Tom Wilson
April 4, 2013
Wanting a closer look at the complex fuel systems in the latest Detroit muscle cars, Kenne Bell took time off from its standard diet of supercharger development to build this fuel system test bench. It accepts the entire fuel system, from the fuel pump assembly in the lower left to the injectors at the far end, where bench designer Ken Christly is standing.

Power-adder Coyote fans are a happy bunch. The free-breathing V-8 eagerly responds to power inducements, and enjoys well-engineered cooling and fuel systems. Up to the octane limits of pump gasoline—about 540 rear-wheel horsepower—making Coyote power is about as easy as howling at the moon.

But as tuners have reached above pump-gas levels, they've run into Coyote fueling trouble. The old tricks of increasing pressure for more flow haven't worked, with many opting for multi-pump fuel systems or fuel-pump voltage boosters. Kenne Bell was one of those ambushed by the '11-and-newer Mustang GT fuel system's quirks. After toying with the usual tricks, the Kenne Bell crew went back to basics. It was the only way to understand how the new Mustang's hardware and software works and how it is best modified.

The Kenne Bell team quickly concluded that the new fuel system design coupled with the more complex '11 fuel circuit made on-car testing difficult. To better study the system, they opted to build a fuel bench. This is an involved bit of custom cabinetry with fuel-flow meters, pressure gauges, and some electronic measuring instruments for things such as voltage and amperage. It allows duplicating the entire Mustang fuel system on the bench so that pressure and flow measurements can be made at any point in the system. The fuel bench also allows standalone testing of any single component, such as the fuel pump, lines, fuel rails, injectors, and because it's Kenne Bell, a Boost-A-Pump fuel pump voltage augmenter.

Kenne Bell discovered that the '11-and-later Mustang fuel system fundamentally differs from its predecessors. Of course, the '10-and-up system is returnless, but with the addition of a mechanical pressure-relief valve in the fuel tank. Having a pressure-relief valve means you can do whatever you want—the latest Mustang fuel system caps fuel pressure at 58 psi. If the pump exceeds this value, the mechanical pressure regulator simply dumps the excess fuel pressure back into the tank just before the gasoline enters the line leading to the engine.

In other words, you could replace your Mustang's fuel pump with a space shuttle turbo-pump and you still wouldn't get over 58 psi coming out of the tank. It's no wonder raising fuel pressure to increase fuel flow hasn't worked well, causing tuners to essentially resort to replacing the fuel system in big-power cars.

Another discovery is that the stock Mustang fuel pump is a surprisingly capable piece. Amped up to 21 volts on the bench, it showed a capacity to free up to 900 hp worth of fuel at the system's 58-psi maximum fuel pressure.

Now, in the real world, there are many losses—to things such as the fuel line length and cooling orifices, so there is no way a stock Coyote fuel pump is going to support 900 rwhp. In the car using the stock 13.5 volts and nominal 55-psi fuel pressure, and not having to work against supercharger boost, Kenne Bell figures the stock pump is good up to 281 liters per hour, or 753 rwhp. Obviously this is one heck of a pump, and you don't have to buy it or install it because your Mustang came off the showroom floor with it.

Kenne Bell reasoned that increasing fuel flow without increasing fuel pressure was a good answer for heavily supercharged 5.0 Mustangs, and an easy, cost-effective way to do this is to increase the flow rate by way of larger injectors and supply them with more fuel by boosting the electrical power to the stock fuel pump. The fuel pressure doesn't rise because the larger injectors act as a larger hole at the end of the fuel line, but fuel flow does increase and thus more power is supported. Obviously some electronic tuning is required to reign in the big injectors at idle and low-speed driving. A set of larger injectors, Boost-A-Pump, and tune will do the job for most applications.

What terminates such thinking with highly boosted Coyotes is that eventually there is too much boost pressure in the intake manifold for the 58-psi fuel pressure limit to overcome. The flow requirements are so high with hugely boosted engines that it ultimately becomes necessary to step up to a bigger pump. But this is well above bolt-on power levels, so it isn't a concern for most of us.

To see how various pumps performed at stock and elevated voltage levels, we hit the bench. Our photos detail Kenne Bell's Fuel Flow Dyno 3000, as the fuel bench is dubbed, but the reason that compelled the KB team to build the bench is worth reinforcing. Until the '11 Mustang GT (and other late-model Detroit hot rods) came along, Kenne Bell relied on in-car testing or an older, less sophisticated fuel bench they had built many years ago. However, the new fuel systems were unknown and complex enough that in-car testing could no longer show sufficient detail of what was going on in the fuel system.

The goal was to build a bench that would support the entire fuel system out of the car, allowing the measurement of flows and pressures at many points in the system. This allows easy testing of the fuel system, both as a whole and as individual parts. Kenne Bell says the fuel bench has proven invaluable as an engineering development tool, and also supplies sales leverage because it gives them the data to prove how its equipment works.

If electrically boosting the stock fuel pump is the real-world solution for the vast majority of Coyotes, there are still plenty of pre-Coyote and hero-class power applications where a larger fuel pump is the way to go. And seeing how Kenne Bell has already tested nearly every pump available on its new fuel bench, we're presenting some of the data on four popular fuel pump options to put numbers to their performance. What follows is data showing the pumps at both the stock 13.5 volts and augmented to 21 volts by way of a Boost-A-Pump.

First of the popular options is the stock Coyote pump. Here's a detailed look at it, flowing by itself on the fuel bench at four voltages. We're showing the 12-volt readings to illustrate what happens to fuel flow should there be a voltage drop (as could happen without the BAP), and the 17.5 volt numbers better extrapolate fuel flow at a moderate voltage increase:

Again, it is obvious the stock pump has a generous flow capacity—remember when Fox Mustangs came with 88-lph pumps? And the relationship between electrical power supplied to the pump and the resulting flow and pressure is easily seen.

The other three pumps we'll look at are the Aeromotive Stealth, the TI 400, and the venerable Walbro 225. This last pump has been around for about a decade and is used in many performance applications, so it's a familiar Mustang upgrade.

On the other hand, the TI Automotive 400 is new. Its official name is the TI 90000262, and it's a real squirter that puts out big numbers. Likewise, the Aeromotive Stealth is another well-respected pump from that long-time racing fuel system specialist:

Horse Sense: Boost-referenced systems work OK until the system voltage drops and the fuel pump slows down, causing the engine to lean out and detonate. A BAP keeps voltage elevated to guarantee sufficient fuel supply. Curiously, headlights are the primary culprit of voltage drops.

Stock Coyote Pump at 12 Volts
40 9.1 261
50 9.9 238
60 10.6 217
70 11.3 193

Stock Coyote Pump at 13.5 Volts
40 10.0 318
50 10.8 294
60 11.5 268
70 12.3 245

Boost-A-Pump Primer
So, what is this Boost-A-Pump box that Kenne Bell sells? The short answer, it's an aluminum box of electronics that augments an electric fuel pump's output, mainly by speeding it up. The colloquial description from Kenne Bell main man Jim Bell is “it's a pump supercharger.”

The more accurate assessment, also from Jim Bell, is it's a voltage amplifier and regulator. In other words, the BAP not only increases voltage to the fuel pump as necessary, it also supplies a steady voltage to the pump. So, despite the input voltage, it regulates to an exact output voltage, either what is preset or what a controller (the engine computer or Fuel Pump Driver Module) commands. This delivers consistent fuel-pump performance.

Kenne Bell references its BAP's via either vacuum in naturally aspirated applications, or more commonly, to boost in supercharged cars. In the later case, it activates at 3 pounds of boost, so fuel-pump operation is stock unless significant boost is present. That means the fuel isn't being heated uselessly 99 percent of the time.

Jim notes the stock fuel pump circuit is not regulated for voltage other than the overall charging system voltage of the car. This varies somewhat with headlights, air conditioning, stereo, electric steering assist, dying batteries, and other electrical loads, which can be troublesome when high electrical demands coincide with high fuel demand. A BAP eliminates this issue by keeping fuel pump voltage rock steady, a trick it can perform with as low as 10 volts coming from the charging system.

As you'd guess, the BAP keeps voltage steady by varying amperage (electrical volume of flow). In other words, when the fuel-pump demands more power to maintain its set voltage (and hence pump speed), the BAP supplies the necessary increased amperage. And the BAP can be used to “supercharge” the fuel pump by increasing the voltage set-point determined by the engine management computer.

Kenne Bell offers two Boost-A-Pumps—a 17-20 amp and a 21-40 amp—plus a dual BAP pairing for Mustangs with dual Fuel Pump Driver Modules, namely GT500s. All are adjustable for voltage using control inputs.\

Stock Coyote Pump at 17.5 Volts
40 13.5 500
50 14.2 463
60 15.0 435
70 15.6 409

Stock Coyote Pump at 20.8 Volts
40 16.0 609
50 17.0 583
60 17.3 549
70 18.2 518

Walbro 225
13.5 Volts 21 Volts
40 7.6 248 11.0 452
50 8.0 227 11.8 446
60 8.9 206 12.7 440
70 9.9 193 13.6 433

Aeromotive Stealth 340
13.5 Volts 21 Volts
40 12.8 348 20.5 562
50 13.4 327 21.2 547
60 14.2 304 21.9 531
70 14.9 287 22.6 515

TI Automotive 400
13.5 Volts 21 Volts
40 14.2 422 22.1 624
50 15.0 388 22.8 594
60 15.8 352 23.7 568
70 16.5 318 24.0 524

From the data, it's clear the stock Ford pump slots in with the familiar performance pumps. From a flow standpoint, there is little or no gain from replacing the stock Coyote pump with another single pump. Dual or triple pumps are another story.

We emphasize this data is no more than a snowflake atop the iceberg of Kenne Bell pump and fuel system data gained from its flow bench. If you have something special in mind with your KB blower application, speaking directly with KB tech support could save much time and expense.

It's tough not to conclude Ford has stepped up the Mustang's fuel system to the point where there's not a lot to be done to support meaningful power increases. It's much the same with the suspension, brakes, steering, and driveline, so we guess it shouldn't be too much of a surprise. Guess we'll just have to aim for making even more power!

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.

Jet Pumps
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.

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.