How To Increase Horsepower - You Can Build It!

In reality, explaining how to build an engine one step at a time is beyond the scope of any single magazine story. Instead, we’ll walk you through the logistics process that every engine build must go through before a single wrench is turned. The first step involves establishing a budget, then determining horsepower and streetability targets. From there, it’s a matter of deciding upon a cubic-inch tally that best suits your needs, then selecting a block, rotating assembly, camshaft, valvetrain, and cylinder heads to make it all happen. Then comes the fun part: turning a pile of parts into a running engine. As anyone who has ever built a motor will attest, there’s nothing like cracking the throttle on an engine you built yourself for the very first time.

Keeping it Practical

Race motors are interesting specimens of engineering that boast insane horsepower. The only thing more insane is how much they cost to build, and the amount of maintenance they require. Sure, it’s easy to be wowed by the 11,000-plus rpm that NHRA Pro Stock engines turn while producing nearly 3 hp per cubic inch. However, the only reason teams must abuse their motors so thoroughly by turning so many rpm is due to rules and restrictions that limit maximum displacement. The good news for street cars is that there is no sanctioning body that limits the number of cubic inches you can pack into your street machine. Since larger motors produce more torque, and horsepower is nothing more than torque multiplied by rpm, they can make the same horsepower as smaller displacement motors without having to turn as many rpm. The net effect of keeping the rpm down is vastly improved valvetrain reliability, smoother idle quality, and the ability to run taller gearing for more relaxed freeway cruising and increases in fuel mileage. Likewise, turning fewer rpm means that you can get by with a less aggressive camshaft, which substantially improves driveability in addition to fattening up low and midrange torque. Consequently, if you use the largest cubic-inch short-block that you can afford as the foundation for your engine build, it’s hard to go wrong. “Our goal is to design a motor to be as efficient as possible for the rpm range we want to run it in, then to use the best cylinder heads available,” Ben Smeding explains. “That way, you don’t need to run a big cam, which makes an engine combination much more streetable.”

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.

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

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.

It’s important to keep in mind that there is a right and wrong way to prioritize parts selection within the rotating assembly. Since the pistons bear the brunt of the abuse inside an engine, and are therefore the most likely components of the rotating assembly to fail, it’s not practical to spend big bucks on a forged crank and rods only to top everything off with hypereutectic pistons. A more effective allocation of any budget would be investing in high-quality forged pistons and rods, and matching them up with a cast crankshaft. Of course, if you can afford it, an all-forged rotating assembly is the ultimate in durability.

Machining

At a minimum, most engine blocks will require, boring, honing, and deck resurfacing. Depending on how badly the main caps are worn, a block may also need to be align-bored and honed. Furthermore, rusty or excessively greasy blocks can benefit from hot tank cleaning, and all rotating assemblies must be balanced prior to installation. Rates vary widely from shop to shop, but it’s not uncommon to shell out over $1,000 for these services.

Granted, quality machine work costs money, but the procedures themselves are easy to understand. Boring is typically performed on a machine like the Rottler FA, which has several cutters that rotate around a boring bar. As the bar moves down the bore, it slowly removes material until the machinist stops it when it’s within a few thousandths of an inch of the final overbore size. From there, the block is honed on machines like the Sunnen CK-10, which not just smooths out the bore surface, but also machines superfine peaks and valleys into the bore surface. Quality honing is essential for proper oil control and ring seal. Generally, race motors use a superslick finish to reduce friction at the expense of oil consumption, while street motors prefer a rougher surface for superior oil control. The align-boring and honing process is similar to boring and honing the cylinder bores, but instead it’s performed on the main caps.