Like the rest of the performance community, we were very excited when Dart announced its new Special High Performance (SHP) line of engine components and assemblies last year. With such a great starting point at its disposal with its blocks, it was only natural that they use them as the basis for impressive short-block assemblies. Now throw in Dart's heads and intake manifolds and you have what amounts to a seriously stout engine.
Looking over the specs, the Dart SHP short-blocks were assembled using all the right stuff. Naturally, we couldn't wait to get our hands on one and strap it to the dyno. On the Chevy side, Dart offers both small- and big-block combinations. Looking for large power numbers, we opted for not only a big-block, but the biggest one offered in the SHP line up. Displacing 540 ci, the SHP 540 was a result of combining a 4.50-inch bore with a 4.25-inch stroke. The Dart Sportsman Big M block was more than up to the task of our intended abuse, but we specified upgrades to the reciprocating assembly to suit our needs. Since both nitrous (and blowers) were in the cards, we upgraded the cast crank to a 4340 forged unit. The SHP short-block already featured 4340 forged I-beam rods (with 7/16 cap screws), but we stepped up to the forged (flat-top) pistons as well.
In order to keep costs reasonable, the SHP short-blocks are available with cast cranks and hypereutectic pistons, which are more than strong enough for most normally aspirated combinations. Since our blower applications would likely exceed 1,000 hp, the forged components were deemed mandatory. With the Big M block stuffed to the gills with a forged reciprocating assembly, the Dart short-block was ready for action.
Mild 540 Combo
We planned on illustrating both the strength and possibilities offered by the Dart SHP short-block by building mild and wild normally aspirated combinations. We figured there would be enough power here for just about any '55-57 Chevy fan. But just to keep things interesting, we decided to throw in a little nitrous. First up was the mild combination. The term mild only compares to the wild combination, as the mild version exceeded 630 hp and 640 lb-ft of torque. Hardly mild by most standards, but the numbers get pretty big when you are starting with 540 ci. By keeping the short-block the same, we were able to replace the power producers, namely the heads, cam and intake manifold and dial up the power output. For the mild combination, we chose a set of as-cast, bolt-on aluminum heads, a streetable (but powerful) hydraulic roller cam and a single-plane intake manifold. Some may question the choice of a single-plane intake on our mild combo, but were this motor in a street car, you'd have no shortage of low-speed torque that might require a dual-plane design. Remember, we are talking about 540 ci!
One area of concern during the build up process was the use of flat-top pistons (with valve reliefs). This combined with the near 120cc combustion chambers on most performance BBC heads created a static compression ratio near 9.0:1. For a dedicated normally aspirated combination, we'd recommend a bit more squeeze, but our multi-purpose motor was to receive boost at a later date and as such, a compression compromise was necessary. The low compression ratio did mean that this power output was easily doable on pump gas and very likely on 87 octane if no premium is available.