Chevrolet stunned the muscle car world during the 1960s with some of its forbidden fruit engines, such as the mighty ZL1 V8 that was brought over into a series of road-going Camaros from the SCCA racing series. It continued its mad experiments into the 1970s, though a looming threat would pretty much put a stop to this overnight.

HotCars decided to take a look back at one of the last engines Chevrolet made before it had to consolidate its engine range for the rest of the decade, a powerplant that would ultimately become the rarest the brand produced in the 1970s.

The Fire-Breathing LS6 V8 Is The Rarest Engine Chevy Made In The '70s

Chevy LS6 V8

(Source: RM Sotheby's, Hagerty)

With the muscle car wars of the 1960s spilling over into the start of the 1970s, Chevrolet kept up its relentless quest to offer the most powerful performance cars on the road. For the 1970 model year, the brand offered the meaty 454 package for its Chevelle coupe and El Camino pickup truck, which consisted primarily of an LS5 V8 that made 360 hp and 500 lb-ft of torque.

The engine was also available on that year's Corvette, though it wasn't the range-topper for the muscle cars. Both the Chevelle and unusual El Camino also came with the option of an LS6, which was an upgraded version of the 454 ci LS5. For an extra premium of $263, the LS5 already being a $503 option, the LS6 boosted power all the way up to 450 hp, while torque remained the same.

While the base cast-iron block remained the same, it was adorned with lightweight forged aluminum pistons, forged steel connecting rods, and crankshaft, as well as solid lifters. The compression ratio of the unit was also increased to 11.25:1, allowing power levels to reach far loftier levels than the still-potent LS5.

Due to the LS6-equipped vehicles costing over $6,000 more than the base model, Chevy struggled to sell enough to justify keeping the option going beyond 1970. As per RM Sotheby's, only 4,475 vehicles featuring the engine were sold that year. This figure includes all Chevelle's El Caminos and a handful of Malibus that were also graved with the powerplant.

When totaled up with the 188 Corvette models that got the engine in 1971, just 4,663 examples of the LS6 cruised out dealers during its brief two-year stint. This makes it easily the rarest engine that Chevrolet produced during the 1970s.

Mighty LS6 Helped Create One Of The Rarest Performance Cars Of The 1970s

The insane LS6 was dropped following the 1970 model year, partly due to low sales, leaving the LS5 as the top option for the rapid Chevelle. The primary reason was the impending restriction put upon vehicles by the US Government as it tried to combat global warming. Chevy's parent company General Motors would impose an 8.5:1 engine compression limit in 1972 as the final nail in the coffin of the limitless muscle car era, with the axing of the LS6 the start of these measures just over a year earlier.

The manufacturer was keen to give it one last big send off though, as it introduced it to a series of Corvette models during 1971. Just 188 examples of the standard Corvette LS6 were shifted that year, with a total of just two of these being christened the ZR2. The ZR2 was placed alongside the existing ZR1 model, which featured the all-aluminum 350 ci V8, which also failed to find many buyers.

The blazing ZR2's LS6 V8 was mated to a close-ratio Muncie M22 four-speed manual gearbox, while a limited-slip differential was also included to ensure the car could achieve optimal traction under acceleration. Alongside the commanding powerplant, the ZR2 also received a heavy-duty disc braking system and an F41 performance suspension setup. The package included new springs and shocks, which were tuned to give the ZR2 a stable platform through the bends, as well as front and rear sway bars to give the chassis more rigidity.

The reason so few ZR2s were sold is almost certainly down to its high price. While a standard C3 Corvette cost around $5,500 at the time, which translates to $44,500 in 2024, the ZR2 came in at a little over $7,600, according to Silodrome. Over $61,500 today, and bearing in mind it didn't come with creature comforts like air conditioning or a radio, most Corvette customers elected they didn't need what was essentially a road-going racer in their lives.

Two of the 12 customers did try to get over the lack of air conditioning issue by choosing the convertible model, making it one of the rarest performance cars ever made.

The LS7 Would Have Been The Cream Of The Crop, Had It Not Been Axed

Chevy LS7 V8

(Source: Hemmings)

While the LS6 was the rarest and most powerful production engine that Chevy made during the 1970s, this nearly didn't end up being the case. The marque was looking at introducing an even meaner 454 ci big block V8, named the LS7, at the same time the LS6 came on the scene in 1970.

It was meant to be for the Chevelle and Corvette model lines and was essentially a cross between the ZL1 and LS6 engines. While keeping the base cast-iron block that featured in the LS6, the LS7 was given the solid lifters from the ZL1, while its compression ratio was raised to 12.25:1. It was also graced with forged high-dome racing-style pistons, a competition camshaft, and was set up to use potent 100 octane fuel.

As per Hemmings, the LS7 would have been capable of producing north of 500 hp and 600 lb-ft of torque. This would have blown any previous Chevy muscle car, as well as anything its competition could come up with, firmly into the weeds. The tightening US Government legislation, as well as rising fuel and insurance costs, scared Chevy enough that they decided to mothball any idea of featuring the LS7 in its production models.

Rather than hide the engine away forever, the manufacturer instead elected to sell them as crate motors. While it's unknown how many were sold, some still crop up on auction sites to this day. A bone-stock version that had been left preserved in its original crate since it was built in 1974 was sold for a hefty $31,500 through Hemmings in 2023.

The engine came with everything needed to install and get it running in its new host, allowing its lucky owner to build up the Chevelle or Corvette that could have been sold through dealers just over 50 years ago.

The LS engines represent the final hurrah of Chevy's IDGAF phase, with even its most potent engines being severely restricted in terms of power beyond 1972. Due to the wonders of modern technology, high-capacity V8s, like the one in the 2024 Corvette Z06, have made a comeback during the new millennium. The LS' though, will always have the accolade of being Chevy's most commanding powerplants of their time.