Those of you who have been following Project C3 Triple-Ex have undoubtedly been anxiously awaiting our decision as to which powerplant would be placed between the framerails. In previous issues we outlined our planned engine build, which was instigated in part by Editor Heath's challenge to build an old-school, first-generation small-block Chevy to similar specs as the Z06 LS7, but with a higher output. While this goal might seem easily attainable, we knew that building a 505-horse (or higher) small-block that would run on pump gas presented several daunting obstacles. This month we'll outline how we elected to overcome these obstacles, and show you our final engine assembly and dyno results.
It's no secret that GM's LS series of V-8 powerplants offers superior design in several ways, particularly in terms of cylinder-head flow. Let's face it: The small-block Chevy has been around since the '50s, and engine technology has come a long way since then. By virtue of a better valve angle (15 degrees in the standard LS, 12 degrees for the LS7), along with larger ports and massive 2.200-inch titanium intake valves, the CNC-ported LS7 head is simply a better-engineered piece than most heads available for the old-school SBC. With that in mind, we knew our work was cut out for us when deciding which parts to use for our project engine.
Starting with the basics, we decided that since the LS7 is built with an aluminum block and heads, our small-block should meet the same criteria. After choosing a cast-aluminum block from Dart Machinery, we had Auto Performance Engines (APE) in nearby Auburndale, Florida, machine it to a final bore size of 4.125 inches, identical to the LS7. We then filled the block with a Scat 4-inch-stroke forged crankshaft, Scat 6-inch forged I-beam connecting rods from Summit Racing Equipment, and forged-aluminum dish pistons from Mahle.
While the LS7 has lighter titanium connecting rods and cast pistons, we felt the exotic rod material didn't justify its cost in our case. Forged pistons will also ensure our engine is durable, and they won't give any discernible power advantage over cast units. We chose not to add a dry-sump oil system to our engine, even though the LS7 does have this feature. Admittedly we're giving up some power here, but the simplicity of a wet-sump system from Stef's Fabrication Specialties will make final installation into our '71 Stingray somewhat easier.
To top this engine, Dart's aluminum Pro 1 CNC cylinder heads with 227cc intake runners were chosen for their proven performance and relatively reasonable cost. While we're sacrificing a fair amount of runner volume, flow, and valve size to the LS7 heads, we felt that the 23-degree Pro 1s would meet our needs while not requiring any exotic parts. Even better, these heads come from Dart fully assembled and ready to bolt on, eliminating the need for expensive cylinder-head machine work (although we did find it necessary to have APE angle mill them to achieve our required compression ratio of 11.1:1, very close to the LS7's 11:1 ratio).
We're giving up quite a bit of flow here, as Dart claims a 309-cfm intake flow (at 0.700-inch lift), compared with some 360 cfm for the LS7 heads, but achieving comparable flow would require fully ported race heads. Additionally, the best valve angle available for the SBC is 18 degrees, well short of the 12-degree LS7. As you're likely figuring out, we aren't making our job any easier, but if we can achieve our performance goals using less exotic 23-degree heads, it will keep costs down and more closely resemble an engine that the average enthusiast might build for his (or her) car.