5.0 Mustang & Super FordsHow To Engine
Fuel Pump Flow Test - Power Hungry
Going inside modern Mustang fuel systems on Kenne Bells' fuel flow bench
Pressure & Flow
Low-, medium-, and high-flow circuits are provided to improve measurement accuracy in the fuel bench. The low-circuit flows from 0 to 303 liters per hour; the medium circuit sees a lot of use at 174-871 lph; while the high circuit accommodates everything from 291 to a rather racy 1,457 lph. That’s 385 gallons per hour, or around 4,600 hp worth.
This white plastic assembly is the fuel pump basket from an ’11 Mustang GT. It houses the fuel pump, fuel-pressure regulator valve, and the fuel-level sender, plus various hose nipples, filters, and other accessories. One of its main purposes is to form a reservoir of fuel around the fuel pump to avoid fuel starvation. One of its largest practical considerations is that its many jobs and complex construction makes it difficult to modify or replace. Here Ken has his left hand on the fuel level float and his right on one of the stainless steel rods that hold everything together.
Broken down into some of its major parts, it’s easy to see how tightly packed the fuel basket is. Any replacement fuel pump needs to be an exact fit to work in the basket.
Here it is, the fuel pressure regulator. A mechanical part, it has a simple job—to vent any fuel back into the tank once fuel pressure reaches 58 psi. Given that it is an integral part of the packaging of the fuel basket, simply removing this pop-off valve is not a reasonable option.
Mustangs have used three systems to control fuel flow and pressure since the advent of the Foxes in 1979. In early cars, fuel was pumped to the engine, through the fuel rails, and then back to the fuel tank—hence the return system moniker. A fuel-pressure regulator at the end of the second fuel rail pinched off fuel flow back to the gas tank to regulate the fuel pressure. This system was used until 1998.
Because a return fuel system heats the fuel, restricts flow through long tubing lengths, and promotes higher evaporative emissions, it was replaced by a return-less system in 1999. Pressure and flow volume control is provided by pulsing the electric fuel pump.
One characteristic shared by the return and return-less systems is tuners have the ability to raise fuel pressure—and hence increase flow at the injectors. That was done with a manually adjustable fuel pressure regulator on '79-'95s, and varying the pump duty cycle on later cars.
Starting in 2010, the third system hit the showrooms. It too is a returnless system, but crucially, it adds a non-adjustable, difficult-to-bypass pop-off mechanical regulator in the fuel tank. It vents fuel pressure so it never rises above 58 psi. Ford controls the fuel system pressure and flow with a combo of varying the fuel pump and injectors duty cycles.
Work around the Coyote fuel system and it won't be long before someone mentions the “jet pumps,” as if everyone present knows what the heck is meant.
The answer starts with the shape of the S197 fuel tank, which is shaped like a pair of saddle bags laid over the driveshaft. Fuel is trapped on either side of the central tunnel, and the jet pumps are simply nozzles shooting a stream of gasoline aimed to scoot fuel from the passenger side of the tank over the central tunnel, to the driver side and the single fuel pump. It's like using a high-pressure water hose to bail water out of a boat.
This saves the cost, weight, and complexity of multiple fuel pumps or fuel pickups around the nooks and crannies of the fuel tank, but it also means some of the fuel pump capacity is spent making fountains inside the gas tank. This reduces the pump's output to the engine, which is one reason the Coyote fuel pump is such an animal.
Previously when Ken tested Boost-A-Pumps, the team used battery power on the early Kenne Bell fuel bench. This drained the battery enough to vary the input voltage. On the new bench, KB installed this 30-volt, 100-amp power supply. “It’s a real Frankenstein,” says Ken, who notes he used 4-gauge wire to feed the BAP test station as the new generation of fuel pumps really suck up the electricity.
Kenne Bell referred us to a two-page summary sheet of pump data it had acquired while testing at least 12 different pumps flowing by themselves. The test variable was input voltage, resulting in 741 data points of flow, pressure, and current draw information. We narrowed that to these four pumps of greatest interest to Mustang tuners—from left, a stock ’11 Mustang GT, Aeromotive Stealth, Walbro 225, and Ti400.
Because even the gnarliest fuel systems benefit from a clean power supply with variable voltage capability, Kenne Bell is developing a mega Boost-A-Pump. Dominating the standard BAP, as shown here, the experimental BAP puts out an arc-welding 60 amps! There’s no word on if or when the bigger BAP will hit the market.