Compacted graphite iron (CGI) is very popular material used in top racing series like NHRA Pro Stock and NASCAR Sprint Cup. The advantage of using CGI in a block over standard gray iron is that it essentially doubles the strength of the iron without increasing weight. This means that it can withstand extreme cylinder pressures and stress. The disadvantage is that it takes a lot more time to machine, and this is really where the cost lies. Any experienced machine shop will tell you that Dart blocks take longer to machine than stock-blocks because of the hardness of the iron, and our CGI blocks take about three times longer than a normal block. Dart uses Class 32B iron for all our blocks except the SHP. This alloy has a 3,200-psi minimum tensile strength, and a Brinell hardness range of 207-255. The SHP block is cast from a gray iron alloy similar to an OE compound, which has 3,000 psi of minimum tensile strength and a Brinell hardness range of 187-241. All Dart blocks, except the SHP, can be special ordered in CGI.
GM produced several different generations of big-blocks over the years. Each generation had subtle changes, so it’s important to know the differences among them when selecting an aftermarket block for your engine build. Rat motors come in Mark IV, Gen V, and Gen VI variants. The Mark IV is the big-block we all know and love. Pretty much all the aftermarket parts are designed for use with the Mark IV. The Gen V differs mainly in that it uses a one-piece rear main seal, and it doesn’t have a fuel pump boss. The Gen VI also uses a one-piece seal along with a different timing cover pattern, a different oil pan pattern, and provisions for the OE hydraulic roller cam and lifters. Dart offers blocks for each generation of big-block Chevys.
Dart’s Big M and Big M Sportsman blocks are based on the same casting, and are designed around standard Mark IV dimensions. They are significantly strengthened compared with a factory block and allow far greater capacity for upgrades. The main difference between them is that the Sportsman version uses ductile iron main caps, while the Big M uses billet steel caps. The Big M Pro is a completely different casting which has the camshaft location raised 0.600-inch higher than stock, and the oil pan rails spread 0.750 inch to increase rod clearance in long-stroke crankshaft applications. A short 9.600- or standard 9.800-inch deck height keeps the casting weight low, and these blocks can be bored up to 4.625 inches for large displacement.
The “Gen 7” name is something Dart came up with to describe our evolution of the GM 8.1L Vortec engine. This was the last version of the big-block Chevy, and it was used in trucks, boats, and motorhomes. There is really no interchangeability between the 8.1L platform and the well-known Mark IV big-block. The 8.1-liter uses all metric fasteners and has a symmetrical port cylinder head design similar to the LS-series small-block. These motors only produced about 300 hp from 496 ci. Dart got involved with these engines to service industrial markets, but there are some performance implications as well. We are now producing blocks for this application, which allow for greater displacement. Likewise, we have developed a redesigned cylinder head with improved ports and chambers, as well as provisions to upgrade the valvetrain. While we have made it possible to upgrade these engines, I don’t see them taking over for the traditional big-block.