This look back article discusses the difference in historic and modern horsepower ratings, and how they differ. Were the “good old days” of muscle cars really the peak of horsepower production? Read on to find out!

Originally published October 15, 2010

Yesterday, I was reading an article in a Mustang Monthly about a 1972 Mach I and it raised the question that I have always asked about factory horsepower ratings.

The older Mustangs (and muscle cars in general) were advertised with, until recently, stratospheric and unsurpassed horsepower ratings, but why are the new cars equal to or faster than their higher rated HP predecessors?

Fast question, what was the fastest reported 1/4 mile time for a factory stock vintage Mustang?

A quick survey of google reports the 1971 BOSS 351 ran a 13.8, which is the quickest recorded of any of the ’73 and older cars. Sorry no vintage numbers for the BOSS 429 and other very heavy hitters, but regardless the 1971 BOSS 351 was advertised at 330 BHP @ 5400 rpm. This car in street trim was reported to weigh about 3220 lbs.

Just another quick pull from the same site takes the 2005 Mustang GT through the traps at 13.5, a full 3 tenths faster than the BOSS. We all know that the S197 is a heavier platform than even the “big” 71-73 years, and I wouldn’t be far off the mark with a 300lb difference. Further the 05 car was geared with 3.55, taller 29″ tires and a 5 speed transmission while the BOSS powered a close ratio 4 speed and 4.10 – 4.30 gears with a short 26″ tire.

So a heavier car, with less horsepower runs faster than the vintage unit, cry foul? Think not.

Something between 1971 and 2005 had certainly changed in Factory horsepower reporting ratings, but anyone who as been in the performance game for awhile knows that this is no surprise. Enter the term SAE, the real focus of this article. The Society of Automotive Engineers (SAE) is; (from wikipedia)

“SAE’s original purpose was to promote the use of standards in the nascent automobile industry (initially in the United States) and to promote the better interchange of ideas and expertise, in a similar manner to many other technical societies.”

One area where a SAE exercised it’s influence was a standardization of engine testing and reporting of results, this was done for very good reason. It would not take much for an engine producer to “skew” their horsepower results for interested purposes. Think about it, even today, Horsepower does sell cars, and if there were no standards of testing, then any marketing people worth employing would certainly report as much as possible. Not convinced that HP matters, take a look at the arguments of a Mustang Dyno vs a Dynojet online, people will always slap down the higher number reported and scoff at a lower one. After seeing the nonsense from that argument, imagine in a modern time if manufacturers were offering their own “certified” in house horsepower testing standards?

It would be difficult to make an apples to apples comparison between engines to say the least.

In the days of pre-SAE engine testing things were a bit muddy, read these wiki excerpts below:

“Brake horsepower
Brake horsepower (abbreviated bhp) is the measure of an engine’s horsepower without the loss in power caused by the gearbox, differential, transfer case, or other drivetrain related losses. Brake horsepower is a standard measurement used by manufacturers to represent the horsepower developed at the crankshaft of an engine with it’s “production” accessories. This is opposed to the older “gross” horsepower measurement taken with no drive accessories, as was the norm in the 1960’s and 1970’s. “Brake” refers to a device which was used to load an engine and hold it at a desired RPM. During testing, the output torque and rotational speed were measured to determine the “brake horsepower”. Horsepower was originally measured and calculated by use of the indicator (a James Watt invention of the late 18th century), and later by means of a De Prony brake connected to the engine’s output shaft. More recently, an engine dynamometer is used instead of a De Prony brake. The output delivered to the driving wheels is less than that obtainable at the engine’s crankshaft.”

Moving on wards we have the first example of SAE Horsepower:

“SAE gross horsepower
Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower (bhp), frequently referred to as SAE gross horsepower, because it was measured in accord with the protocols defined in SAE standards J245 and J1995. As with other brake horsepower test protocols, SAE gross hp was measured using a stock test engine, generally running with few belt-driven accessories and sometimes fitted with long tube (test headers) in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature for testing were relatively idealistic.”

For 1972 we seen this:

“SAE net horsepower
In the United States the term “bhp” fell into disuse in 1971-72, as automakers began to quote power in terms of SAE net horsepower in accord with SAE standard J1349. Like SAE gross and other brake horsepower protocols, SAE Net hp is measured at the engine’s crankshaft, and so does not account for transmission losses. However, the SAE net hp testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold. The change to net hp effectively deflated power ratings to assuage the auto insurance industry and environmental and safety lobbies.”

Comparisons between SAE Gross and SAE Net Horsepower have often draw my original question a bit further. If the testing became more stringent and a subsequent reduction in numbers were reported, on average how much difference did this make? Reading the Mustang monthly article about the 72 Mach 1 cleared this up somewhat, it said the featured 72 Mach 1 came with an R-code 351 which was virtually identical to the last year’s BOSS. The non-Boss 351 was rated at 275 bhp versus the BOSS 351’s 330 which they claimed was mainly attributable to the new for 72 SAE standard.

Pulling out my abacus I can see that we have to account for 55 SAE Gross horsepower between the two ratings. Based from that change of 16.7% a rough scalar could be drawn. 16.7% is significant, that makes a 500 Horsepower car rated in 1971 only 420 by 1972 standards, don’t put away the calculators yet because things changed again.

“SAE certified horsepower
In 2005, the SAE introduced a new test protocol for engine horsepower and torque. The new protocol eliminates some of the flexibility in power measurement, and requires an independent observer present when engines are measured. The test is voluntary, but engines completing it can be advertised as “SAE-certified”.

Many manufacturers began switching to the new rating immediately, with multi-directional results; the rated output of Cadillac’s supercharged Northstar V8 jumped from 440 horsepower to 469 horsepower under the new tests, while the rating for Toyota’s Camry 3.0 L 1MZ-FE V6 fell from 210 horsepower to 190 horsepower. The first engine certified under the new program was the 7.0 L LS7 used in the 2006 Chevrolet Corvette Z06. Certified power rose slightly from 500 horsepower to 505 horsepower.”

For note, the 2005 and forward Mustang was also tested to this standard, and it’s SAE Certified 300 Horsepower is a great number, so what happens when we try to guess to pre 1971 horsepower rating for a new engine?

With the above changes I believe the GM gains are possibly attributed to typical under-rating of power, that generally occurs now to be on the safe side of marketing. For fun let’s adopt the Toyota Camry’s loss as our rounding figure.

190 vs 210 accounts for a 9.5% drop in rated HP so we will apply that to the Mustang’s 300 SAE to get the estimated SAE Net number

300 X 9.5% = 28.5 HP

328.5 SAE Net HP is our estimated 1972 test standard Number

Now let’s apply the same theory to SAE Net to step back to SAE Gross

328.5 X 16.7% = 54.9 HP

383.4 is our theoretical pre-72 factory HP rating for the 4.6L 3V engine.

Looking back we recall the;

1971 BOSS 351 rated at 330 HP ran a 13.8
2005 Mustang GT rated at (383.4) HP ran a 13.5

Interesting stuff eh?

I cannot call this bit of maths anything more than a exercise, but the numbers are close~ish enough for discussion.

Hope you had fun and learned something along the way, but is this the end of the discussion? Certainly not, another time we’ll cover the real life difference between Peak HP and Average HP.