With the computer code finally worked out, several chips were tried to dial in the fuel curve. All of these were at the 550-560hp level, of course, as no power improvements had been tried.
 Here's how the Cobra looked...  Here's how the Cobra looked on its final, 604hp test with the 12-inch conical air filter. The box on the right fender is the disconnected Boost-A-Spark. It was tested but found unnecessary as the Cobra and Denso spark-plug combination had plenty of spark. Again, keep in mind that the big 604hp peak was only visible on the dyno graphs, not the charts we're presenting here, so don't e-mail us complaining. |
 This is the 12-inch air filter...  This is the 12-inch air filter and tubing Kenne Bell will sell in its '03 Cobra kit for inner fender mounting. The corrugated tubing looks restrictive, but KB has used this slightly larger-than-4-inch tubing in many kits and says it flows well and won't pose a restriction. |
Our next power-improvement trick was to disconnect the after-cat (we wanted to try much more with the exhaust, but a lack of time, parts, and rounded-off nuts held us to simply dropping the after-cat). This halved the backpressure from 30 to 14.5 pounds, but the power remained right at 560 hp. This looked fishy at first, but then we noticed the boost had also dropped about 1 pound (to 16.6 pounds), which is about a 10hp loss, and the A/F ratio richened back to where it had been in the immediately prior test (chip and after-cat connected), which explained a 23hp loss. We added it up and figured if the boost was raised back to 17 pounds and the mixture leaned with a chip, we'd get to 583 hp.
So, with the fuel situation back in control, we simply went from the 3.25- to the 3.11-inch pulley and reburned the chip. We got back up to 17.3 pounds of boost at the peak and hit 565 hp with 16 psi of backpressure in the exhaust. Not quite what we had hoped for, but a step in the right direction. With 600 hp just in front of us and the day getting late, the 3.11 pulley was swapped for a 2.87-incher, for another 1.6-pound gain to 18.9 pounds of boost (that's discharging from the blower and measured before the intercooler--after the intercooler it was 17.9 pounds of boost). This ran great, but by then Brent was becoming gun-shy and lifted at 5,000 rpm just to take a look at the data lest we burn up something.
Everything looked fine, so all we had to do was get used to the idea of a more-or-less-stock Mustang making 600 hp. Jim said to heck with it--it was getting late by now--and he added a couple of gallons of leaded, 116-octane race fuel. Yes, the cats were still on the car, but Jim was adamant 2 gallons would not hurt them, and he was still big on detonation avoidance. You can't blame him.
Jim also ordered the 2.87-inch pulley off and the 2.66-inch pulley on. Brent climbed back in and held the loud pedal down for a full run--19.8 pounds of boost, 16.5 pounds of backpressure, 2 inches of vacuum on the naturally aspirated side of the blower, and a blazing 616.3 hp and 604.9 lb-ft of torque. That's 725 flywheel horsepower from 281 cubic inches if you use a reasonable 15-percent driveline loss figure. As it was, we had gained 252 hp from the Eaton baseline, and as Jim prognosticated after playing with his calculator, "With 25 pounds it will make 667 hp, guaranteed!"
We had to stop a few minutes and simply soak it in. At least momentarily sated in our horsepower quest, there were smiles all around, punctuated by the most amazing part of all--what hadn't been done. This staggering power was through the stock mass air meter, stock throttle body, stock engine, stock cast-iron exhaust manifolds, and stock cats. Six hundred horsepower at the tires. Unreal.
For a denouement, we asked Jim to remove the bazooka and run an air filter. Saying the 9-inch conical filter we tried earlier would be too small at 600 hp, Jim selected a 12-inch conical. Clamped right to the air meter, the 12-inch cone knocked the boost down to 19.4 pounds and the power to 604 hp.