How To Increase Horsepower - You Can Build It!

By doing your homework, you can breathe hundreds of horsepower to life with your own two hands.

Stephen Kim Feb 23, 2012 0 Comment(s)
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More Cubes, Please

For as long as people have been racing cars, they have been building stroker motors. Stroking a short-block for additional displacement is a rather straightforward process that involves increasing the length of the crankshaft stroke and enlarging the cylinder bores. A block’s deck height, bore spacing, cylinder wall thickness, and pan rail spacing are the primary factors that determine how much an engine can be stroked. These dimensions vary for not just small- and big-blocks, but also for aftermarket blocks when compared to stock units.

For a standard 4.000-inch bore 350 block, stroking it out to 383 ci is incredibly popular. That’s because they can easily handle an extra 0.030 inch of bore, and increasing the stroke from 3.480 to 3.750 inches requires minimal block clearancing. Increasing the stroke even farther to 3.8750 or 4.000 inches is possible—for displacement totals of 396 and 408 ci, respectively—but it creates far more clearance issues among the rods, block, and camshaft.

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On the big-block front, the ubiquitous 454 Rat motor—with its 4.250-inch bore and 4.000-inch stroke—can net an easy 42 extra cubes with a 0.060-inch overbore and a 4.250-inch stroker crank. This yields a total of 496 ci, which may be old-school by today’s standards, but it’s still a very capable combination. Stepping up from a production 454 block to either a GMPP 502 block, or an aftermarket unit with thicker cylinder walls, allows opening up the bore to 4.500 inches for a total of 540 cubes.

To keep things simple, we’ve focused mainly on production GM blocks, but suffice it to say that aftermarket blocks are entirely different beasts. With thicker cylinder walls, taller deck heights, raised camshaft tunnels, and oil pan rails that are spread way out, they can accommodate truly absurd displacement figures. These days it’s possible to build a 454ci small-block Chevy, and Rat motors that pack 665 ci. As no surprise, the biggest drawback of aftermarket blocks is price. While second-hand production blocks can be had for less than $200, aftermarket units can be a bit more.

Rotating Assembly

When it comes down to it, what’s actually doing the stroking in a stroker motor is the rotating assembly, which consists of the crankshaft, connecting rods, and pistons. Options abound when selecting a rotating assembly, and the first decision that needs to be made is the target displacement of your engine build, as this will determine crankshaft stroke and piston diameter. Once those critical dimensions have been established, it’s time to decide whether to opt for forged hardware, or stick with less expensive cast parts. This decision comes down to the intended use of the motor, and the builder’s budget.

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Today’s cast-steel crankshafts are remarkably robust, capable of handling 500-plus horsepower in small-blocks, and more than 750 hp in big-blocks. Most manufacturers recommended forged steel cranks in applications that exceed those horsepower figures. An added bonus is that the lower grades of steel, such as 5140 and 4130, that were popular just a decade ago have been almost entirely replaced by the popular 4340 steel alloy. These forgings will take as much power as most people can reasonably throw at them. Price-wise, cast cranks can be had for as little as $200-$300, while forgings cost more than twice as much.

Moving up the assembly to the connecting rods, just about any aftermarket unit you purchase today will be a forged steel piece, with 4340 the most prevalent alloy. A mere $250 gets you a brand-new set of I-beam forgings from companies like Scat and Eagle, which will handle roughly 500 hp in small-block applications and 700-plus in big-blocks. Most manufacturers that source their rods from overseas offer H-beam rods as an upgrade to their entry-level I-beams. For $500-$600, they can endure loads well in excess of 1,000 hp, making them perfect for big-horsepower power adder combos. That’s not to say H-beams are superior in design to I-beams, since companies like Oliver, Crower, and Lunati offer domestically produced I-beams that have long been held as the gold standard of durability.

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