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Bullitt 5094's 9-Second Ride - Power Struggle Part 3
The Struggle Over Street-Worthy 9-Second Power Is Over, And We Won.
Armed with a better-suited converter, we were ready to get back to the track-except for one small issue. Our subject Bullitt has been using a stock Mach 1 automatic ECM to handle all of the systems on the car, including automatic-transmission shift points during passes. At this performance level, we're pushing the stock EEC-V computer capabilities, and it's taken some research and testing to get it to work as well as it has. As much as we tried to prevent a WOT shift to Overdrive during the car's 130-mph passes, the EEC was overriding our best lock-out efforts and calling for a 3-4 shift just before the traps. While we do lock up the converter going down the track, we don't let the car go into Overdrive.
A WOT Overdrive shift is a very bad thing for a 4R70W passing 700 hp, and the computer snafu necessitated a trip from Florida to the BC Automotive digs in Indianapolis, Indiana, for a repair/rebuild/upgrade. Since the initial Bullitt 4R70W upgrade, Burch has come up with more innovative tricks to strengthen the internal workings of the transmission, including the intermediate clutch assembly. According to Burch, the intermediate clutch pack is the Achilles' heel of the 4R-series transmissions. With the gearbox apart, Burch confirmed that the OD drum was toast and replaced that as well. We also managed to come up with a solution to prevent a 3-4 WOT shift at the track.
With the driveline back together, it was time for some testing and tuning. With the old converter, Watson got the best 60-foot times by flashing the converter from a dead idle off the line. The Circle D converter, with its 3,000-rpm-stall speed, worked best by powerbraking the engine to 2,000-2,500 rpm and then punching it. With this technique, the car was pulling mid-to-low 1.4-second 60-foot launches that were dead straight, thanks to a recent fortification of the 8.8 axle. The rebuilt 8.8 included a full Moser spool that spins the same 4.10:1 cogs as before, as well as 33-spline axles. Jim Britts at Southern Speed in Orange Park, Florida, installed the 9-inch-style ends to eliminate the C-Clips.
Once back at the track, the 9.98 was produced with a much improved 60-foot time, but the 133 mph was unexpectedly low, actually 5 mph down from an earlier pass that day. We discovered we had some belt slippage problems, signified by a haze of black dust covering the intake. At least we finally had a single-digit timeslip.
With a new belt in place and some additional changes in the transmission shift/lockup schedule, we bested the 9.98 with a 9.81 at 140.7 mph (1.47 60-foot), and backed that up on the next run with a 9.87 at 140.4 mph. Both passes were achieved with only 16 degrees of timing and 15 psi of boost. We added two degrees of timing on the next pass: The car left like a scalded dog and then shook the tires hard. After lifting out of the throttle, the blower belt promptly shredded itself. But even that doesn't diminish the fact that we finally accomplished our goal of making Bullitt 5094 a solid 9-second ride. Just as important, the new setup had completed 30-plus dyno pulls and approximately 25 quarter-mile passes with no sign of head-gasket problems. This is a huge improvement over the old Two-Valve. Our recollection is the car would make a maximum of 10 passes or dyno pulls before we would see water in the catch can. It's nice when great advice and hard work turns into positive results.
If you're interested in more detailed information on Bullitt 5094, Bob Watson's personal accounts of the build, or would like to see videos of some of the track passes, visit www.01bullitt.com. The site also includes a forum for the original '01 Bullitt Edition Mustang owners to discuss these rare cars.