All '03 Mustang Cobras are...
All '03 Mustang Cobras are equipped with dual 160-lph fuel pumps and should never need more pump capacity. However, to achieve the necessary 80-psi fuel pressure in the 550hp-and-up range, more voltage to the existing pumps is required. Kenne Bell used its Boost-A-Pump to increase the pump voltage. The BAP mounts in the left rear inner fender where this voltage reading is being taken. A 30-amp fuse in the fuel pump circuit also proved necessary.
Easy Horsepower
The next meaningful event was the smaller 3.25-inch blower pulley, which took the Cobra to nearly 15 pounds of boost and 508 hp while breathing through the stock air filter. Then the bazooka was substituted, which allowed the boost to climb to a tick shy of 17 pounds, and power to 560 hp, but otherwise all this was through the stock throttle body, cats, and so on.
Wanting to see a run using equipment you could close the hood on, we asked for a run using an air filter instead of the bazooka. A 9-inch conical was attached directly to the mass air meter, which knocked the boost off a piddly 0.4 pound and the power 12 hp. Clearly you don't want to be running the stock air filter at this power level (unless you really don't want to hear a muted blower scream through the more open element air filter for some reason). To resume testing, the bazooka was refitted.
It Gets Tough By now we were almost getting numb to the staggering power the Cobra was dishing out, and greedy power-mongers that we are, we were all eyeing the 600hp mark. Getting past 560 hp, however, proved more difficult from a fuel-delivery standpoint. And in a rare, lucid moment, we concluded the mid-500hp level is where most Cobra owners might want to go with their Kenne Bell blowers. Given the IRS and limits of street tires, any more power is just more tire smoke at these elevated power levels anyway.
Pulleys? Kenne Bell has pulleys....
Pulleys? Kenne Bell has pulleys. Unlike the Eaton design, the KB pulleys easily swap using a single bolt and a simple two-bolt restraining bar. It takes just a minute to change the pulley.
Jim still had plenty of smaller pulleys, though, so we pressed on. But Ken was having increased difficulty navigating the complex Ford software. Working with late-model Ford computers is much more complex than it was just last year, and it took Ken several attempts to arrive at a chip that worked.
Another concern was exhaust backpressure. Obviously 560 hp through stock cats, mufflers, and tailpipes was not representative of typical hot-rodding practice, and it certainly was contributing to excessive backpressure. Was there enough backpressure to block any power increase?
To find out, the exhaust was drilled and tapped before the cats to accept a pressure gauge calibrated in pounds per square inch. Jim said workable backpressure with supercharged engines runs around 5 psi, with the worst he'd seen being 10 psi on a blown 351 (non-Lightning) F-150. He predicted the Cobra would beat that, and he was right by a factor of three. With 30 psi of backpressure, the Cobra might as well have had a potato stuffed up each pipe, and we instantly salivated at the thought of long-tube headers and open collectors. However, in the interest of sticking to a single variable, the stock exhaust remained until Ken solved the fuel-delivery issue.
To shorten a two-day-long story, Ken's challenge was not the pump or really even the injectors at 560 hp--it was still electronic. Add fuel pressure and the EEC pulled injector pulse width. Lengthen the pulse width and the computer pulled fuel pressure! It took some hunting on Ken's part to find the correct tables hidden in the computer code, but ultimately he did, and in the end a combination of his chip and a 17-volt kick in the pumps via the Boost-A-Pump got the necessary fuel flowing.