Tips on finding the best torque monster for your next project.
A torque-monster Chevrolet big-block with simple bolt-ons can make 550 lb-ft of torque with a yawn, and they do it with low-octane fuel! Now, that’s something an LS can’t do.
HOT ROD has proved it before; for example, Evan Perkins installed a top-end kit on a tired 454 from a motorhome and made 567.2 hp and 537 lb-ft of torque with 87-octane. We thought this is a good time to revisit an old favorite. Here’s a quick history lesson on the Chevrolet big-block and which one you should use for your next project.
Chevrolet Big-Blocks by Generations
Mark I “W Motor”
Common Sizes: 348ci, 409ci, 427ci
During the 1950s, Chevrolet needed an engine with more torque but still fit inside the inner fenders of a passenger car. It needed interchangeable compression between trucks and cars, and it had to work well on low-octane fuel while producing gobs of torque.
Engineering nicknamed it the W Motor for the shape of the valve covers resembling an upside-down W. It features 4.81-inch bore centers, two-bolt mains, and side oiling. Read 40 Years Of 348 And 409 W-Engines for more information.
348ci: It features a 4.125×3.25-inch bore and stroke with a 9.5:1 compression ratio, resulting in 250 to 350 hp, depending on the year and performance package. It was available from 1958–1961 (through 1964 in trucks). The 348ci is 3 inches wider and 1.7 inches longer than the 265/283ci small-block of the time.
409ci: The 409 was introduced at 360 hp and phased out after reaching a 425hp rating. It features 4.31×3.5-inch bore and stroke. It was available from 1961–1965.
427ci: The rare RPO Z11 427ci is a version of the 409, with the stroke increased by 0.150 inches. It features larger main bearings and domed pistons and was available from 1962–1963.
Mark IV “Rat Motor”
Common Sizes: 396ci, 402ci, 427ci, 454ci
Years: 1965–1990 (until 1974 in passenger cars)
The Mark IV Rat Motor is the traditional big-block we know today. Many of the fundamental elements are similar between the mark IV and modern generations (excluding the Gen VII/L18).
Customers were used to the W-engine’s ample torque, but they desired more horsepower and higher revs. The original Mark I engines steeply fall off after 6,500 rpm, but the Mark IV features a conventional wedge chamber with the combustion chamber in the head. Now, Chevrolet could develop a variety of heads for different applications, such as low-compression, torquey ones for trucks and high-flowing ones for Corvettes.
366ci: Found in medium trucks and school buses of the era, the first Mark IV features 9.4:1 compression and 4×3.62-inch bore and stroke. Today the 366/396ci is not desirable for hot rodding, as they’re small-bore, although HOT ROD did build one once.
396ci: First introduced as a high-output version for the 1965 Corvette, the 396 is rated at 425 hp with a solid-lifter cam. It uses the same bore as the 366 but a longer 3.760-inch stroke.
402ci: The 396 was bored to 4.126 inches to make 402 ci in 1970, although Chevrolet continued to market it as the 396.
427ci: The 427 can be found with hydraulic flat-tappet lifters in family cars or with high-revving solid lifters in the Corvette. It was rated as high as 435 hp and 460 lb-ft of torque. It features the big-bore block at 4.25 inches but the same 3.760-inch stroke as the 366/396.
454ci: Chevrolet added the 454 in 1970, with the same bore but slightly more stroke than the 427 at 4 inches. An LS7 version—not the modern LS model—offered in the Corvette and as a dealer performance option is rated at 465 hp and 610 lb-ft. It was phased out in 1976.
454ci/L19/EFI: Chevrolet phased big-blocks out of passenger cars as emissions and fuel standards went into place in the late 1970s. In 1987, a new 454 Mark IV hit the market with EFI, exclusively in trucks.
Gen V, introduced in 1991, included a superior one-piece main rear seal, new oiling passages closer to the cam, and all blocks received four-bolt mains. However, the engine dropped any mounting bosses for a mechanical fuel pump and the valvetrain is non-adjustable with hydraulic flat-tappet lifters. GM offers rocker studs (PN 10198929) and nuts (PN 10198930) when running a high-lift cam.
The Gen V gained cast-aluminum valve covers and front cover—stamped steel on the Mark IV—for superior sealing. Today, certain crankshafts are interchangeable between the Mark IV and Gen V when accompanied by an aftermarket adapter kit. The same goes for the heads, which require the correct head gaskets. Visit the Fel-Pro Performance catalog for more than 30 available head gaskets.
The front of the block features extruding bosses for the water pump and front accessory mounting, an excellent way to spot a bare Gen V block. The smaller 4-quart oil pan is ideal for swaps (PN 12495360), compared to the larger pan.
454ci/L19: The mark IV has a sluggish rating of 230 hp and 380 lb-ft of torque, largely due to the small “peanut-port” heads. It ended production after 1995.
Introduced in 1996 model year, the Gen VI features hydraulic-roller lifters, multi-port fuel injection, and a PCM. It is the last step of evolution until the LS hit the market in 1998 (introduced in trucks 1999). You can spot these blocks by a six-bolt aluminum front cover, compared to the Mark IV and Gen V’s 10-bolt front cover.
454ci/L29: Rated at 290 hp, The Vortec 7400 is the last generation 454, with the same 4.5×4-inch bore and stroke as before. Like the Gen V, all engines feature four-bolt mains.
454ci/L21: This commercial version is much rarer. However, if you can score one, you’ll be pleased to find forged pistons and crank. It ceased production in 2001.
Which Generation Is Best?
The jump from the Mark IV to Gen V saw a significant redesign of the water passageways. The Mark IV water passageways are round, meaning you can drill and tap them for a smaller size, which strengthens the block (not possible on later-model blocks). A good indication that the Mark IV is preferred is that Dart bases their race blocks off this design.
A Gen V or VI can make great street engines, though, with four-bolt mains and improved cooling. “The Gen V was all about making things lighter,” said Dennis Siderko, dyno technician at Pro Motor Engines in Mooresville, North Carolina. “I’d say up to 11:01 compression for a street build and/or Gen V block. When you get into the 13.1, I’d reconsider the block.”
Builders say a stock Gen V block will handle 650 to 750 hp, depending on pump gas or race gas, and a two-bolt main limited to around 600 hp. A Mark IV can get you closer to the 900hp mark.
All Gen V heads are the low-torque, poor-flowing “peanut-port” heads. If you decided on a Gen V/VI build, you should consider a top-end kit for adjustable valvetrain and better heads.
Big-Block Head Basics
There’s no runaway winner, as they all serve their own benefits. “Big and bad, or small and efficient,” said Dennis. The oval-ports provide torque, but not much horsepower. You’ll likely find the smaller ones on the trucks, and they’re good for about 650 hp, with square-port heads capable of 750 hp. Aftermarket options will get you to that horsepower rating a lot quicker, however.
Before the 1980s, manufacturers played fast and loose with performance options. Casting numbers do a great job, but they’re not definite and sometimes misleading.
Casting numbers are on the front of the BBC, above the timing cover. There are also markings on the back of the block, below the passenger-side head.
|0-326711||4-bolt, alum., CanAm, 4.44in bores, steel cylinder liners|
|361959||454||19731990, 2- or 4-bolt|
|399204||509||1970–1971, 4-bolt, Alum., CanAm, 4.5in bores, steel cylinder liners|
|495102||4-bolt, alum., CanAm, 4.5in bores|
|3855961||396||1965–1966, 2- or 4-bolt|
|3869942||427||1966–1967, 2- or 4-bolt|
|3902406||396||1967, 2- or 4-bolt|
|3904351||427||1967, 2- or 4-bolt|
|3916321||427||1968, 2- or 4-bolt|
|3916323||396||1968, 2- or 4-bolt|
|3935439||427||1968–1969, 2 or 4 bolt|
|3935440||396||1968–1969, 2 or 4 bolt|
|3946052||427||1969, 4-bolt, alum. ZL-1, Mark IV|
|3946053||427||1997-up, 4-bolt, alum. ZL-1, 2nd version, Mark IV|
|3955270||427||1969, 2- or 4-bolt|
|3955272||396||1969, 2- or 4-bolt|
|3963512||427||1969, 2- or 4-bolt|
|3963512||454||1970–1976, 2- or 4-bolt|
|3969854||396||1969, 2- or 4-bolt|
|3969854||402||1970–1972, 2- or 4-bolt|
|3999289||454||1971–1979, 2- or 4-bolt|
|3999290||396||1968–1969, 2- or 4-bolt, truck|
|3999290||402||1970–1972, 2 or 4 bolt, truck|
|3999290||402||1972, 2- or 4-bolt, passenger|
|10051107||454||4-bolt, Bow Tie, Mark IV, 9 8in deck, siamesed bores, 4.25in bore|
|10069282||366T||1990–1991, 4-bolt, Mark IV|
|10069284||427T||4-bolt, Mark IV|
|10069286||454||1990–1991, 4-bolt, Mark IV, short deck|
|10114182||454||1991-up, 4-bolt, Gen V|
|10114183||366T||1991-up, 4-bolt, Gen V|
|10114184||427T||1991-up, 4-bolt, Gen V|
|10134366||454T||4-bolt, Bow Tie, Gen V|
|10185050||454||4-bolt, Bow Tie, Gen V, short deck|
|10237297||454||1996-up, 4-bolt, Vortec 7400in, inL-29in Gen VI|
|10237299||427T||1996-up, 4-bolt, Gen VI, 7 0L, truck|
|10237300||502||1996-up, 4-bolt, Gen VI, 4.466in bore|
|12550312||427T||1996-up, 4-bolt, Gen VI, 4.250in bore|
|12550313||454||1991-up, 4-bolt, Gen V crate motor, Gen VI 4.25in bare block|
|12556110||496||2001-up, 4-bolt, Gen VII 8 1L, Vortec 8100, truck|
|12561357||454||1996-up, 4-bolt, Gen VI|
|12561358||502||4-bolt, Gen VI|
|14015443||454||1987–1990, 2- or 4-bolt, Mark IV, truck, motorhome|
|14015445||454||1978–1990, 2- or 4-bolt|
|14044807||454T||4-bolt, Bow Tie, Mark IV, CNC prep, 4 25in bore or 4 495in bores|
|14096859||502||4-bolt, Gen V, H.O.|
|24502504||454||4-bolt, Bow Tie, Gen V, race prep, 9.8in, short deck|
|24502506||454T||4-bolt, Bow Tie, Gen V, Race prep, 10.2in|
|24502572||4-bolt, 4.5in bore Olds DRCE 2, 9.5in|
|25534402||4-bolt, 4.59in bore Olds DRCE 3, 9.25in|
|T = Tall deck, 10.2in deck height|
|List courtesy of MoreTec.com. See the full list here: https://www.mortec.com/bbc.htm|