Donald Farr
Former Editor, Mustang Monthly
January 1, 1998

You can read ’em and weep for a couple of reasons. Weep because Ford inflated the factory horsepower numbers or bawl your eyeballs out because we--meaning you and I and most of the participants in this power showdown--don’t keep our muscle Mustangs tuned to their factory potential. As you’ll see, our collection of muscle Mustangs didn’t uphold their reputations as some of the baddest musclecars ever built. With the advent of the Dynojet chassis dynamometer as a valuable tool for accurately measuring a vehicle’s rear-wheel horsepower and torque, we thought it would be both interesting and fun to strap down a group of factory high-performance Mustangs, all equipped to near-stock specifications, to see how the numbers compared both to each other and to the factory ratings. From the rear-wheel numbers, we would be able to extrapolate, using Dynojet’s percentages for drivetrain losses, the actual flywheel horsepower and torque measurements. For a manual transmission, Dynojet calculates a 17-percent loss. For a nonmodified automatic, it’s a whopping 25 percent.

Our sister magazine, Super Ford, has utilized the Dynojet at Paul&8217s High Performance in Jackson, Michigan, to test modified, late-model Mustangs, so we contacted proprietors Paul and Rhonda Svinicki about running our Muscle Mustang Showdown. They even helped us locate some of the cars with additional help from Detroit-area musclecar enthusiasts Austin Craig and George Huisman. For our test, we rounded up an all-star collection of Mustang musclecars: Boss 429 and 302; Shelby GT350, GT500, and GT500KR; 428 Cobra Jet Mach 1; and a pair of 289 High-Performance K-GTs--a four-speed and an automatic. In a perfect editorial world, we would have included a Boss 351 and a 429 Cobra Jet, but the search for stock versions in the Detroit area proved futile.

During the course of our Dynojet day at Paul’s High Performance, only one car, Mike Riemenschneider’s 1968-1/2 Shelby GT500KR, performed up to par with its factory horsepower rating. The others suffered from either poor maintenance--many because they are seldom driven--or from an incorrect carburetor, distributor, or ignition system. Unlike today’s modern cars that have a computer to compensate for tuning inconsistencies, vintage musclecar engines require constant tinkering and the right combination of parts to keep them in top running shape.

Interestingly, the torque numbers from our test came closer to the factory ratings than the horsepower figures. If you toss out the three Mustangs with tuning or parts problems--the four-speed K-GT, 428 CJ Mach 1, and Boss 429--the calculated flywheel torque averaged only 23 lb-ft less than the factory torque ratings. Horsepower, however, averaged 62 lb-ft less. Does this indicate that the factory provided fairly accurate torque ratings but fudged on the horsepower numbers during the highly competitive musclecar wars? Paul says he's seen similar results from other musclecars, like 4-4-2 Oldsmobiles and Stage One Buicks. Regardless, our muscle Mustangs, with the possible lone exception of Mike's GT500KR, could have benefited from proper tuning and correct equipment. According to Paul, "There was a lot more power in all those cars."

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1965 289 High-Performance Automatic

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Actual Rear Wheel: 141 hp/254 lb-ft

Estimated at Flywheel: 176 hp/318 lb-ft

Factory Rating: 271 hp/312 lb-ft

Jeff Burgy from White Lake, Michigan, has owned his white Mustang K-GT convertible for three years. By Jeff's best estimate, the Mustang was completely restored, including an engine rebuild to factory specifications by a previous owner about 15 years ago. Jeff figures his 289 High-Performance engine has about30,000 miles on it since the rebuild.

Ford rated the solid-lifter 289 High-Performance at 271 hp at the flywheel, but on the Dynojet, Jeff's K-GT generated only 141 hp at the rear wheels. Of course, the automatic soaks up a lot of power, at least 25 percent by Dynojet's calculations, which gives us a 176hp reading at the flywheel. Interestingly enough, Ford's rating came at 6,000 rpm, but Jeff's convertible hit its peak at only 4,500 rpm, then started losing horsepower as the revs climbed. Jeff's convertible produced more net torque, based on the 25 percent calculation, than the factory rating.

To put it into modern perspective, the 141 rear-wheel horsepower from Jeff’s automatic K-GT is less than 20 hp off from automatic-equipped 1996-1998 Mustang GTs’ 4.6L V-8s. With some good tuning to gain horsepower in the higher rpm range, Jeff’s K-GT would no doubt outperform today’s new Mustang GTs.

1965 Shelby GT350 Four-Speed

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Actual Rear Wheel: 202 hp/237 lb-ft

Estimated at Flywheel: 236 hp/277 lb-ft

Factory Rating: 306 hp/329 lb-ft

Rebuilt and even rebodied from a "basketcase" found in the Florida Everglades, Clint Conklin's early-production No. 5R038 Shelby is used for both the street and open track, so it was crisply tuned from a recent outing at Road America. Clint's father, Fred, found and restored the car for 18-year-old Clint, who will get full use of it when he graduates from high school next spring. The Cobra version of the 289 High-Performance engine was recently rebuilt to factory specs with the exception of a smaller 600-cfm Autolite 4100 four-barrel in place of the original 715-cfm Holley. In its Road America open-track form, the GT350 was equipped with Dr. Gas X-pipes and open exhaust.

Although plagued at the start of our testing by a misfire, which was eventually diagnosed as a strange electrical glitch in the starter solenoid, Clint's GT350 ran strong on the Dynojet, even with the small four-barrel, producing 202 rear-wheel hp at 4,800 rpm. Factoring in a 17-percent power loss through the four-speed drivetrain, flywheel horsepower was estimated at 236, still 70 hp off from Shelby's 306hp rating.

1966 289 High-Performance Four-Speed

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Actual Rear Wheel: 144 hp/238 lb-ft

Estimated at Flywheel: 168 hp/278 lb-ft

Factory Rating: 271 hp/312 lb-ft

We felt Eugenia Hardaway’s four-speed 289 High-Performance K-GT would be a good comparison to Jeff’s automatic. But unfortunately, Eugenia’s hardtop--which she bought just a few months earlier--didn’t want to make power above 4,000 rpm. With smoke pouring out the twin GT exhaust trumpets, Paul first ruled out blow-by, then traced the potential problem to the carburetor. Carbmeister Mike Riemenschneider, on standby with his GT500KR, identified the four-barrel as an Autolite from a 1967 Thunderbird. In addition to being smaller than the stock 289 High-Performance Autolite, the carb was running way too rich, a condition that explained the black smoke and the car’s lack of horsepower at the top end. Although Eugenia’s four-speed K-GT produced a tad more rear-wheel horsepower than Jeff’s automatic K-GT, the manual soaked up less power, giving us only 168 hp at the flywheel compared to 176 net hp for the automatic.

1967 Shelby GT500 Four-Speed

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Actual Rear Wheel: 240 hp/354 lb-ft

Estimated at Flywheel: 281 hp/415 lb- ft

Factory Rating: 355 hp/420 lb-ft

In usual Shelby fashion, the 1967 GT500 was a case of Carroll Shelby one-upmanship over the Mustang. While the largest displacement engine available in the Mustang was the 390, Shelby transplanted the passenger car 428 into his GT500 and topped it with a pair of 600-cfm Holleys. From Shelby American, the GT500 was rated at 355 net hp.

For our Muscle Mustang Showdown, George Huisman, who operates Classic Design Concepts in Walled Lake, Michigan, trailered Cary Silver’s 1967 GT500 to Paul’s High Performance. "I told Cary I wanted to borrow his car," George explained, "and he said, ’Go ahead!’" As a former owner of the white-with-blue-stripes Shelby, George knew the car ran well a few years ago, but realized it spends more time in storage these days than on the road. "It probably hasn’t been driven 200 miles in the last 10 years," George said.

No problem. Cary's GT500 seemingly shook ground around the Dynojet on its way to a 240hp rear-wheel reading, which calculated to 281 hp at the flywheel, some 74 hp shy of Shelby's 355hp rating. The calculated flywheel torque, however, was amazingly close: 415 lb-ft at 2,800 rpm compared to Shelby's 420 lb-ft rating. Cary's GT500 was particularly impressive, considering George merely rolled the car off the trailer and onto the Dynojet rollers without touching a thing.

1968 GT500KR Automatic

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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.

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1969 Boss 429 Four-Speed

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.

1969 Mach 1 428 CJ Automatic

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Actual Rear Wheel: 213 hp/266 lb-ft

Estimated at Flywheel: 266 hp/332 lb-ft

Factory Rating: 335 hp/440 lb-ft

When Chris Sackett’s 1969 Mach 1 rolled onto the Dynojet rollers, we knew we were stepping up to some serious big-block horsepower and torque potential. The Mach 1’s 428 Cobra Jet ram-air rating of 335 net hp has led the street-wise to accuse Ford of sandbagging the numbers for drag racing purposes. We figured we’d find out.

Chris, from Bath, Michigan, purchased the blue Mach 1 a year ago, supposedly with about 10,000 miles on a fresh engine rebuild. During the first three Dynojet pulls, the exhaust dumped a lot of dark smoke, indicating an overly rich mixture. Removing the air cleaner for a look revealed an Autolite service replacement four-barrel, a 480-cfm 4300 version, instead of the correct 715-cfm Holley. According to Paul, the secondaries also were opening too soon, well before the engine was ready for the additional fuel, leading to the rich conditions and only 192 rear-wheel hp. Making a pull without the shaker-scooped air cleaner helped, providing the best run at 213 hp. Factoring a 25-percent power loss through the C6 automatic, the engine was putting out 266 hp at the flywheel, way off Ford's 335 rating.

Paul noted an incorrect distributor, noticed during the carburetor inspection, may also have been a factor in the car's lackluster performance. Although it looked factory, Paul explained that non-CJ Ford distributor probably had an incorrect advance curve for optimum Cobra Jet performance.

1970 Boss 302 Four-Speed

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Actual Rear Wheel: 179 hp/209 lb-ft

Estimated at Flywheel: 209 hp/245 lb-ft

Factory Rating: 290 hp/290 lb-ft

We expected a better showing from John Niffin’s 1970 Boss 302. A mostly original, 45,000-mile car, the Medium Lime Boss has belonged to John since 1976. The original engine expired at 18,000 miles--the result of the Boss 302’s piston skirt problem, no doubt--and was rebuilt to factory specs. However, John, who lives in White Lake, Michigan, added a Motorcraft electronic ignition to eliminate the hassle of keeping the factory dual points adjusted.

The Boss sounded stout on the Dynojet, but the first three pulls generated a best of just 169 rear-wheel horsepower. A quick inspection under the hood revealed that the manual choke was partially closed and the initial timing was set at 19 degrees, so Paul quickly shoved in the choke handle and reset the timing to a more realistic 14 degrees. The simple changes resulted in a 10hp jump, up to 179, at an uncharacteristically low 4,700 rpm on an engine recognized for its high-rpm power.

Afterward, Paul noted that electronic ignitions advance the timing slower than point ignitions, and they also dial in up to 45 degrees of advance at higher rpm, which is too much for vintage cars that require only 36-38 degrees. Paul felt that a simple ignition change, back to the original dual-point distributor, would restore the factory performance in John's Boss 302. As a side-note comparison to the Windsor-head K-GTs, which developed their torque peaks between 1,800 and 2,300 rpm, the Boss 302 didn't reach its peak torque, as expected with the large-valve, Cleveland-style heads, until almost 4,000 rpm.