Aluminum engine blocks provide obvious advantages in terms of weight and are mandatory for some racing classes. The Sprint car series, for example, requires a cast block that is finished to a 410-ci displacement. Shown here is a popular Donovan aluminum block.
The Donovan 410 block is fully machined on a CNC machining center, including the exterior sides, which are shaved down to reduce weight. Because of the dedicated racing application, there is no need for side motor mount bosses, fuel pump bosses, a starter boss, filter boss, etc. This is a good example of a racing-only block that is purpose-designed for racing function only, eliminating as much weight as is practical in the process.
HC410 and 410 Block
Popular for sprint car builds, the HC410 block weighs in at a mere 83 to 88 pounds and is designed for dry sump oiling only. The blocks feature a 0.390-inch raised cam location and a fully machined exterior with a standard pan rail width. The 410 version weighs a paltry 80 pounds and features the standard cam location and removal of the fuel pump and motor mount bosses.
350/400 Blocks
This block is available to accommodate either a 350 or 400 main bore and features a 9.025-inch deck height, a fuel pump boss, standard motor mount bosses, and an oil filter boss. It is offered for either a 4.000 or 4.125-inch cylinder bore size. Weight is 100 pounds. Custom options include gear drive accommodation; full, half, or solid castings; and a special cam bore size on request.
The Donovan aluminum 410 block is fully CNC machined except for the rear face, which is the only indication that it’s a cast piece.
Lightweight options are available for all Donovan blocks, such as removal of engine mount bosses and oil filter and/or fuel pump bosses, lifter valley and exterior shaving/profiling, etc., where weight savings is a priority.
Note that the HC designation indicates a raised cam configuration, which raises the cam 0.390 inch to allow for increased stroker crank and rod to cam clearance, which can be sized for either 350 or 400 cranks. Special cam bore sizes are also available. The Donovan blocks are truly custom-order race blocks to accommodate the builder’s preferences.
This Donovan racing block has been CNC machined even in the lifter valley, removing all material that is unnecessary while leaving enough material at the lifter bosses for the builder’s plan to use the lifter diameter of choice.
A close view of the lifter valley in this Donovan aluminum block reveals a series of tapped holes in the center. Since builders of these blocks tend to use needle roller cam bearings, these holes allow the insertion of stopper bolts that prevent the roller cam bearings from walking out of their bores during operation, which is possible due to the thermal expansion rate differences between the aluminum block and the baring cages.
While traditional small-block Chevy main caps provide a mount for a wet sump oil pump, when the builder elects to use a dry sump system, the pump holes in the rear main cap must be plugged. For blocks that are intended for dry sump applications from the very start, block makers provide rear main caps that have no such wet sump oil pump provision.
Main caps for high-performance or racing blocks will feature either nodular iron or billet steel main caps. Some builders prefer steel for its strength, while others prefer nodular iron for its combined strength and ductility, or resistance to breaking. One builder I spoke with, for example, prefers nodular iron because, “They tend to bend instead of break,” he said, citing his experience with regard to extreme-duty engine failures.
When ordering an aftermarket block, in many cases you can specify an option regarding camshaft bore diameter, depending on your planned cam bearing application: standard (2.000 inches), big-block journal (2.120 inches), or roller bearing (50, 55, or 60 mm). In some cases, the block maker will produce the cam bore in a standard size, noting that the bores may be enlarged to accommodate the desired bearings. This example shows a block fitted with roller bearings.
Makers of racing blocks often offer weight reduction options that include the removal of unnecessary material. This Donovan aluminum block is a good example. The block is so extensively CNC machined that it initially appears to have been machined from a solid billet of aluminum.
Aftermarket block makers tend to design their blocks with added mass at the pan rail areas to allow for increased crankshaft stroke. Notice the healthy relief notch on this aluminum block for the big end of the rod’s clearance.
Dedicated racing blocks, such as the Donovan aluminum 410, feature external coolant transfer plumbing due to material removal for weight reduction. The fitting in the center allows the racer to inject hot water into the block before firing the engine.
Chevy Performance Blocks
Chevrolet Performance offers a wide range of blocks specifically intended for racing use that receive more detailed prep compared to the regular street blocks.
Cast-Iron Bowtie Race Block
Features include the use of “premium” castings with thicker deck surfaces, priority main oiling, improved coolant flow, splayed four-bolt steel main caps secured with “premium” fasteners, and precision CNC machining to within 0.005 inch.
Chevy Performance Race Blocks
Racing blocks with applications for NASCAR and NHRA are reportedly CNC machined with closer tolerances than their Bowtie blocks (within 0.001 inch) and are reportedly capable of handling up to around 1,200 hp. Features include priority main oiling, four-bolt splayed steel main caps secured with premium studs, and a minimum cylinder wall thickness of 0.225 inch (at 4.155-inch bore). A sonic bore check data sheet is provided with each block. Nominal wall thickness is 0.340 inch. Extra-thick deck surfaces have blind holes that are not open to water.
Enlarged cam bosses allow custom machining for larger bearings, and extra-thick main bearing bulkheads provide greater strength. Main bearing cap inner bolts are spread 0.210 inch to allow machining for the use of 400 journal cranks. The billet steel rear main cap is designed for wet sump use but can be plugged for dry sump applications. The rear main seals are the two-piece style, requiring pre-1986 style oil pans. Lifter bores are tall, requiring clearancing for some aftermarket solid roller lifters. The oil dipstick boss is not drilled, giving you the option of running a dipstick.
Chevy Performance Iron Bowtie Race Block
The Bowtie blocks, rated at handling 700-plus-hp, are CNC machined to within 0.005-inch tolerance. Designed to accommodate 350-size crankshafts, features include four-bolt splayed steel main caps, a deck height of 9.025-inch, two-piece rear main seals, 2.000-inch OD cam bearings, and 3.980-inch finished bores.