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.
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.
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.
With our thinking in alignment with Maskin's, we embarked on an engine-building project that would test his wisdom. We were looking for a 500-plus inch engine that would offer good dual-use duty on the street and on the drag strip. Maskin pecked out a few numbers on his calculator and a figured a 509-inch combo, with 4.500-inch bores and 454-standard 4.00-inch stroke, would do the trick.
"Based on the 400-inch SHP (Special High Performance) engines we build, it should make 668 horsepower, with all the torque you can use," he says. "It's a simple combination, too, using nothing but off-the-shelf parts."
With Maskin's recommendations, as our blueprint and his in-house engine builders our guides, we set up shop at Dart's suburban Detroit headquarters and documented the combination's assembly from the installation of the first bearing to the final pull on the engine dyno.
The basics include:Dart's Big M iron cylinder blockDart's 310cc aluminum Pro1 cylinder headsEagle 4340-forged crankshaftEngine Pro H-beam connecting rodsJE pistonsComp Cams hydraulic roller camshaft and rocker armsDart single-plane intake manifoldDemon 850cfm carburetor
Although our 509 engine combination seems straightforward enough, it's not something Dart stocks in its warehouse. In fact, the company doesn't sell crate engines per se, but offers a variety of short-block assemblies and the service of custom engine builds. To mirror that, we've divided this story into a pair of installments; the first focusing on the big-bore, short-stroke short-block and the second on the remainder of the engine assembly and dyno testing.
What that means is you'll see the inside of the short-block in the accompanying photos, with insight into the attributes of the Dart block that support such a generous displacement. You'll have to pick up next month's issue to see whether this Dart engine hits the bull's eye of Maskin's 668hp target.