) from the top of the FMU to the vacuum tree, which is on the intake manifold, at the same place where the stock fuel pressure regulator gets its vacuum. However, the vacuum system is about to get spliced a lot, and must be done in a certain order. It's best to just leave the line nearby and save the splicing for a later step.

And that really and truly is it for installation.

Should you find that you need to change the disk in the FMU (as evidenced by too much or too little fuel at WOT -- the FMU has no effect on normal driving conditions), it's a simple matter. The lid of the FMU is held on by six 5/32" hex head bolts. Directly under the cover are the diaphragm, disk, and ring (the 12:1 set does not have a ring). Remove these and replace with a different set -- larger ratios richen the mixture (12:1) and smaller ratios lean the mixture (8:1). Ultimately, we ended up using 10:1.

When reassembling, don't overtighten the hex bolts. Vortech recommends 24 in-lb.

With the fuel management unit installed, it's time to move on to Vortech's Step 5, relocating the fan relay box!