Before each pass with the...
Before each pass with the drag radials, Smith made sure to do a nice, smoky burnout to not only break in the tires, but clean them off and get them nice and sticky. This aided in securing maximum traction on the starting line, and it dropped our e.t. numbers by 0.017 second over the baseline run. While this may not seem like a big improvement, the 12.36 we ran on the drag radials was in worsening air. Additionally, Smith found it hard to do burnouts because the rear end wanted to hop quite badly.
The first bolt-on modification we tried was a fairly basic one as we swapped out the stock rear wheels and tires for a set of drag radials. The Shelby rides from the Flat Rock factory on P285/40ZR18 Goodyear F1 Supercar tires wrapped around 18x9.5-inch machined aluminum wheels. While the F1 Supercars are extraordinary all-around tires, we knew that more could be gained by switching to a set of drag racing- specific shoes.
So off came the factory rolling stock and on went a set of M&H Tires Racemaster drag radials. The new tires were mounted on a set of Cobra wheels that would fit the Shelby. The Racemasters are DOT-approved and feature an exclusive, sticky tread rubber and lightweight design. This makes them perfect for launching a car hard on the starting line and keeping the rear end planted. We wanted to have a decent-sized footprint for the car to leave on, so JDM ordered 325/45/R17 tires. The tire diameter itself was taller and the width of the Racemasters are bigger than the stock meats. After we bolted on the tires, Shaun Lacko of JDM torqued down the new rims and set the tire pressure to 16 psi. It was time to see what the drag radials could do for the Shelby.
As soon as the clutch was dropped, we knew there would be an improvement in performance. After recording a 1.949-second 60-foot time, the Shelby motored down track, stopping the timers in 12.36 seconds. The improved traction provided by the Racemasters allowed the Shelby to short-time better, resulting in the best elapsed time of the day up to that point (keep in mind the temperature was up about 15 degrees over the 12.38 with the stock tires). If we throw out the first run and look at the second and third runs, which were extremely close to one another, the tires lowered the elapsed time by about 0.150 seconds, a con-siderable amount.
Of interest with the tire change, however, was the lowered trap speed of the car. After running a best speed of 115.38, which was also recorded on the first pass of the day, the speed on the pass made with the car shod in the drag radials was down to 113.49. Even the speed marks on the 12.5-second passes were nearly a full tick higher. So why did the elapsed time improve while the speed dropped? The answer is as simple as looking at the numbers on the sidewall of the tire. With the taller tire, the effective gear ratio was changed slightly, and that, compounded with slightly worse air when compared to the earlier run, caused the speed to drop. Overall, however, the drag radials gave the Shelby the ability to hook harder on the starting line, thus improving our elapsed times.
Next on our list of bolt-on...
Next on our list of bolt-on modifications was a JDM custom tune. Thanks to tuning wizard Jim D'Amore, the Shelby showed a remarkable performance improvement. By changing around things such as fuel, timing, and rev limiter, the GT500 improved its e.t. by almost half a second.
Most factory fuel-injected cars, especially performance models such as the Mustang in general and the Shelby GT500 in particular, come with a conservative tune. This allows for a safe margin if you get a bad tank of gas, or if you decide to tow a trailer through the Arizona desert in mid-summer. Often, adding a degree or two of timing and trimming the fuel curve is worth some horsepower at the wheels. [Note: Modifying the factory tune almost always requires that 91-93 octane be used-Ed.] And while the 500hp mark associated with the GT500 is stout, we were certain there was still quite a bit of power left in the ECM.
While the car was cooling down after the fourth run-the one in which we ran our drag radial tire experiment-Jim D'Amore broke out the laptop and a blank SCT XCalibrator 2 tuner. He developed a custom tune that started off by raising the rev limiter, which allowed us to raise the shift points. Before, Smitty was sliding the shifter at 6,000 rpm. Now, he'd be rowing at a more aggressive 6,500 rpm. D'Amore then went into the fuel and timing maps, making some moves there as well.
"Within this area, I changed numerous things," he says. "The air/fuel ratio on the run before (run four) at 5,000 rpm was 12.7:1, which is on the lean side. To combat that, I changed the air/fuel ratio to 12.0:1 across the board from idle to 6,500. I also added a bit of timing to the timing curve, and have the coolant fans and intercooler pump turning on at a lower temperature."
With the new tune loaded, the Shelby was fired up and sent down the track for run number five. Once again launching at 4,500, the car 60-footed much better than the run before, recording a 1.834-second short time. Still equipped with the drag radials, the tune unlocked some power that showed up at the finish line as the Shelby broke into the 11-second zone with an 11.901-second, 116.88-mph shot. We then backed up that run with an 11.932/116.64 effort. With the air comparable to what it was when we tested the sticky tires alone, the slightly modified ECM showed an improvement of nearly one-half second over the already sizeable gain we saw with the tire swap. Talk about huge!
By the time we got to the...
By the time we got to the MagnaFlow after-cat exhaust installation, we were getting a bit hungry. Before we grabbed a bite to eat, however, we jacked up the rear end of the Shelby. Placing the jack directly under the centersection of the rear, we lifted the car up in the air, let it settle on a pair of jackstands, ate some pizza, and let the car chill out while we discussed how much the system would be worth.
One of the keys to the MagnaFlow...
One of the keys to the MagnaFlow system's success is the high-flow mufflers that come with the stainless steel, mandrel bent system. Here Lacko pulls out a stock muffler, eventually replacing it with the MagnaFlow piece. The MagnaFlow has the same diameter tubing as the stock system, but its design offers improved sound and flow.
With the after-cat exhaust...
With the after-cat exhaust system on the car, the Shelby ripped off an 11.78/117-mph best. The 60-foot times also improved thanks to the low-end torque increase the exhaust gave us. Additionally, the car sounded mean going down the track at high rpm.