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Fuel Pump Flow Test - Power Hungry
Going inside modern Mustang fuel systems on Kenne Bells' fuel flow bench
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.