Judging from the trending articles on this site, more than a few of you found the 1,100-horsepower Hurst/Kenne Bell engine build interesting. There’s little wonder as nothing but the best lives inside the 5.2 Coyote-Voodoo hybrid and it promises to hammer out breathtaking power — always a good way to hold a gearhead’s attention.
However, there is the matter of feeding that beast. Inside the team building this monster street mill, attention is currently focused on integrating the engine to the 2017 Mustang chassis. At GTR High Performance Ricardo Topete has taken the whiz wheel to the GT’s upper firewall, a tale we’ll tell after Ricardo has finished shoehorning this long and tall powerplant into position. Simultaneously, Ken Christley at Kenne Bell has been noodling that most bothersome leg of the fire triangle: fuel.
Even 800 horses to the tires is well-travelled territory these days. But exceeding four digits at the crankshaft is right in the gray area where street-bred solutions start running out of capability.
One reason this fueling question is so compelling is it falls right in the cusp between crazy street horsepower and pure race car practice. Fueling a 450-horsepower Coyote is already provided by Ford, and the 1,500-plus-horsepower race car folks long ago figured out a stout, engine-driven pump and custom lawn sprinkler system is what they need. Even 800 horses to the tires is well-travelled territory these days. But exceeding four digits at the crankshaft is right in the gray area where street-bred solutions start running out of capability – but race-bred fuel systems are too pricey and, especially in a street car, bothersome to install.
To be more specific, Kenne Bell calculates its Boost-A-Pump module has the electrical juice to motivate enough fuel flow from the stock Ford fuel pump, lines and rails to support 1,100 horsepower; but providing enough fuel pressure is questionable at the 26 pounds of boost anticipated to make that sort of power from the 5.2-liter “Vooyote.”
The alternative is a multiple-fuel-pump race system — Kenne Bell has selected the triple pump system from Fore Innovations as its Plan B — and the question is more than academic. The Boost-A-Pump is within easy financial reach at $270 and dead simple to install.
If the Fore system is needed, it will add $2,500 or more to the project by the time it is installed, and that installation promises plenty of close-quarters fabrication opportunities in an engine compartment already stuffed with air conditioning, power steering, emission controls and so on. But, it has the muscle to deliver all the fuel an 1,100-horsepower engine could ever want.
Additionally, such a capable fuel system will unquestionably support whatever additional power the owner may desire down the road.
Fuel System Basics
Certainly the system must deliver enough fuel volume or mass to support the power the enthusiast expects from the engine.
Certainly the system must deliver enough fuel volume or mass to support the power the enthusiast expects from the engine, so sufficient fuel flow is required. But we budding fuel system engineers must always have the correct pressure at the fuel injectors. Nozzle pressure is important because it is used to both atomize the fuel into a burnable mist, and to overcome any pressure in the inlet tract.
On a naturally aspirated engine, the pressure in the inlet tract equals or is less than atmospheric pressure; so maximum fuel pressure need not exceed approximately 40 psi for good atomization. But when supercharged, the pressure in the intake manifold rises by the amount of boost and air heat delivered by the supercharger. So, what’s desired is around a 40-psi difference between the fuel pressure and the intake manifold pressure.
If there’s 10 pounds of boost from the blower, then you want 10 extra psi of fuel pressure to overcome the raised manifold pressure, plus the usual 40 psi of fuel pressure to correctly spray the fuel. In other words, you need 50 psi of fuel pressure. Kenne Bell refers to the fuel pressure/intake manifold pressure differential as the delta pressure.
Not to wander too far into the semantic woods, but it’s a shame we hot-rodders talk of vacuum and boost when addressing the intake manifold pressure. It’s easier to think in terms of manifold pressure, or more specifically, Manifold Absolute Pressure, as the engineers and aviators say. MAP covers everything from vacuum to boost, including temperature and density altitude. It is one-stop shopping when talking about what’s going on inside the intake manifold and the resulting load on the engine, but it’s not traditional around cars, especially Fords.
Ford’s Fuel System
How Much Fuel?
When exploring a new corner of performance engineering, it’s important to take a step back occasionally and examine the overall situation. If not, you risk spending much time, money, and energy on dead ends.
These days the general consensus when fueling big-horsepower Mustangs is you need a pair of fire trucks to pump the contents of a fuel tanker through a 5.0-liter every 10 seconds. Caught between the fear of burning prized engine into bacon crisps by running them lean and the perception that the Space Shuttle’s fuel pump’s might barely support 1,000 horsepower, many enthusiasts have forgotten the math and let emotion drive them to wild excess in the fuel supply department. The three-pump Fore Innovations fuel system Kenne Bell is using for the this project car is a good example. It can supply 1,100 horsepower using just one of its three fuel pumps, and with all three pumping at 50 psi the system can supply 1,000 liters of fuel per hour. That’s good for 2,600 hp, or as Ken Christley put it, “That would fill a bucket really quick!”
For the Hurst/Kenne Bell project car, it was primarily the decision to support more than 1,100 horsepower that drove the move to the multi-pump fuel system. When reaching for 1,300 or more horsepower there is no question the need for huge manifold pressure, the inability of the stock fuel system to deliver more than 50 psi, plus the secondary considerations of pumping a column of fuel against strong acceleration loads conspire to make a racing fuel system the correct solution.
If mounting an electric pump in gasoline raises an eyebrow, don’t worry. It’s the industry standard, and has been for decades. Its two main advantages are – vapor lock is nearly impossible, because the pump works by pushing fuel to the engine rather than sucking it from the tank to the engine as with legacy systems – and submerging the pump in fuel also absorbs the obnoxious whining electric pumps make.
Where the pulse and mechanical Ford systems differ is how they modulate fuel delivery. The 2010 and earlier system used an electronic driver to control electrical power to the fuel pump. It pulsed the fuel pump on and off in response to a signal fed back from a fuel pressure sensor mounted on the fuel rails, just above the fuel injectors. The system’s goal was maintaining a constant fuel pressure, which in turn more or less automatically varied the fuel volume delivered to the engine. Tuners could fiddle with injector sizes and duty cycles (how long the injector remained open), and the fuel pressure sensor signal to control both fuel pressure and volume on hot-rodded engines.
On 2011 and later Mustang GTs the fuel-pressure sensor at the fuel rails has been eliminated (reducing cost) and a pop-off valve added to the outlet side of the fuel pump. [The pop-off valve is part of the plastic fuel “basket” the fuel pump sits in.] Thus, whenever the pump exceeds 50 psi the pop-off valve opens and the excess fuel is dumped right back into the fuel tank. So, no matter what you do with the stock Ford pump and basket, you cannot exceed 50 psi of fuel pressure.
There’s still an electronic driver in the 2011 and later fuel pump circuit, but it outputs only low (idle, light cruising) and high signals (power) to the fuel pump, so it provides only coarse fuel control.
So, how does the 2011 and later system provide fine fuel pump control? By referencing the mass air meter voltage and having the engine’s ECU compute accordingly. In other words, instead of fitting the engine with a MAP sensor, like Chevy does, and directly reading manifold pressure, Ford infers MAP from the MAF and other signals. To do this, Ford has to tediously map the engine’s needs on the dyno during vehicle development. That definitely complicates new-car development in Dearborn, but they only have to do it once and it saves the cost of fitting a $5 MAP sensor to several million engines every year. That’s an easy sell to Ford’s accountants but it does complicate our hot rodding.
The take away from the 2011 and later fuel system is it doesn’t maintain the delta pressure like the 2010 and earlier system did. The 2011 system is more dependent on varying injector pulse width. It’s definitely more complicated for a tuner to work with, or as Ken Christley says, “It grows into a huge monster.”
One final tidbit about the 2011 engine management: it always stays in closed loop, except during engine deceleration when the injectors are closed anyway. So the ECU is always in play with the fuel system, even at WOT.
Boosting The Pump
So a tuner, such as Ken, is working with a fuel system administered by the ECU. That computer infers the engine’s fuel needs via readings from the mass air meter and other inputs, then controls the fuel pump via the fuel pump driver which offers just two settings, the low- and high-outputs. The rest is done by varying the injector pulse width — the time they spend open.
Clearly, Ken’s major task is in ECU software, extending things such as injector pulse width, but he also needs more oomph from the fuel pump to physically deliver more fuel to the engine. That’s the job of the Boost-A-Pump. At its heart, the Boost-A-Pump is a voltage amplifier, but it’s also a regulator. So, not only can it increase output voltage from a lower input voltage, but it can also maintain a steady output voltage when the input voltage is wandering around. That lends precision to fuel pump operation, even when the car’s voltage had dropped as low as 10.5 volts.
Mainly, however, the BAP is used to increase the voltage and amperage to the fuel pump, thus increasing the fuel flow, and as a consequence, pressure. But remember, on 2011 and later Mustang GT’s fuel system, pressure rises only to 50 psi and then the mechanical pop-off valve opens, regulating fuel pressure at a steady 50 psi.
The BAP is boost referenced, so when there is no boost, the BAP does nothing and the fuel system operates as stock with around 14.2 volts. When boost reaches 3 psi the BAP outputs 17.1 volts to the fuel pump driver, the pump runs faster and more fuel is pumped to the engine. As for the Ford fuel pump, it has far more pumping capacity than most people think. In fact, it puts more than a few “high-output” fuel pumps to shame. When boosted with a Boost-A-Pump, the stock Ford pump is capable of fueling all but the wildest Coyotes. Both testing and real-world experience has shown there’s no doubt it can safely and reliably supply 1,000-horsepower worth of fuel volume when amped up.
Increasing voltage does not harm the pump, as racers have proven for many years using all sorts of voltage amplifiers. Kenne Bell has sold thousands of BAPs for years, and has seen no pump issues. As noted earlier, the BAP mounts easily outside the fuel tank near the fuel pump, and even features a new plug-and-play harness so you don’t even have to cut and splice any wiring. It makes no noise and doesn’t cost an arm and a leg.
So far Kenne Bell has run Boost-A-Pumps successfully on hundreds of current Mustang GTs and GT500s beyond 800 rear-wheel horsepower. That’s to say high-boost Coyotes or Trinity-powered Shelbys need only larger injectors, the Kenne Bell tune and BAP to make big power and run with stock driveability. No extra fuel pumps are needed and the stock fuel line and stock fuel rails do just fine.
What hasn’t happened — yet — is to supply 1,100-horsepower worth of fuel with just the injectors, tune and BAP. The issue is the Ford fuel system doesn’t allow over 50 psi of fuel pressure thanks to the pop-off valve. At 800 rwhp this leads to a small loss in delta pressure (the fuel pressure and manifold pressure do get closer, but not enough to matter). But Ken’s calculations show the 1,100-horsepower engine will have just 16 psi more fuel pressure than manifold pressure at full chat. Ken thinks this will be enough to work, at least in short blasts (what other kind are there with 1,100 horsepower on the street?), but knows he’s right on the ragged edge of delta pressure. Enter the Fore Innovations fuel system…
More Pumping Capacity
As a complete replacement fuel system, there is no question the Fore Innovations three-pump offering will easily fuel the 1,100-horsepower engine. In fact, it will be loafing well below its maximum capacity. Completely replacing the stock Ford fuel system, the Fore combination of pumps, basket, piping, wiring and so on has definitely proven itself for years at the racetrack.
There are no tuning concerns with the Fore system either. It’s 100 percent user adjustable, so tuning may take some time, but is limited only by the tuner’s understanding.
What does give reason to pause is the $2,000 or more to buy the Fore system (or other similar replacement fuel systems from various manufacturers) and then having to install it. These stand-alone fuel systems are designed for race cars and feature sexy-but-bulky AN hoses and fittings; a surprising amount of heavy gauge wiring and plenty of plumbing bits ranging from junction blocks to fuel rails. Installing all this – every piece requires custom mounting, there’s not a plug-and-play part anywhere — isn’t so bad in a gutted race car, but definitely poses fabricating challenges in a fully equipped and heavily optioned street car such as the Hurst/Kenne Bell Mustangs.
This style of system also unavoidably adds weight thanks to its bulletproof hardware and extra pumps. So much so, we broke out the shipping scale to learn that after removing the stock Ford pump and basket, there’s a net gain of 21.45 pounds with the complete replacement Fore system over the stock Ford system and Boost-A-Pump combination. Now, the Kenne Bell supercharger is 65 pounds all by itself, so this is sort of the kettle calling the pot black, but the blower is a must-have to make 1,100 crank horsepower, while the fuel system might be overkill.
So, Which Fuel Solution?
And there’s the operative word: might. Figured as close as possible it’s not abundantly clear if the simple Boost-A-Pump solution is enough or if the full-monty Fore system is needed to save the day at 1,100 horsepower. Kenne Bell’s extensive in-house test and engineering capabilities says the BAP and stock Ford Mustang GT fuel system is good for 1,100 in every way, save the delta pressure is uncomfortably low at just 16 psi.
Ultimately the decision on fueling the 1,100-horsepower car was made much easier when it was decided too much was never enough, so why not try for 1,300 horsepower? Given that thinking, the easy, affordable Boost-A-Pump solution was left for sub-1,100-horsepower applications and the Hurst/Kenne Bell Mustang was fitted with the Fore Innovations three-pump fuel system.
Exciting stuff to be sure, so stay tuned. We’ll have the dyno results in our next story on this cutting-edge street car.