Time was, to build a bigger-displacement big-block, you did it mostly through the stroke. That's because the classic, Mark IV big-block is limited to about 0.100-0.125-inch maximum overbores. On a 454ci engine with a stock, 4.00-inch stroke, a 0.100-inch overbore delivers a 476-inch displacement, while a 0.125-inch overbore will give you a 481-inch Rat.
A popular combination is the safer 0.060-inch overbore with a longer, 4.250-inch stroke to make a 496-inch combination. But to take the basic 454 beyond the magical 500-inch mark requires stretching the dimensions considerably with a much longer stroke: 4.350-inch bores (0.100-inch over the stock 4.250 dimensions) and a 4.250-inch stroke delivers 505 ci. Longer strokes deliver bigger cubes, but there's a definite limit to the stock block's bore capability.
The foundation for our engine...
The foundation for our engine build is Dart's Big M iron cylinder block that is a beefier replica of the Mark IV-style Chevy big-block casting. It is designed with the standard camshaft height and accepts almost all conventional big-block parts. It is also available in a variety of cylinder-bore and deck-height configurations. Ours uses a standard 9.800-inch deck height. Other unique features include true priority main oiling, a stepped mail oil gallery that delivers more oil to the crankshaft at higher rpm. The block weighs 260 pounds.
As for GM's own big-cube big-blocks-the 502- and 572-inch monsters sold through GM Performance Parts-they use later, Gen V blocks that are designed for much greater bore capacity than the Mark IV blocks of the muscle car era. The 502-inch engine uses 4.47-inch bores with a standard 4.00-inch stroke, while the 572 has big 4.560x4.375 bore and stroke specs. Clearly, there is a range of possibilities when it comes to building large-displacement big-block engines. While additional cubic inches bring a corresponding increase in power, the question has to be asked about where in the rpm band that power is delivered and how it will be used.
Prior to the introduction of the Gen-V block and aftermarket blocks, stroking the big-block was the tried and true method-and what was there to complain about? Stroker engines are all about torque, and presumably, that's what we all love about big-blocks-the spine-tugging, axle-twisting pull that can't be matched in a small-displacement engine, no matter how camshafts or valves per cylinder you give it.
But consider this: Maybe building a big-block with even greater low-end torque isn't the best solution for a street/strip car.
With an already abundant reserve of torque off idle, what does even more give you on the street or strip? You'll be spinning the tires on the street and typically sacrificing high-rpm power on the strip, the venue where it's needed the most. Sure, torque will help launch a heavy car, but big-blocks have inherent low-end pull anyway, so that shouldn't be a huge concern. There's also the parasitic energy penalty of driving a long-stroke rotating assembly.
To build our 509ci engine,...
To build our 509ci engine, the bores on our Big M block measure 4.500 inches in diameter. Besides the capacity to accept such large holes, the block also has the "meat" around them to ensure strength. Dart tells us that even with a monster 4.625-inch bore, the cylinder wall thickness is still a minimum of 0.300-inch. With our smaller, 4.500-inch bores, there's plenty of cushion between the pistons.
So, maybe the big-bore, shorter stroke option is one to be explored. With the added rev capability that comes with a shorter stroke, the engine will make more horsepower at the top end, while sacrificing only the torque that would allow a Chevelle with a blade on its front bumper to qualify as an earth mover.
We recently visited the concept of a big-bore, short-stroke big-block with Dart Machinery's Richard Maskin.
"Big-bore engines breathe better," says Maskin. "They make the most of high-flow cylinder heads because less of the valve is shrouded. And there's the unquestionable benefit of quicker, higher revving that builds horsepower where you need it on the track. Unless you want to spend your time doing burnouts at every stop light, there's not much need for a long-stroke engine."
There are a few other points to consider with a large bore/short stroke combination:
Reduced valve shrouding enables the use of larger intake and exhaust valves for greater airflow efficiency.
A shorter stroke reduces the distance the piston travels, which reduces the friction and drag created by the piston rings against the cylinder wall.
The crankshaft rotates in a smaller arc, reducing crankcase windage.
 The Big M block is just plain...  The Big M block is just plain thicker in key areas than the standard production big-block casting. This is evident in many details, such as the depth of material removed when the lifter bores were machined. |  A standard feature of the...  A standard feature of the Big M block is four-bolt, billet steel main bearing caps, with the center caps featuring splayed outer bolts. Billet steel is more expensive than the production-style iron, but the benefit in strength-especially when secured with those four heavy-duty, half-inch fasteners-is well worth the investment. The block uses standard big-block bearings. |  An Eagle 4340-forged crankshaft...  An Eagle 4340-forged crankshaft is heart of the engine's rotating assembly. By keeping the stroke to a short (for a big-block) 4.000 inches, the engine will sacrifice some low-end torque, but make up for it with higher horsepower and a greater rev range. Also, crank flex won't be a worry as the revs climb. With torque an inherent attribute of a big-block, the few extra lb-ft that would have come with a longer stroke won't be noticed on an engine we intend to use on the street as well as the strip. |