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.