Actual Rear Wheel: 275 hp/336 lb-ft
Estimated at Flywheel: 344 hp/420 lb- ft
Factory Rating: 335 hp/440 lb-ft
Mike Riemenschneider’s 1968 GT500KR convertible demonstrated what good tuning can do for a vintage musclecar. Mike, a retiree who tinkers with carburetor and distributor tuning in his home garage in Plymouth, Michigan, once held the AHRA Pure Stock record from 1969 to 1971 with a Mustang Cobra Jet hardtop, so he knows a thing or two about tuning the CJ engine. His advice works well with any Mustang musclecar: "Just keep everything in good condition." A K&N filter has replaced the stock air cleaner, but Mike says the biggest help came from replacing the original transverse muffler with a pair of turbo mufflers.
On the Paul's High Performance Dynojet, Mike's KR quickly pumped out our best numbers of the day, 275 rear-wheel hp and 336 lb-ft of CJ torque. Add another 25 percent to account for the C6 automatic and the blue convertible's 344 hp at the flywheel actually exceeds the factory's 335hp rating. The calculated flywheel torque at 420 lb-ft was just 20 lb-ft less than Ford's rating. Mike's KR also is equipped with nitrous oxide, so just for fun, Paul made a couple of Dynojet blasts with the juice. The Cobra Jet responded to the cooler charge with 376 hp and 475 lb-ft of torque.
Actual Rear Wheel: 214 hp/324 lb-ft
Estimated at Flywheel: 250 hp/379 lb- ft
Factory Rating: 375 hp/450 lb-ft
Ford rated the Boss 429 Mustang at 375 hp, the highest horsepower rating for any Ford. And with Saraphim Pallas’s 10,500-mile 1969 Boss 429 strapped to the Paul’s High Performance Dynojet, we were looking for some of the best numbers of the day. An all-original car, right down to the factory smog equipment and paint, Saraphim’s Boss 429, KK No. 1489, is arguably one of the best preserved of the breed.
On the first pull, the Boss 429 sounded strong up to 3,000 rpm; then, strangely, the power started falling off quickly. Timing checked out at just 9 degrees initial, so Paul upped it to 14 degrees. On the second pull, with the air cleaner off and Paul's assistant, Bryan Kaywood, peering into the engine compartment to confirm that the Holley's secondaries were opening, the problem persisted. Saraphim pulled an original tune-up kit, still in the vintage Motorcraft can, out the trunk and changed the points and plugs. He then removed the 29-year-old fuel filter. The last run netted just 214 hp at a low, low 3,800 rpm, showing that the car had plenty of potential if it could only rev. The final diagnosis was a bad fuel pump, but with no time or parts for a swap, we were stuck with our disappointing numbers.
For perspective sake and to make all Boss 429 owners feel somewhat better, the 214 hp from Saraphim’s Boss 429--at just 3,800 rpm--was more than the peak rear-wheel horsepower for a stock 1987-1993 5.0 Mustang, which typically peaked at just 202 hp. As another side-note, Saraphim’s Boss 429 generated big-block torque at the rear-wheel, over 300 lb-ft from 1,800 rpm to 3,700 rpm, with a peak of 324 lb-ft at 3,100 rpm. Because the car felt so strong at low rpm--with Saraphim admitting that he’d never pushed the 429 past 4,000 rpm--he didn’t realize there was a problem at higher engine speeds.
What is a Dynojet? Unlike an engine dyno, the Dynojet chassis dynamometer simulates highway conditions with the engine in the car and running through its drivetrain and exhaust. To obtain data, the Dynojet mounts a vehicle's drive tires on a pair of 48-inch drums. By calculating how fast the drums accelerate during a full-throttle assault from 2,000 rpm to the car's redline, the Dynojet delivers precise measurements to a personal computer loaded with Dynojet's software, which calculates rear-wheel horsepower and torque, along with other data. The design of the drum prevents tire slippage, resulting in precise, repeatable information.