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Ford Shelby GT500 Modifications - Shelby Bolt-On Bonanza, Part 2 - Tech
The GT500 Gets Off Its High Horse And Into High Gear With A Few More Bolt-Ons.
When last we left you, we had spent a day at the track, taking a bone-stock Shelby GT500 into the 11-second zone with a few basic bolt-ons. Giddy with success, we spent yet another day wrenching on Ford's new supercar in search of more bolt-on happiness.
Before we delve into Part 2's results, let's rehash our efforts from Part 1. We cruised the 800-mile-old GT500 to MM&FF's resident drag-strip, Old Bridge Township Raceway Park, in scenic New Jersey. Once there, we made three baseline runs that would set the stage for the ensuing test. Bone stock, the GT500 recorded a best elapsed time of 12.38 seconds at 115 mph. At the end of that day, thanks to M&H Racemaster drag radials, a JDM custom tune uploaded into the ECM via an SCT Flash tuner, and a MagnaFlow after-cat exhaust system, the Shelby slammed into the 11-second zone, motoring down to an 11.78 with the speedometer (and the track's clock) reading 117 mph. The vehicle weight with driver was 4,080 pounds.
Just like the average car guy or girl, we wanted more. The bolt-ons we installed trackside in Part 1 showed the potential of not only the Shelby itself, but of the parts, too. If those modifications could pick up two ticks on the speedometer and drop almost 6-tenths of a second off the baseline elapsed time, what could a few more simple bolt-ons do to the 5.4-liter beast? Could we get this blown S197 into the 10-second zone with basic stuff? We couldn't conclusively answer that question without trying, so you can see the road we are going to travel down in Part 2 of this bolt-on bonanza. Cinch down the belts and get ready to ride.
For the second round of bolt-ons, we kept the previously installed parts on the car. We once again drove the car to Raceway Park under its own power and ran it on high-test, 93-octane pump gas. During the week, however, the Shelby graced the JDM Engineering shop as this edition's modifications were installed with the car on the lift. While the following modifications can be performed in your driveway or garage, having the car on the lift with the proper tools makes the job much easier and less time consuming.
Geared to Perfection
It's common knowledge that the solid-axle 8.8-inch rear that comes with the Shelby houses 3.31 gears. While the factory ring-and-pinion will get the job done on the highway, when you want to start shedding elapsed time, the 3.31 cogs are not the best choice. So, changing the gears in the rear was the first item on the checklist. Keep in mind two things when changing the ring-and-pinion gears, however. First, make sure you choose the correct gear ratio. There are many different ratios available, so the proper one is dependent upon application, driving style, and the modifications you have already made and are planning to make. Generally, factory Mustangs don't have enough rear gear ratio to maximize quarter-mile acceleration. Ford under-gears them to enhance fuel economy and engine life and to reduce NVH. These modular engines like to rev, though, and adding gear (numerically) will allow the engine to rev quicker, thus enhancing acceleration. For example, switching from the 3.31s to a set of 3.55s or 3.73s will undoubtedly help a stock Shelby while keeping it driveable. Those 4.88s might not be the best choice for your stock GT500, though. If at all possible, avoid the bottom-of-the-page syndrome and consult a tech person when ordering your ring-and-pinion. With most new Stangs ('05-up), 4.10s or 4.30s make a great choice.
Secondly, when changing the gears in the rear, you are required to recalibrate the speedometer. "If you do not change the calibration, the computer will record a wheel torque error," says JDM Engineering's Jim D'Amore. "This error occurs when the speedometer reading doesn't match the actual vehicle speed. If this happens, the error will cause the car to go into fail-safe mode. To avoid this, you absolutely need to get into the tune and recalibrate the speedometer."
In an effort to better utilize the power we found in Part 1, we decided to swap out the stock 3.31s for a new set of 4.30s from Ford Racing Performance Parts. The idea was to not only get the car to 60-foot better with the numerically higher gears, but also to keep the engine in the powerband while going down the track.