Cars

Horsepower Hijinks: The Story Behind Factory Underrating of Muscle Car Power in the 1960s and 1970s

It’s been said that manufacturers underrated their advertised horsepower to keep insurance premiums at bay. I am somewhat skeptical of this idea, as insurers really didn’t get to sink their teeth into the “problem” until 1969. I do believe there’s much more evidence that manufacturers sometimes underrated horsepower to fit into a more competitive race class with the NHRA, which in turn refactored the rating in the interest of maintaining a level playing field.

What were the methods? I dunno, but in American Performance V-8 Specs: 1963-74, author Rick Rittenberg has an interesting chapter profiling individuals who wrote about the physics of vehicle acceleration. Have you ever heard of physicist Lloyd W. Taylor? His article, “The Laws of Motion Under Constant Power,” appeared in the Ohio Journal of Science in 1930. His equations for the laws of motion under constant force were unique because he included motion under constant power.

Nine years later, engineer Dale Kelly wrote an article for Automotive Industries. He had the benefit of a newfangled accelerometer to determine a vehicle’s power output. Then, in 1973, Kelly wrote a Society of Automotive Engineers article titled, “How Do You Characterize ‘Performance,’” deriving an equation to calculate an average reserve power-to-weight ratio.

Thanks to his access to empirical data, Chrysler engineer George M. Wallace developed several predictive performance equations. One formula corrected measurements affected by temperature, pressure, and humidity; another predicted e.t.’s based on weight and horsepower. You can read more on his formulas in The Smallblock Mopar Handbook.

Earles McCaul wrote an article in the Nov. 1973 issue of Hot Rod titled “Drag Strip Dyno.” His calculations determined the minimum power necessary to accelerate a vehicle based on the vehicle’s weight and e.t.

But the granddaddy of them all was Roger Huntington. In the Jan. 1958 issue of Rod & Custom, the Lansing-based automotive journalist mathematically described how a vehicle’s acceleration rate is directly related to its weight and engine power. He collected data from an accelerometer and developed empirical equations relating e.t.’s, terminal speed, and power/weight. (You may have seen his horsepower estimates in his book American Supercar, which covers his calculations in more detail).

Over time, we have had the benefit of learning from the folks who created these engines. On the Mustang 428 Cobra Jet Registry (428cobrajet.org) there’s an interview with Bill Barr, a retired Ford engineer who was responsible for developing the “428 GT 4V” program. The Cobra Jet’s rating of 335 hp actually came from as-installed conditions, “full production engine, automatic fuel and spark control, air cleaner, alternator (not charging), production exhaust manifolds, minimum exhaust back pressure, 200 mile or 15-hour break-in, power level corrected to 100° F @ 29.5-inch HG,” which is similar to net horsepower. For conditions with “maximum compression rated, maximum clearance, no air cleaner, exhaust headers, minimum exhaust back pressure, maximum carburetor air flow (no choke plate or throttle plates), high-hour engine with no alternator, manual control of fuel and spark: optimum (LBT) fuel and spark (MBT) advance, power level corrected to 60° F at sea level standard barometer,” Barr claimed the CJ put out 411 hp.

Over at General Motors, horsepower ratings had the additional hurdle of being tied to a mysterious edict (through 1969) that dictated midsize and compact models were to have no more than 1 hp per 10 pounds of weight (there is maybe one exception, which was Chevy’s 396/375). This is why some Pontiac Firebird engines featured a lower horsepower rating compared to identically equipped GTOs. In the case of the 1970 Buick 455 Stage 1, former Buick engineer Dennis Manner told MCR in our Sept. 2010 issue that “the reason we [rated the Stage 1 at 360 hp at 4,600] is the shipping weight of the car was 3,603 pounds, and we wanted to get it into a class that factored 10-to-1 pounds per hp.” Manner claimed it actually put out 372 hp at 5,200.

Oldsmobile collector Fred Mandrick has a January 30, 1970, report from the Project Engineering Experimental Laboratories that gives insight on engine power curves. Among the tables and graphs are dyno results presenting actual horsepower measurements. Here are some stats for Rocket 455s—take special note how the nonperformance engines were overrated.

Advertised HP Gross Power Production HP @ MBT* As-Installed HP
Standard 4-4-2 (auto) 365 @ 5,000 365 @ 4,800 317 @ 4,400
330 @ 5,000 (W25)**
288 @ 4,400297 @ 4,400 (W25)
Standard 4-4-2 (stick) 365 @ 5,000 366 @ 4,800 325 @ 5,000342 @ 5,000 (W25) 291 @ 4,800308 @ 5,000 (W25)
4-4-2 W30 (auto) 370 @ 5,200 370 @ 4,800 336 @ 5,000 (racing gas)
360 @ 5,000 (headers)
306 @ 5,000
4-4-2 W30 (stick) 370 @ 5,200 372 @ 4,800 335 @ 4,800381 @ 5,000 (headers) 322 @ 5,200
Standard 98 365 @ 4,600 336 @ 4,400
Delta 88 W33/Police 390 @ 5,000 360 @ 4,800
Standard Toronado 375 @ 4,600 346 @ 4,400
Toronado GT W34 400 @ 4,800 368 @ 4,800

* Maximum Brake Torque
** Oldsmobile code for ram air

So how accurate was Huntington? For the Cobra Jet, he estimated gross horsepower to be 410 at 5,600. He was a bit optimistic for both the Buick with 420 at 5,400 and the W30 at 440 at 5,600.

Plymouth rated the 1971 440 6BBL at 385 gross hp. Roger Huntington felt it was actually 430 at 5,600. A more accurate rating was 330 net.
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Thanks !

Thanks for sharing this, you are awesome !

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