“Chevy’s Mark IV Big Block”
Chevrolet introduced their first modern V8 (they made a V8 back in 1917-1918) in 1955. Of course, we all know how that went. The Chevy Small Block V8, as it came to be called, started out as a humble 265 and grew all the way to 400 ci. It was and is one of the most successful and prolific car engines ever made, having powered nearly everything Chevy and later GM made for seven decades. 93 million were built (not including the modern LT- or LS-series)! 93 million small blocks. That’s one helluva record!
Starting out in 1955 with 265 cubic inches, it made 225 hp tops. Once the bugs were sorted out of the new design, it was quickly expanded to 283 ci in 1957 and horsepower also grew. Even fuel injection was added, a radical move for 1957, which stood out even more because it fostered the first American engine to make one horsepower-per-cubic inch of displacement, 283 hp from the 283 Fuelie. That was impressive. But by 1958, they’d slapped dual 4-barrels on it and made 290 hp.
But it wasn’t enough. Chrysler’s Hemi’s passed the 300 hp-mark long ago and now Ford’s new FE big block engine family arrived in 1958 with a 352 ci V8 making 300 hp and gobs more torque. The fun was over. While they could continually enlarge the small block, they needed a heavy duty big block like Ford and Chrysler, with strong internals and an advanced design.
Enter the W-series in 1958. Starting out as a 348 ci, it too was quickly blown out to 409 ci and became a legendary performance engine in its time. The Beach Boys even did a song about it in 1962. This was Chevy’s first attempt at a big block and it was successful to a point. But it’s limitations were quickly discovered. Strangely, Chevy had decided to machine the deck surface of the block (where the heads bolt to) at an angle, not square with the bore. This, combined with an oddly-shaped combustion chamber and piston crown was supposed to improve flow, cylinder-filling, and fuel burning. It was an interesting experiment and a bold move on Chevy’s part, but it had more flaws than advantages and probably stayed too long.
It was a great engine for Chevy’s full-size passenger car line and their trucks in milder form, and in the early 1960s its performance was considered exceptional. But, it was quickly being eclipsed by its competitors. They’d learned a lot of lessons with the Wseries, that could be applied to a whole new engine, a clean-sheet design.
By 1962, design work had started led by Chevy’s Chief Engineer, Bill Fisher, with heavy input from Zora Arkus-Duntov, the legend, and the Corvette Chief Engineer. Fisher wanted a great street engine that could easily be scaled up as needed, an advanced design that would be easy to produce, and Duntov wanted the Corvette to dominate racing and the street scene. The new engine made it’s debut at Daytona in 1963, dubbed “the Mystery Motor” because no one knew what it was. They were expecting the W-series 409. This new engine was unstoppable.
By 1964 early prototypes were enduring extensive dyno work and durability testing. And by mid-year 1965 they released the first Mark IV big block, the 396. Available in a wide variety of Chevrolets, the midsize Chevelle/Malibu, the full-size Impala line, Chevy Trucks, and the Corvette, the new 396 came in 3 flavors: The L35 with hydraulic lifters and a 10.25:1 compression ratio good for 325 hp; the L37 also with hydraulic lifters but a much hotter cam and making 375 hp (but was only available in the Chevelle SS396); and the L78 with solid lifters and a red-hot cam, rectangular-port heads and an 11.00:1 compression ratio with a whopping 425 hp!
This bold new design shared nothing with the W-series that came before it except its 4.84” bore spacing. It was designed from the start to be a heavily oversquare (big bore, short stroke, relatively speaking), large displacement, super-strong, expandable architecture. The tall deck height allowed for increases in stroke and the wide bore spacing left plenty of room for increases in bore. But the key to performance always comes down to breathing, and this massive engine would need lots of it. The heads are a masterpiece of flow optimization. The massive, heavy cast iron heads had huge intake and exhaust ports with straight paths in and out, and the large valves were arranged in the absolute best position to maximize efficiency. That placed the valves oblique to one another, not parallel on any plane. The intake valves point one way and the exhaust valves point in a completely different direction. They’re not far off from one another, but the difference is quite obvious. They’re called “porcupine heads”, and they were harder to produce than small block heads with all their valves lined up straight. But the added cost and complexity was worth it, because the new big block breathed and performed brilliantly, with enough room to expand by every measure.
And expand they did. In 1966 a new, larger version of the Mark IV big block was introduced with 427 ci, an engine that eclipsed anything that Ford had certainly, and even rivaled the mighty 426 Hemi. The 427 dominated racing from 1967 through 1970. The standard version (L36) had a single 4-barrel and made 390 hp. The L72 had solid lifters and a Tri-Power setup (three 2-barrel carbs) and made 425 hp, and these were the street motors. Chevy also built two wicked race motors rarely seen on the streets, although super-rare examples do exist.
The 1967-1969 L88 (RPO L88) used large rectangular-port heads, a 12.5:1 compression ratio, a radical solid-lifter cam, an 850 cfm Holley with no air cleaner, high-flow exhaust manifolds and 4-bolt mains all built on a special high nickel-iron their advertised horsepower was 430, but it was more like 550+ in the real world.
The 1969-only ZL1 (RPO ZL1) used an all-aluminum block and heads, cutting over 150 pounds, an even more aggressive solid-lifter cam, special lightweight pistons and rods, and dry-sump oiling. Otherwise it was the same internally as the L88. Chevy stated the horsepower at 430, the same as the L88, but in fact it was above 600.
Both engines are incredibly rare, having been made expressly for racing and never intended for street use. However, wily Chevy dealers back then figured out how to game the system by plugging the right option codes into the COPO (Central Office Production Order) system. They could get Novas, Chevelles and Camaros with 427s, including these crazy racing engines. Chevy went along with this because SCCA and NHRA rules required that a certain number be sold to the public for homologation.
Cars from Baldwin Motion and Yenko were sold new with financing and factory warranties. Most were intended for racing, but some made it onto the street. Among the rarest is the 1969 ZL1 COPO Camaro with the all-aluminum 427, just 69 were built. Today a genuine Yenko of any kind brings top money at auction.
1970 was a big year, many things seemed to turn on it. In 1970, GM dropped it’s decade-long ban on engines larger than 400 ci in their midsize cars. In 1970, the 396 got bumped to 402 ci with a slight increase in bore, but Chevy continued to call it a 396. And in 1970, the 427 got punched out to 454. The standard 454 (LS5) had a hydraulic cam and 10.25:1 compression, made 365 hp and was a $295 option. The top engine costs four times as much, at $1,221, but boy was it worth it!
Not including race car motors like the L88 or ZL1, the ultimate Chevy big block had to be the 1970 454 ci LS6. It was strong with 4-bolt mains, forged steel crank and rods, forged alloy pistons. It breathed well, with rectangular-port ‘open chamber’ heads (casting #3994028), 2.19” intake valves and 1.88” exhausts, an aluminum low-rise dual-plane intake manifold mounting a single Holley 4150 800 cfm 4-barrel carburetor with mechanical secondaries. And it made big power with an 11.25:1 compression ratio and a radical solid-lifter cam. Chevy claimed they made 450 hp at 5,600 rpm and 500 lb-ft of torque at 3,600 rpm. Big numbers to be sure, bigger even than those claimed by Chrysler for their much-vaunted 426 Hemi. But they were all unnaturally low. Most experts agree that the 1970 LS6 made well north of 500 hp at higher rpms than 5,600. Chevy didn’t want to alarm the insurance companies with such lofty claims, so they simply cut off their dyno tests at 5,600 rpm. But bigger power awaited up around 6,500.
Funny thing, though. This ultimate big block, this top dog muscle car motor, the pinnacle of V8 technology at the time was only available in the 1970 Chevelle SS454. Not Chevy’s full-size cars, and not even the Corvette. That’s unusual since Chevy always reserved its best performance engines for the Corvette first. But, for 1970 the only big block option for the Corvette was the LS5 454 which made 365 hp. The LS6 wouldn’t come to Corvette until 1971 but by then increasing smog regs dropped output to 425 hp. Only 188 were built.
1970 was the last good year, then the Malaise hit in 1971. The combination of all the new smog, gas mileage and safety regulations, the removal of lead from the gasoline and rising insurance costs was killing performance. GM went on a wholesale campaign to neuter all its engines in an effort to meet the new standards.
Big block options became limited, compression ratios fell, cams became milder, carburetors got smaller, tuning was dialed back and endless new gadgets began appearing on these already-choked off engines. And horsepower ratings dropped dramatically and continued to drop for years.
The last year for an optional big block was 1972 in Corvette, Camaro and Chevelle. The last year for Chevy’s full-size cars was 1976, by that time down to just 225 hp. From then on the Mark IV 454 big block soldiered on in Chevy and GMC trucks and SUVs through the first half of the 1991 model year.
That’s when the next generation of big blocks came out. The new Mark V was known as the Vortec 7400 and alongside Chevy’s venerable small block, and some 6-cylinders powered every full-size truck and SUV that GM made, until 1999. In 2000 Chevy punched its mighty big block out one more time to 496 ci, making it the Mark VI and named it the Vortec 8100. This 761-pound monster powered many of GM’s biggest trucks through 2010. From then on it was LS or diesel all the way. Well, until lately, now they’re putting turbo 4-cylinders in full-size trucks. It’s a travesty.
It would seem that the age of the big block has come and gone. None remain. Chrysler’s B and RB engine families (383, 413, 426, 440) died in 1972. Ford’s FE big block (352, 390, 427, 428) was gone by 1970, replaced by the 385-series big block (429, 460), itself gone by 1978 in cars and 1997 in trucks. Today’s modern V8s, Ford’s Coyote, Chrysler’s Gen 3 Hemi and GM’s LS/LT-series can make all the power and torque of the classic big blocks with less displacement, better fuel economy, cleaner emissions and 300 pounds less weight.
But Chevy’s amazing Mark IV big block lives on in racing of many different types. It’s still the most favored engine in drag racing (Hemi fans may argue this). And it’s very strong in the classic and custom car world. Classic Chevelles, Impalas, Camaros and Corvettes with big blocks from the factory get all the attention at shows and command top dollar in auction.
If you look at the lifespans of America’s other big blocks, the Chevrolet Mark IV has had the longest life, by far. Chevy’s own W-series lived from 1958 to 1965 (8 years). Ford’s FE big block lasted from 1958 to 1970 for cars and 1976 for trucks (19 years). Chrysler’s B and RB big blocks were around from 1959 through 1978 (20 years). Ford’s 385-series big block, the 429 and 460 went from 1968 through 1997 (30 years). Chevy’s big block was produced from 1965 through 2010 (46 years). No else even comes close. That’s how good an engine it was and is, what a great design it had from the start. And it’s still revered to this day.
For being billed as GM’s ‘bargain-priced brand’, the lowest in it’s ascending ladder of brands, Chevrolet really took the lead in performance, and not just at GM, but out there on the street. And up against some daunting competition from Chrysler and Ford, both strong on engineering. Chevrolet gambled with the Corvette, then stuck with it when it struggled, and endured low sales volumes because they knew it was important. Important to the Chevrolet brand and important to the world at large. Corvette is something special. It always has been. And as far as engines go, Chevy’s Mark IV big block is also something special. Powerful, indestructible, beautiful (if you can call a brute like that ‘beautiful’), truly one of the world’s great engines.
So good in fact that 15 years after they went missing in new cars and trucks, Chevrolet still sells big blocks as crate motors. And talk about expandable, you can buy brand new big blocks in sizes 427 ci, 454 ci, 502 ci, 572 ci and 632 ci. 632 cubic inches? From Chevy?! Holy cow!
So, why call it a Mark IV? The W-series 409 was considered the Mark I. The Mystery Motor that first appeared at Daytona in 1963 was the Mark II. The Mark III was a 1963 design study that would have changed bore centers, but was never produced. When the new engine went into production in 1965 it was called Mark IV. And what an engine!