Muscle Mustangs & Fast Fords
Packing More ProCharger Performance Part 1 - ProCharger D-1SC Supercharger
We upgrade the supercharger on our 331 Mustang-the right way
So the fuel system needs to be able to support all of the horsepower produced in the cylinder, not just what's left over at the flywheel, and with forced induction you need to be able to deliver at higher, boost-referenced, fuel pressures.
With most "bolt-on" supercharger kits, a second fuel pump, placed in series with the stock pump in the tank, is the standard arrangement. Upgrading the in-tank pump can help to increase available flow. "Two pumps, plumbed in series, are able to maintain the same flow at higher pressure than a single pump alone, but the larger external pump can't make the in-tank unit flow more overall," Clow says.
He adds, "The secondary pump usually fails prematurely in these systems because it starves during normal driving, causing cavitation. This releases tremendous energy inside the pump, shocking parts and shortening pump life. Another quick fix with some systems is a fuel pump voltage booster, used to increase the torque of the pump motor, increasing pump rpm under pressure to provide more flow. Unfortunately, this is yet another shortcut and is just one more thing that can go wrong, and it has shown to reduce the longevity of a pump."
To provide both high flow and long pump life, Aeromotive recommends the opposite approach; that is, installing a pump that is able to supply the engine at normal voltage, then using their fuel pump speed controller to reduce pump speed during normal driving. "A fuel pump speed controller such as our PN 16302, actually lengthens pump life, while recycling less fuel, keeping fuel in the tank cooler," Clow says.
Sumped Tank or Fuel Cell
A proper fuel cell or sumped tank plays a key role in controlling fuel at the pickup point, ensuring the pump always has a supply of fuel at the inlet, which prevents air from entering the system. This is critical to both driveability on the street and all-out racing performance.
If you will be specifically drag racing, and you want to run less fuel in the tank to save weight (gas weighs roughly 6.2 lb/gal), you should seriously consider installing a smaller, racing fuel cell. This will allow the fuel container to be kept full from round to round, avoiding slosh and a dangerous lean condition, while keeping the car as light as possible. If you do a lot of street driving in your Mustang, consider Aeromotive's sumped tank, or if you're the handy type, you can buy the company's new custom sump-box (for steel tanks only) and install it in your own tank.
Clow also warns against using the drop-in pick-up tubes. "Using a pick-up tube is like sucking through a straw, which most people don't do when they're really thirsty. Plus, the Mustang fuel tank draws from a 2-quart reservoir, and you can run into problems with the fuel not replacing itself fast enough. This sort of setup builds more heat in the fuel by recirculating a smaller amount, too. Our sumped tank eliminates all of this as the sump circulates 3 gallons, holding this at the pump inlet all the time."
FMUs-The Good, The Bad, and The Ugly
FMUs, or fuel management units, have been a staple of bolt-on forced induction since the beginning. The purpose of an FMU is to change the rising rate of fuel pressure with boost. Some offer changeable discs and others an adjustable bleeder valve, to "tune" the ratio of boost to fuel pressure increase. Regardless, all raise fuel pressure greater than 1:1. This makes the injectors act bigger than they really are, reducing supercharger kit costs.
"The FMU is a crutch at best," Clow says. "It uses tremendous pressure to force the injector to be big enough, and it comes with a price. Often, midrange performance is poor due to excessive rich conditions, and the fuel pump becomes smaller as excessive pressure reduces pump speed, and flow, to a crawl."
He also warns, "Be careful trying to lean out the midrange; I've seen many try to clean it up by bleeding off boost or changing the ratio too much. Though crisp in the midrange now, this can make it way too lean upstairs, popping a head gasket or worse, the motor."
More Facts on Fuel Delivery
There's no such thing as having pressure but no flow. Pressure is produced as a result of too much flow. Creating and controlling pressure is an act of controlling flow-just look at your bypass regulator to see this in action. As the engine uses more flow, the regulator shuts down the return line, limiting flow back to the tank, to keep system pressure up. When you're out of flow, and the regulator has virtually closed the return line, then the pressure begins to drop.
According to Clow, 99 percent of fuel system problems people experience, where the system should be big enough but isn't getting the job done, are traced to wiring and plumbing problems. Failing to install the correct line sizes, particularly on the inlet side of the pump, or installing a fuel filter that is not rated to flow with the pump, can damage your fuel system and possibly the engine.