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’s 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."