Just when the Bullitt's rate of fire was enough to take out the competition, it would seize in the barrel and spit out head gaskets. That's what Bullitt 5094 owner Bob Watson got for spending a lot of money, time, and research on his 5.0L Two-Valve Mustang. Having this sort of thing happen once is something you can recover from, but having it happen several times could convince anyone to bite the bullet.
We couldn't let that happen, so to ensure that Bullitt 5094 would penetrate the 9-second quarter-mile zone reliably, we pulled the spent Two-Valve and swapped it out for a smaller, more potent 4.6L DOHC powerplant. Over the last few issues, we've documented this swap, which was far more involved than a normal Two-Valve to Four-Valve changeover due to the car's unique Bullitt components.
While we gave up a bit of...
While we gave up a bit of displacement (4.6L versus 5.0L) with the DOHC engine, the smaller-bore engine has proven to be dead reliable, and fast too. Check out the beautiful Kooks headers with Jet Hot's Extreme Sterling high-temperature coating.
This month we explain some of the driveline changes we made to the car, and get the project payoff both on the chassis dynamometer and at the dragstrip.
With the Four-Valve Bullitt up and running, we strapped it to the nearest Dynojet rollers and turned it loose. Said Dynojet belongs to HP Performance in Orange Park, Florida, which is where the 5.0L Two-Valve engine experienced its baseline dyno testing for our "Safety First" tech article (Dec. '07). While utilizing the Snow Performance water/methanol injection system, the car produced 602 rwhp and 531 rwtq with 26 degrees of total timing and 18 psi of boost. We opted not to use the water/meth injection on the 4.6 until we had it running just the way we wanted.
With the new 4.6L Four-Valve engine between the Bullitt's fenders, the Mustang made 693 rwhp and 551 rwtq. This was with only 16 degrees of timing and 15 psi of boost. The Four-Valve can probably handle more timing, but we didn't push it given the sizable gains we had already achieved. The drop in boost pressure shows how much the airflow through the engine has increased over the previous Two-Valve, as we were using the same pulley size on the Vortech T-Trim that we employed in our baseline tests. Also, where the mass airflow meter that we used on the Two-Valve was working in its operating range, the extra flow through the Four-Valve made it tap out in a hurry.
When the factory ECM decided...
When the factory ECM decided to make it's own transmission shift changes, we found ourselves with a roughed-up 4R70W transmission. Here Darrin Burch of BC Automotive takes the opportunity to install some new updates since he first put the transmission together.
With our newly acquired 90 extra horsepower, surely 9-second e.t.'s were only a couple of passes away, but that wasn't the case. The first track sessions only offered a handful of mid-to-low 10s-not too much of an improvement over the Two-Valve mill's effort. From the timeslips, we deduced that the car's 60-foot times were 0.3 slower than previous runs, and slicing that 0.3 off again would knock over a half-second off our elapsed times.
Though we were making more horsepower, we were lacking torque down low in the rpm range. The short/straight runner intake and smaller displacement needed a looser converter to let the engine rev higher at the hit than it did with the tight 2,500-rpm-stall speed converter we were using. With that in mind, we contacted Darrin Burch with BC Automotive. Burch had provided the 4R70W transmission that was in the car, and after filling him in on our recent track outing, we decided to replace the old converter with a new design from Circle D Specialties in Houston, Texas.
We called Circle D owner, Chris Sehorn, and told him we needed a lock-up converter for a 4R70W. We also told him that the car was making 700 at the wheels. These are pretty high numbers for a 4R70W installation, and Sehorn's first question was, "You're leaving lock-up open at the track, right?" Nope-we need wide-open throttle (WOT) lock-up capabilities. Surprisingly Sehorn answered, "No problem."
He took information from the dyno sheets and notes on how the car was performing at the track, and sent us a prototype converter to try out. All we had to do was promise to return it for inspection/rebuild after we abused it sufficiently. To date, we haven't felt inclined to remove it, as it has worked flawlessly. (Bob promises to send it back sometime, Chris.)
 The torque converter stall...  The torque converter stall speed was simply too low in the rpm range, and the smaller engine didn't have the torque to get the car moving fast enough. Circle D Specialties set us up with a 3,000-stall lock-up torque converter that has worked perfectly. Sixty-foot times have dropped 0.2-0.3. |  Here you can see the baseline...  Here you can see the baseline for the 5.0L Two-Valve versus the 4.6L Four-Valve engine. The smaller displacement combined with the short-runner intake took a hit in average torque, but there's no denying the 90hp gain at the top, which is what we were looking for since this car sees far more track time than street time. Should we add more manifold pressure, we would expect to see gains across the board. |  The new intercooler setup...  The new intercooler setup didn't make that big a difference on peak horsepower, but peak torque made a huge jump thanks to the reduction in inlet air temperature. |