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20 Useful High-Performance Engine-Building Questions Answered

Understanding the Difference Between Forged and Cast Parts and a Whole Lot More About Building a High-Performance Engine

Barry Kluczyk Apr 30, 2015
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Unless you’re the reincarnation of Smokey Yunick, you probably don’t know everything about engines or building them. That’s just fine, because few people do—and even those who build them for a living are continually learning new things to make them run better and produce more power.

That said, there are plenty of engine-building procedures and components most of us take for granted, even if we don’t necessarily understand the reasoning behind them. This story aims to enlighten those of you—and us—who read about and overhear the same terms, references, and adages tossed about in magazines, at car shows, and in the staging lanes at the dragstrip. They cover the basics of block machining and the difference between forged and cast parts, to specifications such as camshaft lobe separation angle, and more.

Knowledge is power, and even if you don’t open an engine shop tomorrow or next week, you’ll be better equipped to tackle and engine assembly yourself or discuss it with a professional builder. You’ll also have a better understanding of what those other guys are talking about—even if they’re not entirely sure themselves!

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01. Why are four-bolt mains better for performance? In a word: strength. The main caps hold the crankshaft in place while cylinder pressure and high rpm want to throw it out of the bottom of the engine. Conventional two-bolt main bearings on early small- and big-block engines are suitable for mild-performance builds, but upgrading to four-bolt mains provides extra clamping strength and simply more “meat” in the main caps for higher-power applications. Unfortunately, core engines with factory four-bolt mains are relatively rare these days, but almost all of Chevrolet Performance’s new small- and big-block castings feature four-bolt mains. Two-bolt blocks can also be machined to accept aftermarket four-bolt main caps.

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02. So, then what are splayed main caps? They are four-bolt main caps that have the outer bolts of each cap installed at angles of up to 15 degrees. The idea here is even greater clamping strength than mains with all four bolts installed at the conventional 90-degree angle. No factory blocks came with splayed main caps and the machining procedure is relatively expensive. Unless you’re aiming for 1,000 horsepower or more, don’t worry about needing splayed main caps.

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03. What is align-honing and why is it important? The oil clearance between the crankshaft and the main bearings needs to be spot-on. It was good when the engine was built at the factory, but during a rebuild or working with a brand-new block and main caps, it’s important to perform the machining process known as align-honing to ensure the main bearing carriers in the block and main caps form perfectly round circles. That enables the crankshaft to rotate within the main bearings freely and with an optimal, evenly distributed cushion of oil. It also helps the engine run smoother.

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04. Why is a forged steel crankshaft better than a cast crank? Without diving into a dissertation on metallurgy, a forged steel crankshaft’s high-performance advantage lies with the strength of the steel from which it’s made—and not necessarily the manufacturing process itself. Forging generally makes the crankshaft stiffer and less susceptible to shattering under extreme loads and pressures. That’s not to say cast cranks are weak, however. They’re stronger than ever; and a well-prepared, naturally aspirated street engine that sees occasional strip duty will live just fine with a less-expensive cast crank.

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05. Is it the same deal with forged versus cast pistons? Pretty much. A forged piston generally offers greater resistance to shattering, but unlike cast-iron versus forged steel crankshafts, pistons—whether cast (including production-type hypereutectic) or forged—are pretty much made of the same materials: aluminum and silicon. The ratio of silicon varies, affecting strength, while the manufacturing process for forged pistons generally makes them more malleable, which enables them to withstand detonation better than cast pistons. That’s why you want to use forged pistons in a power-adder combination, where detonation is more of a concern.

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06. Why are forged parts more expensive than cast? It’s a good question, particularly because there are often similarities in the raw materials used—especially pistons. It basically comes down to manufacturing complexity. As the name implies, a cast part is produced when molten material is poured into a mold. It’s a process that requires comparatively minor follow-up machining. Forged parts are shaped by forcing the material into a shape, which requires more steps, more time, and more finish machining. And as the adage goes: Time is money.

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07. Why are main and head studs better than bolts? Studs enable more accurate torque values because, unlike conventional, production-type bolts, they don’t twist during tightening. Also, because they remain stationary during tightening, the studs stretch in one axis alone, providing more even, accurate clamping forces. It’s not such a big deal in a stock-type rebuild, but if your budget allows, go with studs in a high-performance engine.

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08. Why is a roller camshaft better? There are two reasons and they are biggies: longer life and lower friction. A roller-type camshaft is used with roller lifters, which allows the lifters to gently glide up and down on the ramps of the camshaft lobes. With the old-school flat-tappet design, the cam lobes smack the bottom of the flat-profile lifters. The reduction in friction with a roller camshaft/lifter setup enhances performance and the consequential reduction in wear greatly prolongs the life of the camshaft.

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09. Do roller rocker arms have to be used with a roller camshaft? No. In fact, GM’s LS engine family, which use roller camshafts and spin to rpm levels unheard of in the old muscle car days, use conventional, non-roller stamped-steel rocker arms. Why? Mostly because they’re inexpensive. More-expensive roller rocker arms definitely offer a friction reduction benefit, but their use has nothing to do with whether the cam and lifters are roller-type.

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10. Why is the camshaft lobe separation angle (LSA) such a big deal? Even if they don’t understand the methodology for determining optimal camshaft lift and duration spec, most enthusiasts understand the physics behind a camshaft’s lift (how high the valve comes off the seat) and duration (the degrees of crankshaft rotation for which the valve is held open). Lobe separation angle is a little trickier to comprehend. It is the angle in camshaft degrees between the maximum lift points or centerlines on the intake and exhaust lobes. It affects the amount of overlap—the brief period of time when both the intake and exhaust valves are open—and myriad other performance factors. Generally speaking, a “tighter” LSA provides more overlap, a lumpier idle, and a narrower, more specific powerband. A wider angle reduces overlap and offers better idle quality. Supercharged engines typically benefit from a wider LSA because they don’t need as much overlap like a naturally aspirated engine does for exhaust scavenging.

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11. What the heck is the rod/stroke ratio? It’s an important factor when building a stroker engine such as the classic small-block 383, which increases the 3.48-inch stroke on a 350 engine to 3.75 inches. The rod/stroke ratio is length of the connecting rod divided by the stroke and it must be optimized to minimize the friction-inducing side-loading force on the thrust side of the piston. In other words, as the piston moves up in the bore, it will naturally push against the cylinder wall. If the rod is too short for the stroker combination, the piston will generate too much load against the cylinder wall, increasing friction and wear, and vibration at higher rpm. It may sound counter-intuitive, but using a longer connecting rod alleviates the problem. A builder should aim for a rod/stroke ratio of 1.55 or greater.

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12. What is the difference between an I-beam connecting rod and an H-beam? The names are derived from the general profile if they were cut in half. The I-beam—used in most production engines—looks like a capital letter I and the H-beam looks like the letter H. Beyond that, the performance differences are in their respective strength capabilities. H-beam rods typically offer greater strength and stiffness and are best suited for very high-power engines, but they’re heavier than a comparably sized I-beam rod. Consequently, there’s a penalty in rpm and rev capability. For high-performance engines used mostly on the street, I-beams are sufficient.

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13. What is the compressed thickness of a head gasket? The compressed thickness, or “crush,” is the distance the head gasket creates between the cylinder head and cylinder block when the heads are bolted in place. The typical head gasket compressed thickness for a small-block Chevy is around 0.040-inch and it’s important because even that tiny distance can affect the compression ratio and performance. Some builders will go with thinner gaskets to gain a little compression. But if you simply wondered what the term meant, now you know!

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14. Do aluminum heads have an inherent performance advantage over iron heads? With all things being equal, the answer is no. If the head designs, including the port configurations, chamber volumes, and valve sizes are the same, there is no inherent airflow advantage with the aluminum head. However, an aluminum head dissipates heat much faster than an iron head, which can allow for more aggressive ignition timing to enhance performance. Rapid heat dissipation also has the effect of lowering cylinder pressure. Consequently, a higher compression ratio can—and often should—be used with an aluminum head. Of course, aluminum heads are also lighter, which can enhance a vehicle’s overall balance and performance.

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15. What is the valvespring installed height and why is it important? Installed height is the height of the valvespring with the valve is closed, measured from where the spring meets the bottom of the retainer to where it rests on the cylinder head. Every camshaft has a recommended installed height, and making sure it is correct helps ensure the just-right spring pressure. Too little pressure will cause valve float at high rpm and too much pressure can hurt the camshaft.

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16. So, what then is valvespring coil bind? It’s the state where the valvespring’s coils touch one another and can no longer compress—and it’s not good. It means the camshaft and valvetrain components aren’t properly matched. If coil bind occurs, the resulting mechanical interference can severely damage the valvetrain components and the camshaft. As a rule of thumb, the spring travel should be at least 0.060-inch more than the full lift of the valve.

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17. What is a “cold” spark plug? The heat range of a spark plug refers to the speed with which the spark plug can transfer heat from the combustion chamber to the cylinder head. A “hotter” plug retains more heat in the firing tip, while a “colder” plug transfers heat quicker to the head. In a high-performance engine—especially one with a supercharger or turbocharger—colder plugs help stave off detonation. Unfortunately, the heat range index for spark plugs isn’t universal, so it’s important to research the ratings among your manufacturer of choice.

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18. What’s the difference between single-plane and dual-plane intake manifolds? A dual-plane manifold has a divider in the plenum directly beneath the carburetor, with each side feeding half of the cylinders. A single-plane manifold, also known as an open-plenum manifold, has no divider and the plenum feeds all eight cylinders. Generally speaking, dual-plane manifolds are better suited for street engines, because they promote a wider power range and work better at lower rpm. Single-plane manifolds are less restrictive and promote more maximum high-rpm horsepower, but typically at the expense of low-rpm performance and driveability. There are exceptions to these rules, but generally you’ll want to use a single-plane intake at the track and a dual-plane for street/strip engines.

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19. Why is a carburetor with vacuum secondaries better for a street engine? Compared to the classic, high-performance Holley double-pumper, a carb with vacuum secondaries is easier to live with because they kick in automatically for more consistent performance and they usually deliver better fuel economy. Vacuum secondaries are also tailored for the typical street-engine combination, such as a dual-plane intake and a “smaller” camshaft, as well as an automatic transmission—all of which typically provide better vacuum signals. A double-pumper with mechanical secondaries is controlled strictly by your right foot. It uses more gas, but is more accommodating to big cams, single-plane intakes, and other vacuum-limiting factors. These qualities generally make the double-pumper better suited to high-rpm combinations used mostly at the track.

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20. Will an EFI conversion automatically deliver more power? Generally speaking, there won’t be a huge increase in either a stock or otherwise unaltered engine. The injection system may foster more efficient combustion, but it doesn’t necessarily translate into noticeably more horsepower. You’re looking at maybe a 5 percent increase in horsepower, tops, because changing to fuel injection doesn’t alter the airflow aspects of the intake manifold or cylinder heads. It does, however, lay the foundation for greater performance capability and, of course, offers improved driveability attributes.

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