To outsiders looking in, hot rodding can seem rather juvenile. We are talking about grown men who still play with blocks, for goodness sakes. Rather than groveling in shame, however, it’s time to embrace our natural gravitation to these large lumps of iron (or aluminum) and learn all there is to know about them. Let’s face it: Stock cylinder cases are fine for many engine builds, but keep on pushing the limits of horsepower, and sooner or later you’re going to need an aftermarket block. Aside from the irrefutable advantages in strength that they offer, aftermarket blocks are packed with goodies that promote easy cubic inches and unparalleled reliability. Thicker cylinder walls, priority main oiling systems, four-bolt main caps, and commodious crankcases are common fare on any aftermarket block, and the ante can easily be upped with taller decks and raised cam tunnels. The one downside to all these options is that the selection process can get confusing. That’s why we called up Jack McInnis at Dart to sort through it all.
Dart is a company that needs no introduction. Founded by legendary Pro Stock engine builder Richard Maskin, the Dart brand has been synonymous with high-end engine components for decades. Although the company’s rugged aftermarket blocks have been staples of potent street and all-out race cars for decades, it has recently ramped its block program way up. Not that long ago, the Dart block line was comprised of the Little M and Big M blocks. However, in just a few short years Dart has diversified its portfolio substantially. These days, there are multiple variants of Big M and Little M in addition to tall- and short-deck blocks, siamesed blocks, full-water blocks, aluminum blocks, spread-bore blocks, race series blocks, and even a Vortec 8100 series block. That’s a whole lot of blocks. In addition to explaining the differences between them, we used this opportunity to explore the different types of alloys used during the casting process, and also picked up some useful tips on how to keep extreme cylinder pressure sealed in the bores.
Aftermarket Block Basics
When GM was manufacturing small- and big-block Chevy engines, the typical production rate for an assembly plant was one car coming off the assembly line every minute. This meant that every part of the car, including the engine block, was designed with reducing manufacturing time and cost as top priority. In contrast, Dart blocks are designed and manufactured with performance as the number-one priority. The alloys, casting designs, and machining methods are all geared toward performance. Our blocks are heavier than stock because more metal is added to strengthen critical areas, and provisions for upgrades are integrated into the block as well. For instance, all Dart blocks have a true priority main oiling system that supplies oil to the main bearings first, then feeds the cam and lifters. This is the opposite of a standard OE small-block oiling system. The standard system was designed to simplify high-speed production and works adequately for normal passenger car use, but for high performance using a priority main system is vastly superior. Additionally, Dart’s four-bolt main caps feature splayed outer bolts, which go into the strongest part of the bottom end. The stock four-bolt design used straight bolts that actually weaken the main webs, which is why many engine builders prefer two-bolt blocks over the OE four-bolt design. All our blocks also have expanded water jackets for better cooling, and blind head boltholes, which don’t go through into the water jackets, eliminating leaks from the head bolts.
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.
Big M lineup
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.
Siamesed vs. Full Water
When selecting an aftermarket Dart block, engine builders can choose between both siamesed and full water big-blocks. Like OE blocks, a full water block has water jackets between each cylinder, whereas siamesed blocks do not. The advantage of removing these cooling passages is that it allows for much thicker cylinder walls. As a result, siamesed blocks offer cylinder walls that can be bored out much larger. For instance, a full water Dart big-block can only be bored to 4.310 inches, while a siamesed block can go up to 4.625 inches. A common concern is that siamesed blocks are prone to overheating, but this really isn’t an issue as we have incorporated expanded water jackets in these blocks. The reason we developed the full-water version was to address the needs of the industrial environment, where engines run 24 hours a day. For these applications, our customers specifically asked for full water around the bores. This feature does reduce the weight of the casting, and we can use it for small-bore big-blocks like 396 and 427 replacements. The downside is that you can’t go to the big 4.500- and 4.600-inch bores like you can with siamesed bores.
Aluminum is seen by many hot rodders as a very exotic material when used in a block. There’s always a trade-off between weight savings and strength when compared to an iron block, but aluminum blocks are still plenty stout for most applications. A cast-iron Dart Big M block weighs from 250 to 280 pounds, depending on deck height and bore size. On the other hand, a Big M aluminum block weighs from 140 to 160 pounds. As you can see, there’s typically a bit over a 100-pound difference in weight. Similarly, an Iron Eagle small-block weighs from 208 to 224 pounds, while an aluminum Dart small-block weighs 105 to 120 pounds. Again, the difference is roughly 100 pounds. The aluminum blocks will handle substantial power when set up properly, but iron is much stronger ultimately. With an aluminum small-block, once you go over about 900 hp it’s time to go to an iron block. With a big-block that figure is more like 1,100 hp. There are a lot of variables, of course, but those are pretty safe estimates.
Dart’s diverse range of Mouse motor blocks—in ascending order of strength—includes the SHP, Little M, and Iron Eagle. The SHP was designed as an upgraded stock replacement block. It is cast from gray iron and incorporates all the most sought-after features of the various evolutions of the Gen 1 small-block such as siamesed cylinder bores, priority main oiling, and four-bolt main caps on the center three saddles. The SHP has a standard deck height of 9.025 inches along with standard 350 main bearings and standard cam bearings. It also has provisions for OE-style roller lifters and spider. For applications that require more durability, the SHP Pro block uses billet steel four-bolt main caps on all five saddles that are anchored with ARP studs. Furthermore, the SHP Pro has a big-block Chevy cam bore and larger 0.904-inch lifter bores. These features greatly enhance stability in high-rpm usage.
Race Mouse Blocks
For the ultimate in small-block durability, Dart has two options: the Little M and the Iron Eagle blocks. The Little M is a true race block that is cast from a premium high-strength alloy and is substantially reinforced. As such, it weighs about 30 pounds more than an SHP block. The Little M comes with a standard deck height, either 350 or 400 mains, and steel four-bolt main caps on all five saddles. Furthermore, it’s compatible with dry-sump oiling with front and rear crossovers, and has provision for restrictors. The Little M can also be upgraded to 50mm roller cam bearings. On the other hand, the Iron Eagle block is a much more specialized race block. It has the cam location raised 0.391-inch higher than stock to allow the use of large base circle cams, and it comes standard with big-block cam bearing journals, which can be upgraded to 55mm roller bearings. The oil pan rails are spread 0.400 inch per side to clear long stroke cranks. Deck heights from 8.200-9.325 inches are available, providing tremendous versatility. Mains are either 350 or 400 size, and the Iron Eagle has steel four-bolt main caps on all five saddles. Dry-sump oiling and restrictor provisions with front and rear crossovers are also included. The Iron Eagle does not have an oil filter pad, so it requires a remote filter.
Little M Options
Dart has recently expanded the Little M block line to include three variants: the Little M Sportsman, the Little M, and the Little M Pro. The Sportsman version really kind of falls in between the SHP and the Little M. It is cast from premium high-strength iron alloy, and has four-bolt ductile caps on the center three mains and two-bolt caps on the outer mains. It allows for more upgrade possibilities than the SHP as well. In contrast, the standard Little M has four-bolt billet steel caps on all five mains. It is also machined with provisions for dry-sump oiling with front and rear oil crossovers. Stepping things up to the next level, the Little M Pro is essentially the Little M block with a selected menu of upgrades. It can be outfitted in several different ways with options such as larger cam bores and lifter bores, and block lightening options.
Spread Bore Blocks
From the factory, the small-block Chevy measures 4.400 inches from bore center to bore center, while the big-block Chevy measures 4.840 inches. Dart now offers 4.500-inch bore space small-block Chevy blocks, and 4.900-inch bore space big-block Chevy blocks, including a billet only 5.00-, 5.200-, and 5.300-inch bore spacing. The main advantage of spreading out the bore spacing is that it allows you to use a bigger cylinder bore and still have adequate sealing area between the bores. It is always better to have a large-bore, short-stroke engine combination because it un-shrouds the valves, makes room for larger valves, and reduces piston speed. Most of the 4.500-inch bore space small-blocks are using a 4.250-inch big-bore piston configuration. This enables displacement well in excess of 454 ci while retaining thick cylinder walls for durability and head sealing. The 4.900-inch bore space big-block traces its roots to Pro Stock, and currently most of the spread bore big-blocks have 5.000- or 5.200-inch bore centers. Quite a few custom components are required for these engines in addition to the block itself. They require cylinder heads, intake manifolds, crankshafts, camshafts, pistons, headers, and gaskets are all specific to the spread bore architecture.
For those seeking maximum cubic inches, Dart’s race series tall-deck big-block Chevy block provides many interesting possibilities. This tall-deck block falls under our Big M Pro category with nonstandard architecture. The casting has a 0.600-inch raised cam location, oil pan rails spread 0.750-inch apart, and deck heights ranging from 10.600 to 11.100 inches. To put this in perspective, the factory block has a 9.800-inch deck height. Additionally, the Dart tall-deck block is also available in 4.900-inch bore spacing. These features allow you to build an engine up to 763 ci.
A stock big-block Chevy has five head bolts per cylinder. This works fine for most applications, but Dart offers the option of adding an additional bolt or stud on the top side of each cylinder to improve sealing. It is a necessity in any power-adder application, and is a good idea in any engine making over 1 hp per cubic inch. Plus, it’s cheap insurance when you take into account the problems that can be associated with blown head gaskets. Dart’s Big M blocks have the extra boltholes machined in, and we offer a simple stud kit that attaches to the heads. To take advantage of the additional clamping force, all Dart big-block heads are machined to accept the additional bolthole. CHP