How could muscle cars, the most powerful affordable cars the world had seen, lose their oomph so quickly? What caused them to disappear? I'm not a private eye, not remotely like one, but this was one case I had to solve.

It happened back in the 70's, but the evidence was still there. It was the case of the disappearing horsepower and this is what happened.

In the era of the muscle car power was everything. It didn't matter what it was, sports car, family car, pickup; it had the biggest V-8 possible stuffed under the hood. Cubic inches were king and advertised power was astronomical. These cars could kick sand in the windscreens of anything else on the road.

But then horsepower seemed to disappear overnight!

Take my favourite muscle car, the Ford Mustang. The macho models had V-8s, though meeker models came with an inline six. The biggest six had 200 cubic inches and 155 hp in 1969/70.

What did the V-8s punch out? The most powerful 351 gave 300 hp in 1970 and the 427 gave a massive 390 hp in 1968. But by 1973 the most powerful Mustang had a 351 V-8 with just 156 hp. Almost half what it had in 1970, and only one horsepower more than the 200 cu in six of 1970! As for the Mustang II of 1974, we won't even go there.

The story was similar with the other manufacturers. What was going on? It just didn't add up. Could I trust the figures?

My detective mentor, Agatha Christie, taught me that when you're solving a case you can't trust anyone. Murderers do lie. In this case it wasn't murder though it was the death of the muscle car, and it wasn't so much of an outright lie as not telling the whole truth. And outside forces were at play.

I had to dig deeper. I had to find the facts. Why would horsepower virtually halve?

It turned out there were a few reasons. Salesmanship was one. Horsepower was everything so why not measure it in a salesman friendly way? Gross SAE horsepower was used. Power was measured at the flywheel with no power-hungry accessories attached. Only the bare essentials were used.

In 1972 SAE Net measurements were phased in. Power was still measured at the flywheel but all the accessories were installed including the full exhaust system, emission controls, all pumps and the alternator. SAE Net can't be compared exactly to SAE Gross because there are just too many variations in measuring, but it is down around 80%. So power ratings dropped. In 1973 horsepower ratings went down again as power sapping emission controls were tightened.

Gross SAE horsepower had pushed the listed power up. So did the advertised horsepower some car companies used. What's wrong with a little rounding up of the numbers for the brochure? Surely that would help sales too.

All this horsepower galloping around got noticed and not just by young guys.

Safety legislators noticed, and so did insurance companies who started charging more for insurance. The word on the street is that in 1967 a young guy under 25 with a clean driving record would have paid $700 a year for GTO coverage. Ouch! Some car companies lowered their advertised horsepower ratings.

Muscles peaked in 1970, and by 1971 they were starting to get flabbier. Engines were being detuned and within another year bigger engines were being dropped.

In 1973 many muscle cars were a shadow of their former selves. And they were finished off by the oil crisis of late '73. Long lines at gas stations and soaring prices were a real shock, and so was a 55 mph national speed limit. Gas guzzlers were irresponsible, expensive and unwanted, it didn't matter how much fun they were.

So there you have it. I now knew what had happened to all that brute power. Some exaggeration had pushed listed horsepower up. A fairer, more accurate measuring system brought it down. Emission controls brought it down more, and soaring insurance costs made ground-thumping power too expensive to own. The oil crisis finished the muscle car off. This case was solved.

About The Author: Warren Newson is editor of the-classic-sports-cars.com , where you'll find sports car articles, pictures and collectibles.