January 26, 2023


Still car-crazy after all these years

Semi-Hemi "Rat" Motor

Semi-Hemi "Rat" Motor

The Porcupine, Semi-Hemi, Mystery Rat Motor

The big block Chevrolet V8 is known by a variety of names, but for most Chevrolet racers and hot rodders, it will always be simply the rat motor. How did such a popular engine come to be labeled as a rat?
The Chevrolet Big Block V-8 : AKA the Rat Motor
The Chevrolet Big Block V-8 is affectionately known as the rat motor to Chevy high performance fans.

One of the displays at the 1968 Detroit Autorama was this jewel-like big-block Chevy, AKA the Rat Motor. How did the big Chevy come to be known as a rat?

To a hot rodder or racer any Chevrolet big block V-8 is a rat motor. The slang term goes back over four decades and few enthusiasts will use any other term to identify the big Chevy power-plant.

Generally, the expression rat is less than complimentary. From James Cagney’s famous line “…that dirty, double-crossin’ rat” to the much-maligned rat finks of the fifties, calling someone or something a rat is the ultimate insult. When something is old, used up and falling apart, it is “ratty.”

The Chevrolet big block is a marvel of engineering and is the most powerful automotive engine ever mass-produced by General Motors. How did such a masterpiece of automotive design come to known-by the unsavory term of rat?

In the early 1960s, long before the first rat motor was introduced, a David and Goliath confrontation was taking place on the nation’s dragstrips. The Chrysler Hemi had become the dominate force in the Top Fuel and Top Gas categories, gradually replacing the Cadillac, Lincoln on Oldsmobile engines that previously controlled these categories.

The early Chrysler Hemi: Elephant Motor

While the Chrysler Hemi produced gobs of power, it was also physically larger and heavier than other V8 engines. The huge V-8 quickly earned the soubriquet of Elephant motor due to it’s size. An elephant motor with a GMC supercharger looked amazingly huge when installed in the typical tube frame sling-shot dragster chassis. It was enormous.

Although the big elephants were the de-facto standard in Top Fuel, there was another engine that could hold its own against the hemi. Many chose to run the legendary small-block Chevy in their dragsters. As often as not, they didn’t bother with a supercharger, preferring a simple set of injectors.

On paper it was an unequal contest. The blown hemi was far more powerful than the miniscule Chevy, and it seemed that the Chevy didn’t have prayer against the Chrysler in a quarter-mile sprint.

The Chevy, however, did have an edge. Tire-technology, or rather the lack of tire technology was on the Chevy’s side.

While the big elephant motors developed a huge amount of horsepower, the tires of the day were unequal to the task of delivering that power to the asphalt. As a result, much of that power went up in smoke, as the big Chrysler rails burned the tires almost the full length of the drags-tip.

The Chevrolet powered rails could burn their tires as well, but their lower power allowed the tires to hook up and gain traction while the big hemis were still spinning their wheels. The combination less wheelspin, lighter overall weight and higher RPMs allowed the little Chevy powered cars to be competitive and often best the more powerful hemis.

Enter the Chevy Mouse Motor

While the hemi teams weren’t actually afraid of the little Chevy dragsters, the small blocks tended to make them more than a little nervous. There was no shame for a blown hemi machine to lose to another blown hemi, but it was positively embarrassing to lose to the much smaller Chevy.

Some wag remembered the fable that the only creatures elephants are afraid of are mice, and started referring to the small Chevy as the mouse motor. Thus the elephant and mouse motors vied for racing honors on dragstrips throughout the country.

The mouse motor’s role as elephant-killer was short lived. Tire and clutch engineers continued to expand their technology, and the big elephants were eventually able to deliver more power to the track. By the early sixties, the mouse motor could no longer plague the big Chryslers in the Top Eliminator ranks. The small block Chevy was still hugely popular in the altered, gas coupe and modified classes. Even though it seldom ran against the big elephant motors any longer, the engine was still known as the mouse motor.

During that period, Chevrolet and General Motors were following a strict non-involvement policy toward auto racing. It is well known that many GM engineers didn’t agree with this policy, and flaunted the official stance by helping various race teams without management’s knowledge or approval.

The Mystery Engine

Legend has it that during the early sixties, certain Chevy drivers showed up at the race track with a strange new engine under the hood of their race cars. It wasn’t a mouse motor, and it wasn’t the big 409 V-8 that the Beach Boys immortalized in song. It was a mysterious, all new Chevy V-8.

The mystery only lasted a short while, as in 1965, Chevrolet formally announced a new big block V8. Available first as a 396 cubic inch version, the engine eventually evolved to displacements of 402, 427 and 454 CI. Eventually, enthusiasts could buy a 572 cubic inch crate motor based on the same engine block.

Even after the engine was introduced, many people still continued to refer to it as the mystery engine. When the valve covers were removed, hot rodders noticed that, unlike the mouse motor which had the valve stems neatly lined up in a row, the valves in the new engine were canted at various angles. Because of this, the engine also gained the nick-name of the porcupine V8. When engine builders started modifying the engine for racing, they noted that the cylinder heads had a somewhat hemispherical design. The engine wasn’t a true hemi, but it wasn’t exactly a wedge head, either. Thus the engine earned another name: the semi-hemi.

Look at photos of Chevrolet race cars from the era, and you find many examples of cars with the terms Mystery Engine or Semi Hemi painted on the hood.

The Rat Motor, big brother to the Mouse

For most rodders and racers, however, there was only one legitimate moniker for the new engine. If the small Chevy V-8 was a mouse motor, it’s bigger, more powerful, fiercer sibling could only be a RAT motor.

The name has stuck. Today, only old school hot rodders and nostalgia buffs still employ the terms mouse motor and elephant motor. The name rat motor, however, is still universally used to describe the big block Chevy V8. Far from being a derogatory term, the expression rat motor is much-loved by Chevy fans and is used as a title of respect even by those who build and race other brands.

And that is how the Chevy V-8 became known as a rat motor.

Show your love for the big block Chevy rat motor with a Tee Shirt!:

Click Here!

Tom Bonner