This month's installment deals with the short-block and its components. The NASCAR Winston
Editor's Note: This month we dig into the short-block and its major companion parts. Included will be crankshafts, connecting rods, pistons, and rings. Let's begin with the block itself.
The Block The block is the foundation for the engine, so inspection of the casting is critical. Sonic-checking is a service available at most machine shops today and is critical to ensure stability of the block. The small-block Chevy and small-block Ford each have areas where the castings are traditionally thin. You should rely on someone with experience with your type of engine to check the block to make sure it is acceptable. For example, a small-block Chevy is traditionally thin at the base of cylinder No. 7 and should never be any thinner than 0.140 inch for Late Model Stock applications. Engines with less power can afford to be 0.125 inches at the minimum.
Lifter bores must be checked for proper position since "misplacement" or poor "indexing" is common in as-cast conditions, thereby affecting valve timing and valvetrain geometry. Blocks that were not specifically designed for motorsports applications are particularly problematic. Line bore and hone the mains to proper size and make sure the deck is at a true 90-degrees-to-the-crank centerline. If the block you have is of unknown history, it is also a good idea to check the distance from the crank centerline to the cam centerline because this can move (or change) with repeated line boring.
Cylinder deck height should also be checked. The deck height can be adjusted within a small window to optimize a given engine configuration. For example, when hood height severely restricts the induction system performance, running the minimum deck can create some room. This is usually not an option for budget engine builds. Deburr the block and lighten it as much as the rules allow.
Crankshaft Buy the best parts that fit into your budget. Higher engine speeds or an engine that must run a full season without a rebuild require better parts. Remember, when working on a limited budget, the money should go into valvetrain, cylinder heads, and intakes for power. Lighter cranks will allow the engine to accelerate faster and can ease the load on the bearings, but they tend to cost more and may have a shorter service life.
How Rotational And Reciprocating Mass Affect Engine Acceleration
Think of rotating crankshaft and reciprocating piston/rod assemblies as flywheels. The greater the total mass, the more resistance offered to changes in system acceleration (or deceleration). Also, the shorter the stroke, the less the mass moment of inertia or resistance to a change in rpm. In the so-called "slider-crank" mechanism of the crankshaft (weight and stroke length), connecting rod (length and weight), and piston (weight and pin position), changing the energy state of this system is a function of weight and leverages. Keep in mind that changes of rotational or reciprocating speed requires energy that might otherwise be available to accelerate a race car.
Journal Sizes The trend in most forms of motorsports is to run smaller and smaller journal sizes where rules permit. This not for budget builds but will benefit high-speed engines the most. The reduction in journal radius reduces the speed of the journal at any engine speed and can reduce friction. NASCAR has limited rod journals to a minimum of 1.847 inches. If this size will work to support an 800hp Cup engine, it may be suitable in yours. Only forged cranks specifically designed for these journals can handle the loads, so don't try cutting down your cast crank to run Honda bearings.