In 1965, Chevrolet began offering the big block Chevy Mark IV—better known as the Rat.

This wasn’t Chevy’s initial go-around with big block engines as its Gen. I W-series big blocks were first offered in 1958. However, the Mark IV engines (originally called Gen. II) improved upon the design of the original W-series engines and helped escalate the battle of the Detroit Big Three into an all-out war in the 1960s and 70s.

Like the small block “Mouse” introduced before it, the “Rat” came in a variety of configurations and horsepower levels. Likewise, it was an option in a lot of different vehicles, including some of the most iconic American muscle cars and performance vehicles of all time.

When a Chevrolet enthusiast says “big block,” this is the engine they’re talking about!

A Little History

Unlike the small block Chevrolet, which was basically designed from scratch by Ed Cole and company, the Mark IV big block was loosely based on its predecessor, the W-series engine. And while the small block was built as sort of an everyman’s engine for road, the big block was developed for NASCAR competition.

In 1963, a secret, experimental engine called the Mark II was used inside Junior Johnson’s Chevy at the Daytona 500. A highly modified version of the original W-engine, this powerplant featured more conventional wedge-shaped combustion chambers and larger, canted valves to better maintain power above 6,500 rpm. This top-secret 427-cubic-inch engine was essentially the first Rat motor.

Two years later, the Mark IV big block went into production as the 396 Turbo-Jet V8. Soon it was offered in various 427-cubic-inch and 454-cubic-inch configurations. And the rest is truly was history.

The extremely popular and aftermarket-friendly Rat motor has been offered in everything from station wagons and mundane passenger cars to asphalt-eating drag cars and muscle machines. Fifty years later, it continues to be a go-to mill for today’s hot rodders. We’ve seen some pretty stout, radical big block Chevys over the years, but as far as factory Rats go, here are our Top 5 favorites:

Our Top 5 Big Block Chevys of All Time

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5) 396 L78

This is the original factory big block Mark IV engine from Chevrolet. It was first offered on the 1965 Corvette and spit out about 425 horsepower, with very little trouble performing above 6,500. This L78 version of the 396 featured an 11:1 compression ratio, square port heads, and four-bolt main caps—and oh yeah, it started the big block revolution of the 1960s.

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4) 427 L71 Tri-Power

The 427 big block was introduced as a production engine in 1966 for full-sized Chevrolets and Corvettes. Arguably one of the most famous big blocks of all time, the 427 Tri-Power (also known as the L71) was available on 1967-69 Corvettes. Instead of the single four-barrel carburetor, this engine featured a distinctive triple two-barrel carburetor intake setup. Large-port cylinder heads and a high-lift camshaft helped the engine make a reported 435 horsepower. Even today, the 427 Tri-Power is one of the most sought-after Corvettes ever.

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3) 427 L88

The 427 big block could be had in a variety of configurations, but none make a Chevrolet enthusiast smile as quickly and easily as the L88. The L88 was a competition-grade version of the 427 that used a racing cam, solid lifters, high-flow aluminum heads, and other competition parts. Available from 1967-69, the legendary L88 Corvette produced over 430 horsepower, although that number is thought to be grossly underrated.

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2) 454 LS6

Introduced in 1970, the powerful 454 big block was used exclusively in high performance vehicles like the Camaro, Chevelle, and Corvette. The LS6 was the king of the 454s—and of all big block Chevys, really. The 1970 LS6 was conservatively rated to produce 450 horsepower from the factory, although some estimated that it actually made over 500 horsepower. This is was pretty much the high mark for big block power as tighter emissions standards and the fuel crisis of the 1970s spelled the end for the LS6 and, later, the 454.

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1) 427 ZL1

The ZL1 was basically the L88 above with an all-aluminum block and cylinder heads. It featured all the power of the L88 (and then some) in a lighter-weight package akin to the small block Chevy engine. This power/weight combination made it perfect for racing, but it was actually used in a few production vehicles—namely the legendary 1969 COPO Camaros. It was also stuffed into a couple ultra-rare 1969 Corvettes. The 500-pound (or so) ZL1 powerplant was rated at 430 horsepower but, according to some accounts, made nearly 500 horses.